scsi: docs: convert scsi_mid_low_api.txt to ReST

Link: https://lore.kernel.org/r/881e7741dfed5d6f5f73e1dfc2826b200b8604aa.1583136624.git.mchehab+huawei@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

4 files changed, 1336 insertions, 1281 deletions

diff --git a/Documentation/scsi/index.rst b/Documentation/scsi/index.rst
index 119280f26da6..48337be1c3f1 100644
--- a/Documentation/scsi/index.rst
+++ b/Documentation/scsi/index.rst
@@ -35,5 +35,6 @@ Linux SCSI Subsystem
    scsi_eh
    scsi_fc_transport
    scsi-generic
+   scsi_mid_low_api
 
    scsi_transport_srp/figures
diff --git a/Documentation/scsi/scsi_eh.rst b/Documentation/scsi/scsi_eh.rst
index 341f22f35056..7d78c2475615 100644
--- a/Documentation/scsi/scsi_eh.rst
+++ b/Documentation/scsi/scsi_eh.rst
@@ -5,7 +5,7 @@ SCSI EH
 =======
 
 This document describes SCSI midlayer error handling infrastructure.
-Please refer to Documentation/scsi/scsi_mid_low_api.txt for more
+Please refer to Documentation/scsi/scsi_mid_low_api.rst for more
 information regarding SCSI midlayer.
 
 .. TABLE OF CONTENTS
diff --git a/Documentation/scsi/scsi_mid_low_api.rst b/Documentation/scsi/scsi_mid_low_api.rst
new file mode 100644
index 000000000000..37ee63b147e9
--- /dev/null
+++ b/Documentation/scsi/scsi_mid_low_api.rst
@@ -0,0 +1,1334 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============================================
+SCSI mid_level - lower_level driver interface
+=============================================
+
+Introduction
+============
+This document outlines the interface between the Linux SCSI mid level and
+SCSI lower level drivers. Lower level drivers (LLDs) are variously called
+host bus adapter (HBA) drivers and host drivers (HD). A "host" in this
+context is a bridge between a computer IO bus (e.g. PCI or ISA) and a
+single SCSI initiator port on a SCSI transport. An "initiator" port
+(SCSI terminology, see SAM-3 at http://www.t10.org) sends SCSI commands
+to "target" SCSI ports (e.g. disks). There can be many LLDs in a running
+system, but only one per hardware type. Most LLDs can control one or more
+SCSI HBAs. Some HBAs contain multiple hosts.
+
+In some cases the SCSI transport is an external bus that already has
+its own subsystem in Linux (e.g. USB and ieee1394). In such cases the
+SCSI subsystem LLD is a software bridge to the other driver subsystem.
+Examples are the usb-storage driver (found in the drivers/usb/storage
+directory) and the ieee1394/sbp2 driver (found in the drivers/ieee1394
+directory).
+
+For example, the aic7xxx LLD controls Adaptec SCSI parallel interface
+(SPI) controllers based on that company's 7xxx chip series. The aic7xxx
+LLD can be built into the kernel or loaded as a module. There can only be
+one aic7xxx LLD running in a Linux system but it may be controlling many
+HBAs. These HBAs might be either on PCI daughter-boards or built into
+the motherboard (or both). Some aic7xxx based HBAs are dual controllers
+and thus represent two hosts. Like most modern HBAs, each aic7xxx host
+has its own PCI device address. [The one-to-one correspondence between
+a SCSI host and a PCI device is common but not required (e.g. with
+ISA adapters).]
+
+The SCSI mid level isolates an LLD from other layers such as the SCSI
+upper layer drivers and the block layer.
+
+This version of the document roughly matches linux kernel version 2.6.8 .
+
+Documentation
+=============
+There is a SCSI documentation directory within the kernel source tree,
+typically Documentation/scsi . Most documents are in plain
+(i.e. ASCII) text. This file is named scsi_mid_low_api.txt and can be
+found in that directory. A more recent copy of this document may be found
+at http://web.archive.org/web/20070107183357rn_1/sg.torque.net/scsi/.
+Many LLDs are documented there (e.g. aic7xxx.txt). The SCSI mid-level is
+briefly described in scsi.txt which contains a url to a document
+describing the SCSI subsystem in the lk 2.4 series. Two upper level
+drivers have documents in that directory: st.txt (SCSI tape driver) and
+scsi-generic.txt (for the sg driver).
+
+Some documentation (or urls) for LLDs may be found in the C source code
+or in the same directory as the C source code. For example to find a url
+about the USB mass storage driver see the
+/usr/src/linux/drivers/usb/storage directory.
+
+Driver structure
+================
+Traditionally an LLD for the SCSI subsystem has been at least two files in
+the drivers/scsi directory. For example, a driver called "xyz" has a header
+file "xyz.h" and a source file "xyz.c". [Actually there is no good reason
+why this couldn't all be in one file; the header file is superfluous.] Some
+drivers that have been ported to several operating systems have more than
+two files. For example the aic7xxx driver has separate files for generic
+and OS-specific code (e.g. FreeBSD and Linux). Such drivers tend to have
+their own directory under the drivers/scsi directory.
+
+When a new LLD is being added to Linux, the following files (found in the
+drivers/scsi directory) will need some attention: Makefile and Kconfig .
+It is probably best to study how existing LLDs are organized.
+
+As the 2.5 series development kernels evolve into the 2.6 series
+production series, changes are being introduced into this interface. An
+example of this is driver initialization code where there are now 2 models
+available. The older one, similar to what was found in the lk 2.4 series,
+is based on hosts that are detected at HBA driver load time. This will be
+referred to the "passive" initialization model. The newer model allows HBAs
+to be hot plugged (and unplugged) during the lifetime of the LLD and will
+be referred to as the "hotplug" initialization model. The newer model is
+preferred as it can handle both traditional SCSI equipment that is
+permanently connected as well as modern "SCSI" devices (e.g. USB or
+IEEE 1394 connected digital cameras) that are hotplugged. Both
+initialization models are discussed in the following sections.
+
+An LLD interfaces to the SCSI subsystem several ways:
+
+  a) directly invoking functions supplied by the mid level
+  b) passing a set of function pointers to a registration function
+     supplied by the mid level. The mid level will then invoke these
+     functions at some point in the future. The LLD will supply
+     implementations of these functions.
+  c) direct access to instances of well known data structures maintained
+     by the mid level
+
+Those functions in group a) are listed in a section entitled "Mid level
+supplied functions" below.
+
+Those functions in group b) are listed in a section entitled "Interface
+functions" below. Their function pointers are placed in the members of
+"struct scsi_host_template", an instance of which is passed to
+scsi_host_alloc() [#]_.  Those interface functions that the LLD does not
+wish to supply should have NULL placed in the corresponding member of
+struct scsi_host_template.  Defining an instance of struct
+scsi_host_template at file scope will cause NULL to be  placed in function
+pointer members not explicitly initialized.
+
+Those usages in group c) should be handled with care, especially in a
+"hotplug" environment. LLDs should be aware of the lifetime of instances
+that are shared with the mid level and other layers.
+
+All functions defined within an LLD and all data defined at file scope
+should be static. For example the slave_alloc() function in an LLD
+called "xxx" could be defined as
+``static int xxx_slave_alloc(struct scsi_device * sdev) { /* code */ }``
+
+.. [#] the scsi_host_alloc() function is a replacement for the rather vaguely
+       named scsi_register() function in most situations.
+
+
+Hotplug initialization model
+============================
+In this model an LLD controls when SCSI hosts are introduced and removed
+from the SCSI subsystem. Hosts can be introduced as early as driver
+initialization and removed as late as driver shutdown. Typically a driver
+will respond to a sysfs probe() callback that indicates an HBA has been
+detected. After confirming that the new device is one that the LLD wants
+to control, the LLD will initialize the HBA and then register a new host
+with the SCSI mid level.
+
+During LLD initialization the driver should register itself with the
+appropriate IO bus on which it expects to find HBA(s) (e.g. the PCI bus).
+This can probably be done via sysfs. Any driver parameters (especially
+those that are writable after the driver is loaded) could also be
+registered with sysfs at this point. The SCSI mid level first becomes
+aware of an LLD when that LLD registers its first HBA.
+
+At some later time, the LLD becomes aware of an HBA and what follows
+is a typical sequence of calls between the LLD and the mid level.
+This example shows the mid level scanning the newly introduced HBA for 3
+scsi devices of which only the first 2 respond::
+
+	HBA PROBE: assume 2 SCSI devices found in scan
+    LLD                   mid level                    LLD
+    ===-------------------=========--------------------===------
+    scsi_host_alloc()  -->
+    scsi_add_host()  ---->
+    scsi_scan_host()  -------+
+			    |
+			slave_alloc()
+			slave_configure() -->  scsi_change_queue_depth()
+			    |
+			slave_alloc()
+			slave_configure()
+			    |
+			slave_alloc()   ***
+			slave_destroy() ***
+
+
+    *** For scsi devices that the mid level tries to scan but do not
+	respond, a slave_alloc(), slave_destroy() pair is called.
+
+If the LLD wants to adjust the default queue settings, it can invoke
+scsi_change_queue_depth() in its slave_configure() routine.
+
+When an HBA is being removed it could be as part of an orderly shutdown
+associated with the LLD module being unloaded (e.g. with the "rmmod"
+command) or in response to a "hot unplug" indicated by sysfs()'s
+remove() callback being invoked. In either case, the sequence is the
+same::
+
+	    HBA REMOVE: assume 2 SCSI devices attached
+    LLD                      mid level                 LLD
+    ===----------------------=========-----------------===------
+    scsi_remove_host() ---------+
+				|
+			slave_destroy()
+			slave_destroy()
+    scsi_host_put()
+
+It may be useful for a LLD to keep track of struct Scsi_Host instances
+(a pointer is returned by scsi_host_alloc()). Such instances are "owned"
+by the mid-level.  struct Scsi_Host instances are freed from
+scsi_host_put() when the reference count hits zero.
+
+Hot unplugging an HBA that controls a disk which is processing SCSI
+commands on a mounted file system is an interesting situation. Reference
+counting logic is being introduced into the mid level to cope with many
+of the issues involved. See the section on reference counting below.
+
+
+The hotplug concept may be extended to SCSI devices. Currently, when an
+HBA is added, the scsi_scan_host() function causes a scan for SCSI devices
+attached to the HBA's SCSI transport. On newer SCSI transports the HBA
+may become aware of a new SCSI device _after_ the scan has completed.
+An LLD can use this sequence to make the mid level aware of a SCSI device::
+
+		    SCSI DEVICE hotplug
+    LLD                   mid level                    LLD
+    ===-------------------=========--------------------===------
+    scsi_add_device()  ------+
+			    |
+			slave_alloc()
+			slave_configure()   [--> scsi_change_queue_depth()]
+
+In a similar fashion, an LLD may become aware that a SCSI device has been
+removed (unplugged) or the connection to it has been interrupted. Some
+existing SCSI transports (e.g. SPI) may not become aware that a SCSI
+device has been removed until a subsequent SCSI command fails which will
+probably cause that device to be set offline by the mid level. An LLD that
+detects the removal of a SCSI device can instigate its removal from
+upper layers with this sequence::
+
+		    SCSI DEVICE hot unplug
+    LLD                      mid level                 LLD
+    ===----------------------=========-----------------===------
+    scsi_remove_device() -------+
+				|
+			slave_destroy()
+
+It may be useful for an LLD to keep track of struct scsi_device instances
+(a pointer is passed as the parameter to slave_alloc() and
+slave_configure() callbacks). Such instances are "owned" by the mid-level.
+struct scsi_device instances are freed after slave_destroy().
+
+
+Reference Counting
+==================
+The Scsi_Host structure has had reference counting infrastructure added.
+This effectively spreads the ownership of struct Scsi_Host instances
+across the various SCSI layers which use them. Previously such instances
+were exclusively owned by the mid level. LLDs would not usually need to
+directly manipulate these reference counts but there may be some cases
+where they do.
+
+There are 3 reference counting functions of interest associated with
+struct Scsi_Host:
+
+  - scsi_host_alloc():
+	returns a pointer to new instance of struct
+        Scsi_Host which has its reference count ^^ set to 1
+
+  - scsi_host_get():
+	adds 1 to the reference count of the given instance
+
+  - scsi_host_put():
+	decrements 1 from the reference count of the given
+        instance. If the reference count reaches 0 then the given instance
+        is freed
+
+The scsi_device structure has had reference counting infrastructure added.
+This effectively spreads the ownership of struct scsi_device instances
+across the various SCSI layers which use them. Previously such instances
+were exclusively owned by the mid level. See the access functions declared
+towards the end of include/scsi/scsi_device.h . If an LLD wants to keep
+a copy of a pointer to a scsi_device instance it should use scsi_device_get()
+to bump its reference count. When it is finished with the pointer it can
+use scsi_device_put() to decrement its reference count (and potentially
+delete it).
+
+.. Note::
+
+   struct Scsi_Host actually has 2 reference counts which are manipulated
+   in parallel by these functions.
+
+
+Conventions
+===========
+First, Linus Torvalds's thoughts on C coding style can be found in the
+Documentation/process/coding-style.rst file.
+
+Next, there is a movement to "outlaw" typedefs introducing synonyms for
+struct tags. Both can be still found in the SCSI subsystem, but
+the typedefs have been moved to a single file, scsi_typedefs.h to
+make their future removal easier, for example:
+"typedef struct scsi_cmnd Scsi_Cmnd;"
+
+Also, most C99 enhancements are encouraged to the extent they are supported
+by the relevant gcc compilers. So C99 style structure and array
+initializers are encouraged where appropriate. Don't go too far,
+VLAs are not properly supported yet.  An exception to this is the use of
+``//`` style comments; ``/*...*/`` comments are still preferred in Linux.
+
+Well written, tested and documented code, need not be re-formatted to
+comply with the above conventions. For example, the aic7xxx driver
+comes to Linux from FreeBSD and Adaptec's own labs. No doubt FreeBSD
+and Adaptec have their own coding conventions.
+
+
+Mid level supplied functions
+============================
+These functions are supplied by the SCSI mid level for use by LLDs.
+The names (i.e. entry points) of these functions are exported
+so an LLD that is a module can access them. The kernel will
+arrange for the SCSI mid level to be loaded and initialized before any LLD
+is initialized. The functions below are listed alphabetically and their
+names all start with ``scsi_``.
+
+Summary:
+
+  - scsi_add_device - creates new scsi device (lu) instance
+  - scsi_add_host - perform sysfs registration and set up transport class
+  - scsi_change_queue_depth - change the queue depth on a SCSI device
+  - scsi_bios_ptable - return copy of block device's partition table
+  - scsi_block_requests - prevent further commands being queued to given host
+  - scsi_host_alloc - return a new scsi_host instance whose refcount==1
+  - scsi_host_get - increments Scsi_Host instance's refcount
+  - scsi_host_put - decrements Scsi_Host instance's refcount (free if 0)
+  - scsi_partsize - parse partition table into cylinders, heads + sectors
+  - scsi_register - create and register a scsi host adapter instance.
+  - scsi_remove_device - detach and remove a SCSI device
+  - scsi_remove_host - detach and remove all SCSI devices owned by host
+  - scsi_report_bus_reset - report scsi _bus_ reset observed
+  - scsi_scan_host - scan SCSI bus
+  - scsi_track_queue_full - track successive QUEUE_FULL events
+  - scsi_unblock_requests - allow further commands to be queued to given host
+  - scsi_unregister - [calls scsi_host_put()]
+
+
+Details::
+
+    /**
+    * scsi_add_device - creates new scsi device (lu) instance
+    * @shost:   pointer to scsi host instance
+    * @channel: channel number (rarely other than 0)
+    * @id:      target id number
+    * @lun:     logical unit number
+    *
+    *      Returns pointer to new struct scsi_device instance or
+    *      ERR_PTR(-ENODEV) (or some other bent pointer) if something is
+    *      wrong (e.g. no lu responds at given address)
+    *
+    *      Might block: yes
+    *
+    *      Notes: This call is usually performed internally during a scsi
+    *      bus scan when an HBA is added (i.e. scsi_scan_host()). So it
+    *      should only be called if the HBA becomes aware of a new scsi
+    *      device (lu) after scsi_scan_host() has completed. If successful
+    *      this call can lead to slave_alloc() and slave_configure() callbacks
+    *      into the LLD.
+    *
+    *      Defined in: drivers/scsi/scsi_scan.c
+    **/
+    struct scsi_device * scsi_add_device(struct Scsi_Host *shost,
+					unsigned int channel,
+					unsigned int id, unsigned int lun)
+
+
+    /**
+    * scsi_add_host - perform sysfs registration and set up transport class
+    * @shost:   pointer to scsi host instance
+    * @dev:     pointer to struct device of type scsi class
+    *
+    *      Returns 0 on success, negative errno of failure (e.g. -ENOMEM)
+    *
+    *      Might block: no
+    *
+    *      Notes: Only required in "hotplug initialization model" after a
+    *      successful call to scsi_host_alloc().  This function does not
+    *	scan the bus; this can be done by calling scsi_scan_host() or
+    *	in some other transport-specific way.  The LLD must set up
+    *	the transport template before calling this function and may only
+    *	access the transport class data after this function has been called.
+    *
+    *      Defined in: drivers/scsi/hosts.c
+    **/
+    int scsi_add_host(struct Scsi_Host *shost, struct device * dev)
+
+
+    /**
+    * scsi_change_queue_depth - allow LLD to change queue depth on a SCSI device
+    * @sdev:       pointer to SCSI device to change queue depth on
+    * @tags        Number of tags allowed if tagged queuing enabled,
+    *              or number of commands the LLD can queue up
+    *              in non-tagged mode (as per cmd_per_lun).
+    *
+    *      Returns nothing
+    *
+    *      Might block: no
+    *
+    *      Notes: Can be invoked any time on a SCSI device controlled by this
+    *      LLD. [Specifically during and after slave_configure() and prior to
+    *      slave_destroy().] Can safely be invoked from interrupt code.
+    *
+    *      Defined in: drivers/scsi/scsi.c [see source code for more notes]
+    *
+    **/
+    int scsi_change_queue_depth(struct scsi_device *sdev, int tags)
+
+
+    /**
+    * scsi_bios_ptable - return copy of block device's partition table
+    * @dev:        pointer to block device
+    *
+    *      Returns pointer to partition table, or NULL for failure
+    *
+    *      Might block: yes
+    *
+    *      Notes: Caller owns memory returned (free with kfree() )
+    *
+    *      Defined in: drivers/scsi/scsicam.c
+    **/
+    unsigned char *scsi_bios_ptable(struct block_device *dev)
+
+
+    /**
+    * scsi_block_requests - prevent further commands being queued to given host
+    *
+    * @shost: pointer to host to block commands on
+    *
+    *      Returns nothing
+    *
+    *      Might block: no
+    *
+    *      Notes: There is no timer nor any other means by which the requests
+    *      get unblocked other than the LLD calling scsi_unblock_requests().
+    *
+    *      Defined in: drivers/scsi/scsi_lib.c
+    **/
+    void scsi_block_requests(struct Scsi_Host * shost)
+
+
+    /**
+    * scsi_host_alloc - create a scsi host adapter instance and perform basic
+    *                   initialization.
+    * @sht:        pointer to scsi host template
+    * @privsize:   extra bytes to allocate in hostdata array (which is the
+    *              last member of the returned Scsi_Host instance)
+    *
+    *      Returns pointer to new Scsi_Host instance or NULL on failure
+    *
+    *      Might block: yes
+    *
+    *      Notes: When this call returns to the LLD, the SCSI bus scan on
+    *      this host has _not_ yet been done.
+    *      The hostdata array (by default zero length) is a per host scratch
+    *      area for the LLD's exclusive use.
+    *      Both associated refcounting objects have their refcount set to 1.
+    *      Full registration (in sysfs) and a bus scan are performed later when
+    *      scsi_add_host() and scsi_scan_host() are called.
+    *
+    *      Defined in: drivers/scsi/hosts.c .
+    **/
+    struct Scsi_Host * scsi_host_alloc(struct scsi_host_template * sht,
+				    int privsize)
+
+
+    /**
+    * scsi_host_get - increment Scsi_Host instance refcount
+    * @shost:   pointer to struct Scsi_Host instance
+    *
+    *      Returns nothing
+    *
+    *      Might block: currently may block but may be changed to not block
+    *
+    *      Notes: Actually increments the counts in two sub-objects
+    *
+    *      Defined in: drivers/scsi/hosts.c
+    **/
+    void scsi_host_get(struct Scsi_Host *shost)
+
+
+    /**
+    * scsi_host_put - decrement Scsi_Host instance refcount, free if 0
+    * @shost:   pointer to struct Scsi_Host instance
+    *
+    *      Returns nothing
+    *
+    *      Might block: currently may block but may be changed to not block
+    *
+    *      Notes: Actually decrements the counts in two sub-objects. If the
+    *      latter refcount reaches 0, the Scsi_Host instance is freed.
+    *      The LLD need not worry exactly when the Scsi_Host instance is
+    *      freed, it just shouldn't access the instance after it has balanced
+    *      out its refcount usage.
+    *
+    *      Defined in: drivers/scsi/hosts.c
+    **/
+    void scsi_host_put(struct Scsi_Host *shost)
+
+
+    /**
+    * scsi_partsize - parse partition table into cylinders, heads + sectors
+    * @buf: pointer to partition table
+    * @capacity: size of (total) disk in 512 byte sectors
+    * @cyls: outputs number of cylinders calculated via this pointer
+    * @hds: outputs number of heads calculated via this pointer
+    * @secs: outputs number of sectors calculated via this pointer
+    *
+    *      Returns 0 on success, -1 on failure
+    *
+    *      Might block: no
+    *
+    *      Notes: Caller owns memory returned (free with kfree() )
+    *
+    *      Defined in: drivers/scsi/scsicam.c
+    **/
+    int scsi_partsize(unsigned char *buf, unsigned long capacity,
+		    unsigned int *cyls, unsigned int *hds, unsigned int *secs)
+
+
+    /**
+    * scsi_register - create and register a scsi host adapter instance.
+    * @sht:        pointer to scsi host template
+    * @privsize:   extra bytes to allocate in hostdata array (which is the
+    *              last member of the returned Scsi_Host instance)
+    *
+    *      Returns pointer to new Scsi_Host instance or NULL on failure
+    *
+    *      Might block: yes
+    *
+    *      Notes: When this call returns to the LLD, the SCSI bus scan on
+    *      this host has _not_ yet been done.
+    *      The hostdata array (by default zero length) is a per host scratch
+    *      area for the LLD.
+    *
+    *      Defined in: drivers/scsi/hosts.c .
+    **/
+    struct Scsi_Host * scsi_register(struct scsi_host_template * sht,
+				    int privsize)
+
+
+    /**
+    * scsi_remove_device - detach and remove a SCSI device
+    * @sdev:      a pointer to a scsi device instance
+    *
+    *      Returns value: 0 on success, -EINVAL if device not attached
+    *
+    *      Might block: yes
+    *
+    *      Notes: If an LLD becomes aware that a scsi device (lu) has
+    *      been removed but its host is still present then it can request
+    *      the removal of that scsi device. If successful this call will
+    *      lead to the slave_destroy() callback being invoked. sdev is an
+    *      invalid pointer after this call.
+    *
+    *      Defined in: drivers/scsi/scsi_sysfs.c .
+    **/
+    int scsi_remove_device(struct scsi_device *sdev)
+
+
+    /**
+    * scsi_remove_host - detach and remove all SCSI devices owned by host
+    * @shost:      a pointer to a scsi host instance
+    *
+    *      Returns value: 0 on success, 1 on failure (e.g. LLD busy ??)
+    *
+    *      Might block: yes
+    *
+    *      Notes: Should only be invoked if the "hotplug initialization
+    *      model" is being used. It should be called _prior_ to
+    *      scsi_unregister().
+    *
+    *      Defined in: drivers/scsi/hosts.c .
+    **/
+    int scsi_remove_host(struct Scsi_Host *shost)
+
+
+    /**
+    * scsi_report_bus_reset - report scsi _bus_ reset observed
+    * @shost: a pointer to a scsi host involved
+    * @channel: channel (within) host on which scsi bus reset occurred
+    *
+    *      Returns nothing
+    *
+    *      Might block: no
+    *
+    *      Notes: This only needs to be called if the reset is one which
+    *      originates from an unknown location.  Resets originated by the
+    *      mid level itself don't need to call this, but there should be
+    *      no harm.  The main purpose of this is to make sure that a
+    *      CHECK_CONDITION is properly treated.
+    *
+    *      Defined in: drivers/scsi/scsi_error.c .
+    **/
+    void scsi_report_bus_reset(struct Scsi_Host * shost, int channel)
+
+
+    /**
+    * scsi_scan_host - scan SCSI bus
+    * @shost: a pointer to a scsi host instance
+    *
+    *	Might block: yes
+    *
+    *	Notes: Should be called after scsi_add_host()
+    *
+    *	Defined in: drivers/scsi/scsi_scan.c
+    **/
+    void scsi_scan_host(struct Scsi_Host *shost)
+
+
+    /**
+    * scsi_track_queue_full - track successive QUEUE_FULL events on given
+    *                      device to determine if and when there is a need
+    *                      to adjust the queue depth on the device.
+    * @sdev:  pointer to SCSI device instance
+    * @depth: Current number of outstanding SCSI commands on this device,
+    *         not counting the one returned as QUEUE_FULL.
+    *
+    *      Returns 0  - no change needed
+    *              >0 - adjust queue depth to this new depth
+    *              -1 - drop back to untagged operation using host->cmd_per_lun
+    *                   as the untagged command depth
+    *
+    *      Might block: no
+    *
+    *      Notes: LLDs may call this at any time and we will do "The Right
+    *              Thing"; interrupt context safe.
+    *
+    *      Defined in: drivers/scsi/scsi.c .
+    **/
+    int scsi_track_queue_full(struct scsi_device *sdev, int depth)
+
+
+    /**
+    * scsi_unblock_requests - allow further commands to be queued to given host
+    *
+    * @shost: pointer to host to unblock commands on
+    *
+    *      Returns nothing
+    *
+    *      Might block: no
+    *
+    *      Defined in: drivers/scsi/scsi_lib.c .
+    **/
+    void scsi_unblock_requests(struct Scsi_Host * shost)
+
+
+    /**
+    * scsi_unregister - unregister and free memory used by host instance
+    * @shp:        pointer to scsi host instance to unregister.
+    *
+    *      Returns nothing
+    *
+    *      Might block: no
+    *
+    *      Notes: Should not be invoked if the "hotplug initialization
+    *      model" is being used. Called internally by exit_this_scsi_driver()
+    *      in the "passive initialization model". Hence a LLD has no need to
+    *      call this function directly.
+    *
+    *      Defined in: drivers/scsi/hosts.c .
+    **/
+    void scsi_unregister(struct Scsi_Host * shp)
+
+
+
+
+Interface Functions
+===================
+Interface functions are supplied (defined) by LLDs and their function
+pointers are placed in an instance of struct scsi_host_template which
+is passed to scsi_host_alloc() [or scsi_register() / init_this_scsi_driver()].
+Some are mandatory. Interface functions should be declared static. The
+accepted convention is that driver "xyz" will declare its slave_configure()
+function as::
+
+    static int xyz_slave_configure(struct scsi_device * sdev);
+
+and so forth for all interface functions listed below.
+
+A pointer to this function should be placed in the 'slave_configure' member
+of a "struct scsi_host_template" instance. A pointer to such an instance
+should be passed to the mid level's scsi_host_alloc() [or scsi_register() /
+init_this_scsi_driver()].
+
+The interface functions are also described in the include/scsi/scsi_host.h
+file immediately above their definition point in "struct scsi_host_template".
+In some cases more detail is given in scsi_host.h than below.
+
+The interface functions are listed below in alphabetical order.
+
+Summary:
+
+  - bios_param - fetch head, sector, cylinder info for a disk
+  - eh_timed_out - notify the host that a command timer expired
+  - eh_abort_handler - abort given command
+  - eh_bus_reset_handler - issue SCSI bus reset
+  - eh_device_reset_handler - issue SCSI device reset
+  - eh_host_reset_handler - reset host (host bus adapter)
+  - info - supply information about given host
+  - ioctl - driver can respond to ioctls
+  - proc_info - supports /proc/scsi/{driver_name}/{host_no}
+  - queuecommand - queue scsi command, invoke 'done' on completion
+  - slave_alloc - prior to any commands being sent to a new device
+  - slave_configure - driver fine tuning for given device after attach
+  - slave_destroy - given device is about to be shut down
+
+
+Details::
+
+    /**
+    *      bios_param - fetch head, sector, cylinder info for a disk
+    *      @sdev: pointer to scsi device context (defined in
+    *             include/scsi/scsi_device.h)
+    *      @bdev: pointer to block device context (defined in fs.h)
+    *      @capacity:  device size (in 512 byte sectors)
+    *      @params: three element array to place output:
+    *              params[0] number of heads (max 255)
+    *              params[1] number of sectors (max 63)
+    *              params[2] number of cylinders
+    *
+    *      Return value is ignored
+    *
+    *      Locks: none
+    *
+    *      Calling context: process (sd)
+    *
+    *      Notes: an arbitrary geometry (based on READ CAPACITY) is used
+    *      if this function is not provided. The params array is
+    *      pre-initialized with made up values just in case this function
+    *      doesn't output anything.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int bios_param(struct scsi_device * sdev, struct block_device *bdev,
+		    sector_t capacity, int params[3])
+
+
+    /**
+    *      eh_timed_out - The timer for the command has just fired
+    *      @scp: identifies command timing out
+    *
+    *      Returns:
+    *
+    *      EH_HANDLED:             I fixed the error, please complete the command
+    *      EH_RESET_TIMER:         I need more time, reset the timer and
+    *                              begin counting again
+    *      EH_NOT_HANDLED          Begin normal error recovery
+    *
+    *
+    *      Locks: None held
+    *
+    *      Calling context: interrupt
+    *
+    *      Notes: This is to give the LLD an opportunity to do local recovery.
+    *      This recovery is limited to determining if the outstanding command
+    *      will ever complete.  You may not abort and restart the command from
+    *      this callback.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int eh_timed_out(struct scsi_cmnd * scp)
+
+
+    /**
+    *      eh_abort_handler - abort command associated with scp
+    *      @scp: identifies command to be aborted
+    *
+    *      Returns SUCCESS if command aborted else FAILED
+    *
+    *      Locks: None held
+    *
+    *      Calling context: kernel thread
+    *
+    *      Notes: If 'no_async_abort' is defined this callback
+    *  	will be invoked from scsi_eh thread. No other commands
+    *	will then be queued on current host during eh.
+    *	Otherwise it will be called whenever scsi_times_out()
+    *      is called due to a command timeout.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int eh_abort_handler(struct scsi_cmnd * scp)
+
+
+    /**
+    *      eh_bus_reset_handler - issue SCSI bus reset
+    *      @scp: SCSI bus that contains this device should be reset
+    *
+    *      Returns SUCCESS if command aborted else FAILED
+    *
+    *      Locks: None held
+    *
+    *      Calling context: kernel thread
+    *
+    *      Notes: Invoked from scsi_eh thread. No other commands will be
+    *      queued on current host during eh.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int eh_bus_reset_handler(struct scsi_cmnd * scp)
+
+
+    /**
+    *      eh_device_reset_handler - issue SCSI device reset
+    *      @scp: identifies SCSI device to be reset
+    *
+    *      Returns SUCCESS if command aborted else FAILED
+    *
+    *      Locks: None held
+    *
+    *      Calling context: kernel thread
+    *
+    *      Notes: Invoked from scsi_eh thread. No other commands will be
+    *      queued on current host during eh.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int eh_device_reset_handler(struct scsi_cmnd * scp)
+
+
+    /**
+    *      eh_host_reset_handler - reset host (host bus adapter)
+    *      @scp: SCSI host that contains this device should be reset
+    *
+    *      Returns SUCCESS if command aborted else FAILED
+    *
+    *      Locks: None held
+    *
+    *      Calling context: kernel thread
+    *
+    *      Notes: Invoked from scsi_eh thread. No other commands will be
+    *      queued on current host during eh.
+    *      With the default eh_strategy in place, if none of the _abort_,
+    *      _device_reset_, _bus_reset_ or this eh handler function are
+    *      defined (or they all return FAILED) then the device in question
+    *      will be set offline whenever eh is invoked.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int eh_host_reset_handler(struct scsi_cmnd * scp)
+
+
+    /**
+    *      info - supply information about given host: driver name plus data
+    *             to distinguish given host
+    *      @shp: host to supply information about
+    *
+    *      Return ASCII null terminated string. [This driver is assumed to
+    *      manage the memory pointed to and maintain it, typically for the
+    *      lifetime of this host.]
+    *
+    *      Locks: none
+    *
+    *      Calling context: process
+    *
+    *      Notes: Often supplies PCI or ISA information such as IO addresses
+    *      and interrupt numbers. If not supplied struct Scsi_Host::name used
+    *      instead. It is assumed the returned information fits on one line
+    *      (i.e. does not included embedded newlines).
+    *      The SCSI_IOCTL_PROBE_HOST ioctl yields the string returned by this
+    *      function (or struct Scsi_Host::name if this function is not
+    *      available).
+    *      In a similar manner, init_this_scsi_driver() outputs to the console
+    *      each host's "info" (or name) for the driver it is registering.
+    *      Also if proc_info() is not supplied, the output of this function
+    *      is used instead.
+    *
+    *      Optionally defined in: LLD
+    **/
+	const char * info(struct Scsi_Host * shp)
+
+
+    /**
+    *      ioctl - driver can respond to ioctls
+    *      @sdp: device that ioctl was issued for
+    *      @cmd: ioctl number
+    *      @arg: pointer to read or write data from. Since it points to
+    *            user space, should use appropriate kernel functions
+    *            (e.g. copy_from_user() ). In the Unix style this argument
+    *            can also be viewed as an unsigned long.
+    *
+    *      Returns negative "errno" value when there is a problem. 0 or a
+    *      positive value indicates success and is returned to the user space.
+    *
+    *      Locks: none
+    *
+    *      Calling context: process
+    *
+    *      Notes: The SCSI subsystem uses a "trickle down" ioctl model.
+    *      The user issues an ioctl() against an upper level driver
+    *      (e.g. /dev/sdc) and if the upper level driver doesn't recognize
+    *      the 'cmd' then it is passed to the SCSI mid level. If the SCSI
+    *      mid level does not recognize it, then the LLD that controls
+    *      the device receives the ioctl. According to recent Unix standards
+    *      unsupported ioctl() 'cmd' numbers should return -ENOTTY.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int ioctl(struct scsi_device *sdp, int cmd, void *arg)
+
+
+    /**
+    *      proc_info - supports /proc/scsi/{driver_name}/{host_no}
+    *      @buffer: anchor point to output to (0==writeto1_read0) or fetch from
+    *               (1==writeto1_read0).
+    *      @start: where "interesting" data is written to. Ignored when
+    *              1==writeto1_read0.
+    *      @offset: offset within buffer 0==writeto1_read0 is actually
+    *               interested in. Ignored when 1==writeto1_read0 .
+    *      @length: maximum (or actual) extent of buffer
+    *      @host_no: host number of interest (struct Scsi_Host::host_no)
+    *      @writeto1_read0: 1 -> data coming from user space towards driver
+    *                            (e.g. "echo some_string > /proc/scsi/xyz/2")
+    *                       0 -> user what data from this driver
+    *                            (e.g. "cat /proc/scsi/xyz/2")
+    *
+    *      Returns length when 1==writeto1_read0. Otherwise number of chars
+    *      output to buffer past offset.
+    *
+    *      Locks: none held
+    *
+    *      Calling context: process
+    *
+    *      Notes: Driven from scsi_proc.c which interfaces to proc_fs. proc_fs
+    *      support can now be configured out of the scsi subsystem.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int proc_info(char * buffer, char ** start, off_t offset,
+		    int length, int host_no, int writeto1_read0)
+
+
+    /**
+    *      queuecommand - queue scsi command, invoke scp->scsi_done on completion
+    *      @shost: pointer to the scsi host object
+    *      @scp: pointer to scsi command object
+    *
+    *      Returns 0 on success.
+    *
+    *      If there's a failure, return either:
+    *
+    *      SCSI_MLQUEUE_DEVICE_BUSY if the device queue is full, or
+    *      SCSI_MLQUEUE_HOST_BUSY if the entire host queue is full
+    *
+    *      On both of these returns, the mid-layer will requeue the I/O
+    *
+    *      - if the return is SCSI_MLQUEUE_DEVICE_BUSY, only that particular
+    *      device will be paused, and it will be unpaused when a command to
+    *      the device returns (or after a brief delay if there are no more
+    *      outstanding commands to it).  Commands to other devices continue
+    *      to be processed normally.
+    *
+    *      - if the return is SCSI_MLQUEUE_HOST_BUSY, all I/O to the host
+    *      is paused and will be unpaused when any command returns from
+    *      the host (or after a brief delay if there are no outstanding
+    *      commands to the host).
+    *
+    *      For compatibility with earlier versions of queuecommand, any
+    *      other return value is treated the same as
+    *      SCSI_MLQUEUE_HOST_BUSY.
+    *
+    *      Other types of errors that are detected immediately may be
+    *      flagged by setting scp->result to an appropriate value,
+    *      invoking the scp->scsi_done callback, and then returning 0
+    *      from this function. If the command is not performed
+    *      immediately (and the LLD is starting (or will start) the given
+    *      command) then this function should place 0 in scp->result and
+    *      return 0.
+    *
+    *      Command ownership.  If the driver returns zero, it owns the
+    *      command and must take responsibility for ensuring the
+    *      scp->scsi_done callback is executed.  Note: the driver may
+    *      call scp->scsi_done before returning zero, but after it has
+    *      called scp->scsi_done, it may not return any value other than
+    *      zero.  If the driver makes a non-zero return, it must not
+    *      execute the command's scsi_done callback at any time.
+    *
+    *      Locks: up to and including 2.6.36, struct Scsi_Host::host_lock
+    *             held on entry (with "irqsave") and is expected to be
+    *             held on return. From 2.6.37 onwards, queuecommand is
+    *             called without any locks held.
+    *
+    *      Calling context: in interrupt (soft irq) or process context
+    *
+    *      Notes: This function should be relatively fast. Normally it
+    *      will not wait for IO to complete. Hence the scp->scsi_done
+    *      callback is invoked (often directly from an interrupt service
+    *      routine) some time after this function has returned. In some
+    *      cases (e.g. pseudo adapter drivers that manufacture the
+    *      response to a SCSI INQUIRY) the scp->scsi_done callback may be
+    *      invoked before this function returns.  If the scp->scsi_done
+    *      callback is not invoked within a certain period the SCSI mid
+    *      level will commence error processing.  If a status of CHECK
+    *      CONDITION is placed in "result" when the scp->scsi_done
+    *      callback is invoked, then the LLD driver should perform
+    *      autosense and fill in the struct scsi_cmnd::sense_buffer
+    *      array. The scsi_cmnd::sense_buffer array is zeroed prior to
+    *      the mid level queuing a command to an LLD.
+    *
+    *      Defined in: LLD
+    **/
+	int queuecommand(struct Scsi_Host *shost, struct scsi_cmnd * scp)
+
+
+    /**
+    *      slave_alloc -   prior to any commands being sent to a new device
+    *                      (i.e. just prior to scan) this call is made
+    *      @sdp: pointer to new device (about to be scanned)
+    *
+    *      Returns 0 if ok. Any other return is assumed to be an error and
+    *      the device is ignored.
+    *
+    *      Locks: none
+    *
+    *      Calling context: process
+    *
+    *      Notes: Allows the driver to allocate any resources for a device
+    *      prior to its initial scan. The corresponding scsi device may not
+    *      exist but the mid level is just about to scan for it (i.e. send
+    *      and INQUIRY command plus ...). If a device is found then
+    *      slave_configure() will be called while if a device is not found
+    *      slave_destroy() is called.
+    *      For more details see the include/scsi/scsi_host.h file.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int slave_alloc(struct scsi_device *sdp)
+
+
+    /**
+    *      slave_configure - driver fine tuning for given device just after it
+    *                     has been first scanned (i.e. it responded to an
+    *                     INQUIRY)
+    *      @sdp: device that has just been attached
+    *
+    *      Returns 0 if ok. Any other return is assumed to be an error and
+    *      the device is taken offline. [offline devices will _not_ have
+    *      slave_destroy() called on them so clean up resources.]
+    *
+    *      Locks: none
+    *
+    *      Calling context: process
+    *
+    *      Notes: Allows the driver to inspect the response to the initial
+    *      INQUIRY done by the scanning code and take appropriate action.
+    *      For more details see the include/scsi/scsi_host.h file.
+    *
+    *      Optionally defined in: LLD
+    **/
+	int slave_configure(struct scsi_device *sdp)
+
+
+    /**
+    *      slave_destroy - given device is about to be shut down. All
+    *                      activity has ceased on this device.
+    *      @sdp: device that is about to be shut down
+    *
+    *      Returns nothing
+    *
+    *      Locks: none
+    *
+    *      Calling context: process
+    *
+    *      Notes: Mid level structures for given device are still in place
+    *      but are about to be torn down. Any per device resources allocated
+    *      by this driver for given device should be freed now. No further
+    *      commands will be sent for this sdp instance. [However the device
+    *      could be re-attached in the future in which case a new instance
+    *      of struct scsi_device would be supplied by future slave_alloc()
+    *      and slave_configure() calls.]
+    *
+    *      Optionally defined in: LLD
+    **/
+	void slave_destroy(struct scsi_device *sdp)
+
+
+
+Data Structures
+===============
+struct scsi_host_template
+-------------------------
+There is one "struct scsi_host_template" instance per LLD [#]_. It is
+typically initialized as a file scope static in a driver's header file. That
+way members that are not explicitly initialized will be set to 0 or NULL.
+Member of interest:
+
+    name
+		 - name of driver (may contain spaces, please limit to
+                   less than 80 characters)
+
+    proc_name
+		 - name used in "/proc/scsi/<proc_name>/<host_no>" and
+                   by sysfs in one of its "drivers" directories. Hence
+                   "proc_name" should only contain characters acceptable
+                   to a Unix file name.
+
+   ``(*queuecommand)()``
+		 - primary callback that the mid level uses to inject
+                   SCSI commands into an LLD.
+
+The structure is defined and commented in include/scsi/scsi_host.h
+
+.. [#] In extreme situations a single driver may have several instances
+       if it controls several different classes of hardware (e.g. an LLD
+       that handles both ISA and PCI cards and has a separate instance of
+       struct scsi_host_template for each class).
+
+struct Scsi_Host
+----------------
+There is one struct Scsi_Host instance per host (HBA) that an LLD
+controls. The struct Scsi_Host structure has many members in common
+with "struct scsi_host_template". When a new struct Scsi_Host instance
+is created (in scsi_host_alloc() in hosts.c) those common members are
+initialized from the driver's struct scsi_host_template instance. Members
+of interest:
+
+    host_no
+		 - system wide unique number that is used for identifying
+                   this host. Issued in ascending order from 0.
+    can_queue
+		 - must be greater than 0; do not send more than can_queue
+                   commands to the adapter.
+    this_id
+		 - scsi id of host (scsi initiator) or -1 if not known
+    sg_tablesize
+		 - maximum scatter gather elements allowed by host.
+                   Set this to SG_ALL or less to avoid chained SG lists.
+                   Must be at least 1.
+    max_sectors
+		 - maximum number of sectors (usually 512 bytes) allowed
+                   in a single SCSI command. The default value of 0 leads
+                   to a setting of SCSI_DEFAULT_MAX_SECTORS (defined in
+                   scsi_host.h) which is currently set to 1024. So for a
+                   disk the maximum transfer size is 512 KB when max_sectors
+                   is not defined. Note that this size may not be sufficient
+                   for disk firmware uploads.
+    cmd_per_lun
+		 - maximum number of commands that can be queued on devices
+                   controlled by the host. Overridden by LLD calls to
+                   scsi_change_queue_depth().
+    unchecked_isa_dma
+		 - 1=>only use bottom 16 MB of ram (ISA DMA addressing
+                   restriction), 0=>can use full 32 bit (or better) DMA
+                   address space
+    no_async_abort
+		 - 1=>Asynchronous aborts are not supported
+		 - 0=>Timed-out commands will be aborted asynchronously
+    hostt
+		 - pointer to driver's struct scsi_host_template from which
+                   this struct Scsi_Host instance was spawned
+    hostt->proc_name
+		 - name of LLD. This is the driver name that sysfs uses
+    transportt
+		 - pointer to driver's struct scsi_transport_template instance
+                   (if any). FC and SPI transports currently supported.
+    sh_list
+		 - a double linked list of pointers to all struct Scsi_Host
+                   instances (currently ordered by ascending host_no)
+    my_devices
+		 - a double linked list of pointers to struct scsi_device
+                   instances that belong to this host.
+    hostdata[0]
+		 - area reserved for LLD at end of struct Scsi_Host. Size
+                   is set by the second argument (named 'xtr_bytes') to
+                   scsi_host_alloc() or scsi_register().
+    vendor_id
+		 - a unique value that identifies the vendor supplying
+                   the LLD for the Scsi_Host.  Used most often in validating
+                   vendor-specific message requests.  Value consists of an
+                   identifier type and a vendor-specific value.
+                   See scsi_netlink.h for a description of valid formats.
+
+The scsi_host structure is defined in include/scsi/scsi_host.h
+
+struct scsi_device
+------------------
+Generally, there is one instance of this structure for each SCSI logical unit
+on a host. Scsi devices connected to a host are uniquely identified by a
+channel number, target id and logical unit number (lun).
+The structure is defined in include/scsi/scsi_device.h
+
+struct scsi_cmnd
+----------------
+Instances of this structure convey SCSI commands to the LLD and responses
+back to the mid level. The SCSI mid level will ensure that no more SCSI
+commands become queued against the LLD than are indicated by
+scsi_change_queue_depth() (or struct Scsi_Host::cmd_per_lun). There will
+be at least one instance of struct scsi_cmnd available for each SCSI device.
+Members of interest:
+
+    cmnd
+		 - array containing SCSI command
+    cmnd_len
+		 - length (in bytes) of SCSI command
+    sc_data_direction
+		 - direction of data transfer in data phase. See
+                   "enum dma_data_direction" in include/linux/dma-mapping.h
+    request_bufflen
+		 - number of data bytes to transfer (0 if no data phase)
+    use_sg
+		 - ==0 -> no scatter gather list, hence transfer data
+                          to/from request_buffer
+                 - >0 ->  scatter gather list (actually an array) in
+                          request_buffer with use_sg elements
+    request_buffer
+		   - either contains data buffer or scatter gather list
+                     depending on the setting of use_sg. Scatter gather
+                     elements are defined by 'struct scatterlist' found
+                     in include/linux/scatterlist.h .
+    done
+		 - function pointer that should be invoked by LLD when the
+                   SCSI command is completed (successfully or otherwise).
+                   Should only be called by an LLD if the LLD has accepted
+                   the command (i.e. queuecommand() returned or will return
+                   0). The LLD may invoke 'done'  prior to queuecommand()
+                   finishing.
+    result
+		 - should be set by LLD prior to calling 'done'. A value
+                   of 0 implies a successfully completed command (and all
+                   data (if any) has been transferred to or from the SCSI
+                   target device). 'result' is a 32 bit unsigned integer that
+                   can be viewed as 4 related bytes. The SCSI status value is
+                   in the LSB. See include/scsi/scsi.h status_byte(),
+                   msg_byte(), host_byte() and driver_byte() macros and
+                   related constants.
+    sense_buffer
+		 - an array (maximum size: SCSI_SENSE_BUFFERSIZE bytes) that
+                   should be written when the SCSI status (LSB of 'result')
+                   is set to CHECK_CONDITION (2). When CHECK_CONDITION is
+                   set, if the top nibble of sense_buffer[0] has the value 7
+                   then the mid level will assume the sense_buffer array
+                   contains a valid SCSI sense buffer; otherwise the mid
+                   level will issue a REQUEST_SENSE SCSI command to
+                   retrieve the sense buffer. The latter strategy is error
+                   prone in the presence of command queuing so the LLD should
+                   always "auto-sense".
+    device
+		 - pointer to scsi_device object that this command is
+                   associated with.
+    resid
+		 - an LLD should set this signed integer to the requested
+                   transfer length (i.e. 'request_bufflen') less the number
+                   of bytes that are actually transferred. 'resid' is
+                   preset to 0 so an LLD can ignore it if it cannot detect
+                   underruns (overruns should be rare). If possible an LLD
+                   should set 'resid' prior to invoking 'done'. The most
+                   interesting case is data transfers from a SCSI target
+                   device (e.g. READs) that underrun.
+    underflow
+		 - LLD should place (DID_ERROR << 16) in 'result' if
+                   actual number of bytes transferred is less than this
+                   figure. Not many LLDs implement this check and some that
+                   do just output an error message to the log rather than
+                   report a DID_ERROR. Better for an LLD to implement
+                   'resid'.
+
+It is recommended that a LLD set 'resid' on data transfers from a SCSI
+target device (e.g. READs). It is especially important that 'resid' is set
+when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
+(and possibly RECOVERED ERROR). In these cases if a LLD is in doubt how much
+data has been received then the safest approach is to indicate no bytes have
+been received. For example: to indicate that no valid data has been received
+a LLD might use these helpers::
+
+    scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
+
+where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
+bytes blocks has been received 'resid' could be set like this::
+
+    scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
+
+The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
+
+
+Locks
+=====
+Each struct Scsi_Host instance has a spin_lock called struct
+Scsi_Host::default_lock which is initialized in scsi_host_alloc() [found in
+hosts.c]. Within the same function the struct Scsi_Host::host_lock pointer
+is initialized to point at default_lock.  Thereafter lock and unlock
+operations performed by the mid level use the struct Scsi_Host::host_lock
+pointer.  Previously drivers could override the host_lock pointer but
+this is not allowed anymore.
+
+
+Autosense
+=========
+Autosense (or auto-sense) is defined in the SAM-2 document as "the
+automatic return of sense data to the application client coincident
+with the completion of a SCSI command" when a status of CHECK CONDITION
+occurs. LLDs should perform autosense. This should be done when the LLD
+detects a CHECK CONDITION status by either:
+
+    a) instructing the SCSI protocol (e.g. SCSI Parallel Interface (SPI))
+       to perform an extra data in phase on such responses
+    b) or, the LLD issuing a REQUEST SENSE command itself
+
+Either way, when a status of CHECK CONDITION is detected, the mid level
+decides whether the LLD has performed autosense by checking struct
+scsi_cmnd::sense_buffer[0] . If this byte has an upper nibble of 7 (or 0xf)
+then autosense is assumed to have taken place. If it has another value (and
+this byte is initialized to 0 before each command) then the mid level will
+issue a REQUEST SENSE command.
+
+In the presence of queued commands the "nexus" that maintains sense
+buffer data from the command that failed until a following REQUEST SENSE
+may get out of synchronization. This is why it is best for the LLD
+to perform autosense.
+
+
+Changes since lk 2.4 series
+===========================
+io_request_lock has been replaced by several finer grained locks. The lock
+relevant to LLDs is struct Scsi_Host::host_lock and there is
+one per SCSI host.
+
+The older error handling mechanism has been removed. This means the
+LLD interface functions abort() and reset() have been removed.
+The struct scsi_host_template::use_new_eh_code flag has been removed.
+
+In the 2.4 series the SCSI subsystem configuration descriptions were
+aggregated with the configuration descriptions from all other Linux
+subsystems in the Documentation/Configure.help file. In the 2.6 series,
+the SCSI subsystem now has its own (much smaller) drivers/scsi/Kconfig
+file that contains both configuration and help information.
+
+struct SHT has been renamed to struct scsi_host_template.
+
+Addition of the "hotplug initialization model" and many extra functions
+to support it.
+
+
+Credits
+=======
+The following people have contributed to this document:
+
+	- Mike Anderson <andmike at us dot ibm dot com>
+	- James Bottomley <James dot Bottomley at hansenpartnership dot com>
+	- Patrick Mansfield <patmans at us dot ibm dot com>
+	- Christoph Hellwig <hch at infradead dot org>
+	- Doug Ledford <dledford at redhat dot com>
+	- Andries Brouwer <Andries dot Brouwer at cwi dot nl>
+	- Randy Dunlap <rdunlap at xenotime dot net>
+	- Alan Stern <stern at rowland dot harvard dot edu>
+
+
+Douglas Gilbert
+dgilbert at interlog dot com
+
+21st September 2004
diff --git a/Documentation/scsi/scsi_mid_low_api.txt b/Documentation/scsi/scsi_mid_low_api.txt
deleted file mode 100644
index 2a4be1c3e6db..000000000000
--- a/Documentation/scsi/scsi_mid_low_api.txt
+++ /dev/null
@@ -1,1280 +0,0 @@
-                          Linux Kernel 2.6 series
-                 SCSI mid_level - lower_level driver interface
-                 =============================================
-
-Introduction
-============
-This document outlines the interface between the Linux SCSI mid level and
-SCSI lower level drivers. Lower level drivers (LLDs) are variously called 
-host bus adapter (HBA) drivers and host drivers (HD). A "host" in this
-context is a bridge between a computer IO bus (e.g. PCI or ISA) and a
-single SCSI initiator port on a SCSI transport. An "initiator" port
-(SCSI terminology, see SAM-3 at http://www.t10.org) sends SCSI commands
-to "target" SCSI ports (e.g. disks). There can be many LLDs in a running
-system, but only one per hardware type. Most LLDs can control one or more
-SCSI HBAs. Some HBAs contain multiple hosts.
-
-In some cases the SCSI transport is an external bus that already has
-its own subsystem in Linux (e.g. USB and ieee1394). In such cases the
-SCSI subsystem LLD is a software bridge to the other driver subsystem.
-Examples are the usb-storage driver (found in the drivers/usb/storage
-directory) and the ieee1394/sbp2 driver (found in the drivers/ieee1394
-directory).
-
-For example, the aic7xxx LLD controls Adaptec SCSI parallel interface
-(SPI) controllers based on that company's 7xxx chip series. The aic7xxx
-LLD can be built into the kernel or loaded as a module. There can only be
-one aic7xxx LLD running in a Linux system but it may be controlling many 
-HBAs. These HBAs might be either on PCI daughter-boards or built into 
-the motherboard (or both). Some aic7xxx based HBAs are dual controllers
-and thus represent two hosts. Like most modern HBAs, each aic7xxx host
-has its own PCI device address. [The one-to-one correspondence between
-a SCSI host and a PCI device is common but not required (e.g. with
-ISA adapters).]
-
-The SCSI mid level isolates an LLD from other layers such as the SCSI
-upper layer drivers and the block layer.
-
-This version of the document roughly matches linux kernel version 2.6.8 .
-
-Documentation
-=============
-There is a SCSI documentation directory within the kernel source tree, 
-typically Documentation/scsi . Most documents are in plain
-(i.e. ASCII) text. This file is named scsi_mid_low_api.txt and can be 
-found in that directory. A more recent copy of this document may be found
-at http://web.archive.org/web/20070107183357rn_1/sg.torque.net/scsi/. 
-Many LLDs are documented there (e.g. aic7xxx.txt). The SCSI mid-level is
-briefly described in scsi.txt which contains a url to a document 
-describing the SCSI subsystem in the lk 2.4 series. Two upper level 
-drivers have documents in that directory: st.txt (SCSI tape driver) and 
-scsi-generic.txt (for the sg driver).
-
-Some documentation (or urls) for LLDs may be found in the C source code
-or in the same directory as the C source code. For example to find a url
-about the USB mass storage driver see the 
-/usr/src/linux/drivers/usb/storage directory.
-
-Driver structure
-================
-Traditionally an LLD for the SCSI subsystem has been at least two files in
-the drivers/scsi directory. For example, a driver called "xyz" has a header
-file "xyz.h" and a source file "xyz.c". [Actually there is no good reason
-why this couldn't all be in one file; the header file is superfluous.] Some
-drivers that have been ported to several operating systems have more than
-two files. For example the aic7xxx driver has separate files for generic 
-and OS-specific code (e.g. FreeBSD and Linux). Such drivers tend to have
-their own directory under the drivers/scsi directory.
-
-When a new LLD is being added to Linux, the following files (found in the
-drivers/scsi directory) will need some attention: Makefile and Kconfig .
-It is probably best to study how existing LLDs are organized.
-
-As the 2.5 series development kernels evolve into the 2.6 series
-production series, changes are being introduced into this interface. An
-example of this is driver initialization code where there are now 2 models
-available. The older one, similar to what was found in the lk 2.4 series,
-is based on hosts that are detected at HBA driver load time. This will be
-referred to the "passive" initialization model. The newer model allows HBAs
-to be hot plugged (and unplugged) during the lifetime of the LLD and will
-be referred to as the "hotplug" initialization model. The newer model is
-preferred as it can handle both traditional SCSI equipment that is
-permanently connected as well as modern "SCSI" devices (e.g. USB or
-IEEE 1394 connected digital cameras) that are hotplugged. Both 
-initialization models are discussed in the following sections.
-
-An LLD interfaces to the SCSI subsystem several ways:
-  a) directly invoking functions supplied by the mid level
-  b) passing a set of function pointers to a registration function
-     supplied by the mid level. The mid level will then invoke these
-     functions at some point in the future. The LLD will supply
-     implementations of these functions.
-  c) direct access to instances of well known data structures maintained
-     by the mid level
-
-Those functions in group a) are listed in a section entitled "Mid level
-supplied functions" below.
-
-Those functions in group b) are listed in a section entitled "Interface
-functions" below. Their function pointers are placed in the members of
-"struct scsi_host_template", an instance of which is passed to
-scsi_host_alloc() ** .  Those interface functions that the LLD does not 
-wish to supply should have NULL placed in the corresponding member of 
-struct scsi_host_template.  Defining an instance of struct 
-scsi_host_template at file scope will cause NULL to be  placed in function
- pointer members not explicitly initialized.
-
-Those usages in group c) should be handled with care, especially in a
-"hotplug" environment. LLDs should be aware of the lifetime of instances
-that are shared with the mid level and other layers.
-
-All functions defined within an LLD and all data defined at file scope
-should be static. For example the slave_alloc() function in an LLD
-called "xxx" could be defined as 
-"static int xxx_slave_alloc(struct scsi_device * sdev) { /* code */ }"
-
-** the scsi_host_alloc() function is a replacement for the rather vaguely
-named scsi_register() function in most situations.
-
-
-Hotplug initialization model
-============================
-In this model an LLD controls when SCSI hosts are introduced and removed
-from the SCSI subsystem. Hosts can be introduced as early as driver
-initialization and removed as late as driver shutdown. Typically a driver
-will respond to a sysfs probe() callback that indicates an HBA has been
-detected. After confirming that the new device is one that the LLD wants
-to control, the LLD will initialize the HBA and then register a new host
-with the SCSI mid level.
-
-During LLD initialization the driver should register itself with the
-appropriate IO bus on which it expects to find HBA(s) (e.g. the PCI bus).
-This can probably be done via sysfs. Any driver parameters (especially
-those that are writable after the driver is loaded) could also be
-registered with sysfs at this point. The SCSI mid level first becomes
-aware of an LLD when that LLD registers its first HBA.
-
-At some later time, the LLD becomes aware of an HBA and what follows
-is a typical sequence of calls between the LLD and the mid level.
-This example shows the mid level scanning the newly introduced HBA for 3 
-scsi devices of which only the first 2 respond:
-
-     HBA PROBE: assume 2 SCSI devices found in scan
-LLD                   mid level                    LLD
-===-------------------=========--------------------===------
-scsi_host_alloc()  -->
-scsi_add_host()  ---->
-scsi_scan_host()  -------+
-                         |
-                    slave_alloc()
-                    slave_configure() -->  scsi_change_queue_depth()
-                         |
-                    slave_alloc()
-                    slave_configure()
-                         |
-                    slave_alloc()   ***
-                    slave_destroy() ***
-------------------------------------------------------------
-
-If the LLD wants to adjust the default queue settings, it can invoke
-scsi_change_queue_depth() in its slave_configure() routine.
-
-*** For scsi devices that the mid level tries to scan but do not
-    respond, a slave_alloc(), slave_destroy() pair is called.
-
-When an HBA is being removed it could be as part of an orderly shutdown
-associated with the LLD module being unloaded (e.g. with the "rmmod"
-command) or in response to a "hot unplug" indicated by sysfs()'s
-remove() callback being invoked. In either case, the sequence is the
-same:
-
-        HBA REMOVE: assume 2 SCSI devices attached
-LLD                      mid level                 LLD
-===----------------------=========-----------------===------
-scsi_remove_host() ---------+
-                            |
-                     slave_destroy()
-                     slave_destroy()
-scsi_host_put()
-------------------------------------------------------------
-                     
-It may be useful for a LLD to keep track of struct Scsi_Host instances
-(a pointer is returned by scsi_host_alloc()). Such instances are "owned"
-by the mid-level.  struct Scsi_Host instances are freed from
-scsi_host_put() when the reference count hits zero.
-
-Hot unplugging an HBA that controls a disk which is processing SCSI
-commands on a mounted file system is an interesting situation. Reference
-counting logic is being introduced into the mid level to cope with many
-of the issues involved. See the section on reference counting below.
-
-
-The hotplug concept may be extended to SCSI devices. Currently, when an
-HBA is added, the scsi_scan_host() function causes a scan for SCSI devices
-attached to the HBA's SCSI transport. On newer SCSI transports the HBA
-may become aware of a new SCSI device _after_ the scan has completed.
-An LLD can use this sequence to make the mid level aware of a SCSI device:
-
-                 SCSI DEVICE hotplug
-LLD                   mid level                    LLD
-===-------------------=========--------------------===------
-scsi_add_device()  ------+
-                         |
-                    slave_alloc()
-                    slave_configure()   [--> scsi_change_queue_depth()]
-------------------------------------------------------------
-
-In a similar fashion, an LLD may become aware that a SCSI device has been
-removed (unplugged) or the connection to it has been interrupted. Some
-existing SCSI transports (e.g. SPI) may not become aware that a SCSI
-device has been removed until a subsequent SCSI command fails which will
-probably cause that device to be set offline by the mid level. An LLD that
-detects the removal of a SCSI device can instigate its removal from
-upper layers with this sequence:
-
-                  SCSI DEVICE hot unplug
-LLD                      mid level                 LLD
-===----------------------=========-----------------===------
-scsi_remove_device() -------+
-                            |
-                     slave_destroy()
-------------------------------------------------------------
-
-It may be useful for an LLD to keep track of struct scsi_device instances
-(a pointer is passed as the parameter to slave_alloc() and
-slave_configure() callbacks). Such instances are "owned" by the mid-level.
-struct scsi_device instances are freed after slave_destroy().
-
-
-Reference Counting
-==================
-The Scsi_Host structure has had reference counting infrastructure added.
-This effectively spreads the ownership of struct Scsi_Host instances
-across the various SCSI layers which use them. Previously such instances
-were exclusively owned by the mid level. LLDs would not usually need to
-directly manipulate these reference counts but there may be some cases
-where they do.
-
-There are 3 reference counting functions of interest associated with
-struct Scsi_Host:
-  - scsi_host_alloc(): returns a pointer to new instance of struct 
-        Scsi_Host which has its reference count ^^ set to 1
-  - scsi_host_get(): adds 1 to the reference count of the given instance
-  - scsi_host_put(): decrements 1 from the reference count of the given
-        instance. If the reference count reaches 0 then the given instance
-        is freed
-
-The scsi_device structure has had reference counting infrastructure added.
-This effectively spreads the ownership of struct scsi_device instances
-across the various SCSI layers which use them. Previously such instances
-were exclusively owned by the mid level. See the access functions declared
-towards the end of include/scsi/scsi_device.h . If an LLD wants to keep
-a copy of a pointer to a scsi_device instance it should use scsi_device_get()
-to bump its reference count. When it is finished with the pointer it can
-use scsi_device_put() to decrement its reference count (and potentially
-delete it).
-
-^^ struct Scsi_Host actually has 2 reference counts which are manipulated
-in parallel by these functions.
-
-
-Conventions
-===========
-First, Linus Torvalds's thoughts on C coding style can be found in the
-Documentation/process/coding-style.rst file.
-
-Next, there is a movement to "outlaw" typedefs introducing synonyms for 
-struct tags. Both can be still found in the SCSI subsystem, but
-the typedefs have been moved to a single file, scsi_typedefs.h to
-make their future removal easier, for example: 
-"typedef struct scsi_cmnd Scsi_Cmnd;"
-
-Also, most C99 enhancements are encouraged to the extent they are supported
-by the relevant gcc compilers. So C99 style structure and array
-initializers are encouraged where appropriate. Don't go too far,
-VLAs are not properly supported yet.  An exception to this is the use of
-"//" style comments; /*...*/ comments are still preferred in Linux.
-
-Well written, tested and documented code, need not be re-formatted to
-comply with the above conventions. For example, the aic7xxx driver
-comes to Linux from FreeBSD and Adaptec's own labs. No doubt FreeBSD
-and Adaptec have their own coding conventions.
-
-
-Mid level supplied functions
-============================
-These functions are supplied by the SCSI mid level for use by LLDs.
-The names (i.e. entry points) of these functions are exported 
-so an LLD that is a module can access them. The kernel will
-arrange for the SCSI mid level to be loaded and initialized before any LLD
-is initialized. The functions below are listed alphabetically and their
-names all start with "scsi_".
-
-Summary:
-   scsi_add_device - creates new scsi device (lu) instance
-   scsi_add_host - perform sysfs registration and set up transport class
-   scsi_change_queue_depth - change the queue depth on a SCSI device
-   scsi_bios_ptable - return copy of block device's partition table
-   scsi_block_requests - prevent further commands being queued to given host
-   scsi_host_alloc - return a new scsi_host instance whose refcount==1
-   scsi_host_get - increments Scsi_Host instance's refcount
-   scsi_host_put - decrements Scsi_Host instance's refcount (free if 0)
-   scsi_partsize - parse partition table into cylinders, heads + sectors
-   scsi_register - create and register a scsi host adapter instance.
-   scsi_remove_device - detach and remove a SCSI device
-   scsi_remove_host - detach and remove all SCSI devices owned by host
-   scsi_report_bus_reset - report scsi _bus_ reset observed
-   scsi_scan_host - scan SCSI bus
-   scsi_track_queue_full - track successive QUEUE_FULL events 
-   scsi_unblock_requests - allow further commands to be queued to given host
-   scsi_unregister - [calls scsi_host_put()]
-
-
-Details:
-
-/**
- * scsi_add_device - creates new scsi device (lu) instance
- * @shost:   pointer to scsi host instance
- * @channel: channel number (rarely other than 0)
- * @id:      target id number
- * @lun:     logical unit number
- *
- *      Returns pointer to new struct scsi_device instance or 
- *      ERR_PTR(-ENODEV) (or some other bent pointer) if something is
- *      wrong (e.g. no lu responds at given address)
- *
- *      Might block: yes
- *
- *      Notes: This call is usually performed internally during a scsi
- *      bus scan when an HBA is added (i.e. scsi_scan_host()). So it
- *      should only be called if the HBA becomes aware of a new scsi
- *      device (lu) after scsi_scan_host() has completed. If successful
- *      this call can lead to slave_alloc() and slave_configure() callbacks
- *      into the LLD.
- *
- *      Defined in: drivers/scsi/scsi_scan.c
- **/
-struct scsi_device * scsi_add_device(struct Scsi_Host *shost, 
-                                     unsigned int channel,
-                                     unsigned int id, unsigned int lun)
-
-
-/**
- * scsi_add_host - perform sysfs registration and set up transport class
- * @shost:   pointer to scsi host instance
- * @dev:     pointer to struct device of type scsi class
- *
- *      Returns 0 on success, negative errno of failure (e.g. -ENOMEM)
- *
- *      Might block: no
- *
- *      Notes: Only required in "hotplug initialization model" after a
- *      successful call to scsi_host_alloc().  This function does not
- *	scan the bus; this can be done by calling scsi_scan_host() or
- *	in some other transport-specific way.  The LLD must set up
- *	the transport template before calling this function and may only
- *	access the transport class data after this function has been called.
- *
- *      Defined in: drivers/scsi/hosts.c
- **/
-int scsi_add_host(struct Scsi_Host *shost, struct device * dev)
-
-
-/**
- * scsi_change_queue_depth - allow LLD to change queue depth on a SCSI device
- * @sdev:       pointer to SCSI device to change queue depth on
- * @tags        Number of tags allowed if tagged queuing enabled,
- *              or number of commands the LLD can queue up
- *              in non-tagged mode (as per cmd_per_lun).
- *
- *      Returns nothing
- *
- *      Might block: no
- *
- *      Notes: Can be invoked any time on a SCSI device controlled by this
- *      LLD. [Specifically during and after slave_configure() and prior to
- *      slave_destroy().] Can safely be invoked from interrupt code.
- *
- *      Defined in: drivers/scsi/scsi.c [see source code for more notes]
- *
- **/
-int scsi_change_queue_depth(struct scsi_device *sdev, int tags)
-
-
-/**
- * scsi_bios_ptable - return copy of block device's partition table
- * @dev:        pointer to block device
- *
- *      Returns pointer to partition table, or NULL for failure
- *
- *      Might block: yes
- *
- *      Notes: Caller owns memory returned (free with kfree() )
- *
- *      Defined in: drivers/scsi/scsicam.c
- **/
-unsigned char *scsi_bios_ptable(struct block_device *dev)
-
-
-/**
- * scsi_block_requests - prevent further commands being queued to given host
- *
- * @shost: pointer to host to block commands on
- *
- *      Returns nothing
- *
- *      Might block: no
- *
- *      Notes: There is no timer nor any other means by which the requests
- *      get unblocked other than the LLD calling scsi_unblock_requests().
- *
- *      Defined in: drivers/scsi/scsi_lib.c
-**/
-void scsi_block_requests(struct Scsi_Host * shost)
-
-
-/**
- * scsi_host_alloc - create a scsi host adapter instance and perform basic
- *                   initialization.
- * @sht:        pointer to scsi host template
- * @privsize:   extra bytes to allocate in hostdata array (which is the
- *              last member of the returned Scsi_Host instance)
- *
- *      Returns pointer to new Scsi_Host instance or NULL on failure
- *
- *      Might block: yes
- *
- *      Notes: When this call returns to the LLD, the SCSI bus scan on
- *      this host has _not_ yet been done.
- *      The hostdata array (by default zero length) is a per host scratch 
- *      area for the LLD's exclusive use.
- *      Both associated refcounting objects have their refcount set to 1.
- *      Full registration (in sysfs) and a bus scan are performed later when
- *      scsi_add_host() and scsi_scan_host() are called.
- *
- *      Defined in: drivers/scsi/hosts.c .
- **/
-struct Scsi_Host * scsi_host_alloc(struct scsi_host_template * sht,
-                                   int privsize)
-
-
-/**
- * scsi_host_get - increment Scsi_Host instance refcount
- * @shost:   pointer to struct Scsi_Host instance
- *
- *      Returns nothing
- *
- *      Might block: currently may block but may be changed to not block
- *
- *      Notes: Actually increments the counts in two sub-objects
- *
- *      Defined in: drivers/scsi/hosts.c
- **/
-void scsi_host_get(struct Scsi_Host *shost)
-
-
-/**
- * scsi_host_put - decrement Scsi_Host instance refcount, free if 0
- * @shost:   pointer to struct Scsi_Host instance
- *
- *      Returns nothing
- *
- *      Might block: currently may block but may be changed to not block
- *
- *      Notes: Actually decrements the counts in two sub-objects. If the
- *      latter refcount reaches 0, the Scsi_Host instance is freed.
- *      The LLD need not worry exactly when the Scsi_Host instance is
- *      freed, it just shouldn't access the instance after it has balanced
- *      out its refcount usage.
- *
- *      Defined in: drivers/scsi/hosts.c
- **/
-void scsi_host_put(struct Scsi_Host *shost)
-
-
-/**
- * scsi_partsize - parse partition table into cylinders, heads + sectors
- * @buf: pointer to partition table
- * @capacity: size of (total) disk in 512 byte sectors
- * @cyls: outputs number of cylinders calculated via this pointer
- * @hds: outputs number of heads calculated via this pointer
- * @secs: outputs number of sectors calculated via this pointer
- *
- *      Returns 0 on success, -1 on failure
- *
- *      Might block: no
- *
- *      Notes: Caller owns memory returned (free with kfree() )
- *
- *      Defined in: drivers/scsi/scsicam.c
- **/
-int scsi_partsize(unsigned char *buf, unsigned long capacity,
-                  unsigned int *cyls, unsigned int *hds, unsigned int *secs)
-
-
-/**
- * scsi_register - create and register a scsi host adapter instance.
- * @sht:        pointer to scsi host template
- * @privsize:   extra bytes to allocate in hostdata array (which is the
- *              last member of the returned Scsi_Host instance)
- *
- *      Returns pointer to new Scsi_Host instance or NULL on failure
- *
- *      Might block: yes
- *
- *      Notes: When this call returns to the LLD, the SCSI bus scan on
- *      this host has _not_ yet been done.
- *      The hostdata array (by default zero length) is a per host scratch 
- *      area for the LLD.
- *
- *      Defined in: drivers/scsi/hosts.c .
- **/
-struct Scsi_Host * scsi_register(struct scsi_host_template * sht,
-                                 int privsize)
-
-
-/**
- * scsi_remove_device - detach and remove a SCSI device
- * @sdev:      a pointer to a scsi device instance
- *
- *      Returns value: 0 on success, -EINVAL if device not attached
- *
- *      Might block: yes
- *
- *      Notes: If an LLD becomes aware that a scsi device (lu) has
- *      been removed but its host is still present then it can request
- *      the removal of that scsi device. If successful this call will
- *      lead to the slave_destroy() callback being invoked. sdev is an 
- *      invalid pointer after this call.
- *
- *      Defined in: drivers/scsi/scsi_sysfs.c .
- **/
-int scsi_remove_device(struct scsi_device *sdev)
-
-
-/**
- * scsi_remove_host - detach and remove all SCSI devices owned by host
- * @shost:      a pointer to a scsi host instance
- *
- *      Returns value: 0 on success, 1 on failure (e.g. LLD busy ??)
- *
- *      Might block: yes
- *
- *      Notes: Should only be invoked if the "hotplug initialization
- *      model" is being used. It should be called _prior_ to  
- *      scsi_unregister().
- *
- *      Defined in: drivers/scsi/hosts.c .
- **/
-int scsi_remove_host(struct Scsi_Host *shost)
-
-
-/**
- * scsi_report_bus_reset - report scsi _bus_ reset observed
- * @shost: a pointer to a scsi host involved
- * @channel: channel (within) host on which scsi bus reset occurred
- *
- *      Returns nothing
- *
- *      Might block: no
- *
- *      Notes: This only needs to be called if the reset is one which
- *      originates from an unknown location.  Resets originated by the 
- *      mid level itself don't need to call this, but there should be 
- *      no harm.  The main purpose of this is to make sure that a
- *      CHECK_CONDITION is properly treated.
- *
- *      Defined in: drivers/scsi/scsi_error.c .
- **/
-void scsi_report_bus_reset(struct Scsi_Host * shost, int channel)
-
-
-/**
- * scsi_scan_host - scan SCSI bus
- * @shost: a pointer to a scsi host instance
- *
- *	Might block: yes
- *
- *	Notes: Should be called after scsi_add_host()
- *
- *	Defined in: drivers/scsi/scsi_scan.c
- **/
-void scsi_scan_host(struct Scsi_Host *shost)
-
-
-/**
- * scsi_track_queue_full - track successive QUEUE_FULL events on given
- *                      device to determine if and when there is a need
- *                      to adjust the queue depth on the device.
- * @sdev:  pointer to SCSI device instance
- * @depth: Current number of outstanding SCSI commands on this device,
- *         not counting the one returned as QUEUE_FULL.
- *
- *      Returns 0  - no change needed
- *              >0 - adjust queue depth to this new depth
- *              -1 - drop back to untagged operation using host->cmd_per_lun
- *                   as the untagged command depth
- *
- *      Might block: no
- *
- *      Notes: LLDs may call this at any time and we will do "The Right
- *              Thing"; interrupt context safe. 
- *
- *      Defined in: drivers/scsi/scsi.c .
- **/
-int scsi_track_queue_full(struct scsi_device *sdev, int depth)
-
-
-/**
- * scsi_unblock_requests - allow further commands to be queued to given host
- *
- * @shost: pointer to host to unblock commands on
- *
- *      Returns nothing
- *
- *      Might block: no
- *
- *      Defined in: drivers/scsi/scsi_lib.c .
-**/
-void scsi_unblock_requests(struct Scsi_Host * shost)
-
-
-/**
- * scsi_unregister - unregister and free memory used by host instance
- * @shp:        pointer to scsi host instance to unregister.
- *
- *      Returns nothing
- *
- *      Might block: no
- *
- *      Notes: Should not be invoked if the "hotplug initialization
- *      model" is being used. Called internally by exit_this_scsi_driver()
- *      in the "passive initialization model". Hence a LLD has no need to
- *      call this function directly.
- *
- *      Defined in: drivers/scsi/hosts.c .
- **/
-void scsi_unregister(struct Scsi_Host * shp)
-
-
-
-
-Interface Functions
-===================
-Interface functions are supplied (defined) by LLDs and their function
-pointers are placed in an instance of struct scsi_host_template which
-is passed to scsi_host_alloc() [or scsi_register() / init_this_scsi_driver()].
-Some are mandatory. Interface functions should be declared static. The
-accepted convention is that driver "xyz" will declare its slave_configure() 
-function as:
-    static int xyz_slave_configure(struct scsi_device * sdev);
-and so forth for all interface functions listed below.
-
-A pointer to this function should be placed in the 'slave_configure' member
-of a "struct scsi_host_template" instance. A pointer to such an instance
-should be passed to the mid level's scsi_host_alloc() [or scsi_register() /
-init_this_scsi_driver()].
-
-The interface functions are also described in the include/scsi/scsi_host.h
-file immediately above their definition point in "struct scsi_host_template".
-In some cases more detail is given in scsi_host.h than below.
-
-The interface functions are listed below in alphabetical order.
-
-Summary:
-   bios_param - fetch head, sector, cylinder info for a disk
-   eh_timed_out - notify the host that a command timer expired
-   eh_abort_handler - abort given command
-   eh_bus_reset_handler - issue SCSI bus reset
-   eh_device_reset_handler - issue SCSI device reset
-   eh_host_reset_handler - reset host (host bus adapter)
-   info - supply information about given host
-   ioctl - driver can respond to ioctls
-   proc_info - supports /proc/scsi/{driver_name}/{host_no}
-   queuecommand - queue scsi command, invoke 'done' on completion
-   slave_alloc - prior to any commands being sent to a new device 
-   slave_configure - driver fine tuning for given device after attach
-   slave_destroy - given device is about to be shut down
-
-
-Details:
-
-/**
- *      bios_param - fetch head, sector, cylinder info for a disk
- *      @sdev: pointer to scsi device context (defined in 
- *             include/scsi/scsi_device.h)
- *      @bdev: pointer to block device context (defined in fs.h)
- *      @capacity:  device size (in 512 byte sectors)
- *      @params: three element array to place output:
- *              params[0] number of heads (max 255)
- *              params[1] number of sectors (max 63)
- *              params[2] number of cylinders 
- *
- *      Return value is ignored
- *
- *      Locks: none
- *
- *      Calling context: process (sd)
- *
- *      Notes: an arbitrary geometry (based on READ CAPACITY) is used
- *      if this function is not provided. The params array is
- *      pre-initialized with made up values just in case this function 
- *      doesn't output anything.
- *
- *      Optionally defined in: LLD
- **/
-    int bios_param(struct scsi_device * sdev, struct block_device *bdev,
-                   sector_t capacity, int params[3])
-
-
-/**
- *      eh_timed_out - The timer for the command has just fired
- *      @scp: identifies command timing out
- *
- *      Returns:
- *
- *      EH_HANDLED:             I fixed the error, please complete the command
- *      EH_RESET_TIMER:         I need more time, reset the timer and
- *                              begin counting again
- *      EH_NOT_HANDLED          Begin normal error recovery
- *
- *
- *      Locks: None held
- *
- *      Calling context: interrupt
- *
- *      Notes: This is to give the LLD an opportunity to do local recovery.
- *      This recovery is limited to determining if the outstanding command
- *      will ever complete.  You may not abort and restart the command from
- *      this callback.
- *
- *      Optionally defined in: LLD
- **/
-     int eh_timed_out(struct scsi_cmnd * scp)
-
-
-/**
- *      eh_abort_handler - abort command associated with scp
- *      @scp: identifies command to be aborted
- *
- *      Returns SUCCESS if command aborted else FAILED
- *
- *      Locks: None held
- *
- *      Calling context: kernel thread
- *
- *      Notes: If 'no_async_abort' is defined this callback
- *  	will be invoked from scsi_eh thread. No other commands
- *	will then be queued on current host during eh.
- *	Otherwise it will be called whenever scsi_times_out()
- *      is called due to a command timeout.
- *
- *      Optionally defined in: LLD
- **/
-     int eh_abort_handler(struct scsi_cmnd * scp)
-
-
-/**
- *      eh_bus_reset_handler - issue SCSI bus reset
- *      @scp: SCSI bus that contains this device should be reset
- *
- *      Returns SUCCESS if command aborted else FAILED
- *
- *      Locks: None held
- *
- *      Calling context: kernel thread
- *
- *      Notes: Invoked from scsi_eh thread. No other commands will be
- *      queued on current host during eh.
- *
- *      Optionally defined in: LLD
- **/
-     int eh_bus_reset_handler(struct scsi_cmnd * scp)
-
-
-/**
- *      eh_device_reset_handler - issue SCSI device reset
- *      @scp: identifies SCSI device to be reset
- *
- *      Returns SUCCESS if command aborted else FAILED
- *
- *      Locks: None held
- *
- *      Calling context: kernel thread
- *
- *      Notes: Invoked from scsi_eh thread. No other commands will be
- *      queued on current host during eh.
- *
- *      Optionally defined in: LLD
- **/
-     int eh_device_reset_handler(struct scsi_cmnd * scp)
-
-
-/**
- *      eh_host_reset_handler - reset host (host bus adapter)
- *      @scp: SCSI host that contains this device should be reset
- *
- *      Returns SUCCESS if command aborted else FAILED
- *
- *      Locks: None held
- *
- *      Calling context: kernel thread
- *
- *      Notes: Invoked from scsi_eh thread. No other commands will be
- *      queued on current host during eh. 
- *      With the default eh_strategy in place, if none of the _abort_, 
- *      _device_reset_, _bus_reset_ or this eh handler function are 
- *      defined (or they all return FAILED) then the device in question 
- *      will be set offline whenever eh is invoked.
- *
- *      Optionally defined in: LLD
- **/
-     int eh_host_reset_handler(struct scsi_cmnd * scp)
-
-
-/**
- *      info - supply information about given host: driver name plus data
- *             to distinguish given host
- *      @shp: host to supply information about
- *
- *      Return ASCII null terminated string. [This driver is assumed to
- *      manage the memory pointed to and maintain it, typically for the
- *      lifetime of this host.]
- *
- *      Locks: none
- *
- *      Calling context: process
- *
- *      Notes: Often supplies PCI or ISA information such as IO addresses
- *      and interrupt numbers. If not supplied struct Scsi_Host::name used
- *      instead. It is assumed the returned information fits on one line 
- *      (i.e. does not included embedded newlines).
- *      The SCSI_IOCTL_PROBE_HOST ioctl yields the string returned by this
- *      function (or struct Scsi_Host::name if this function is not
- *      available).
- *      In a similar manner, init_this_scsi_driver() outputs to the console
- *      each host's "info" (or name) for the driver it is registering.
- *      Also if proc_info() is not supplied, the output of this function
- *      is used instead.
- *
- *      Optionally defined in: LLD
- **/
-    const char * info(struct Scsi_Host * shp)
-
-
-/**
- *      ioctl - driver can respond to ioctls
- *      @sdp: device that ioctl was issued for
- *      @cmd: ioctl number
- *      @arg: pointer to read or write data from. Since it points to
- *            user space, should use appropriate kernel functions
- *            (e.g. copy_from_user() ). In the Unix style this argument
- *            can also be viewed as an unsigned long.
- *
- *      Returns negative "errno" value when there is a problem. 0 or a
- *      positive value indicates success and is returned to the user space.
- *
- *      Locks: none
- *
- *      Calling context: process
- *
- *      Notes: The SCSI subsystem uses a "trickle down" ioctl model.
- *      The user issues an ioctl() against an upper level driver
- *      (e.g. /dev/sdc) and if the upper level driver doesn't recognize
- *      the 'cmd' then it is passed to the SCSI mid level. If the SCSI
- *      mid level does not recognize it, then the LLD that controls
- *      the device receives the ioctl. According to recent Unix standards
- *      unsupported ioctl() 'cmd' numbers should return -ENOTTY.
- *
- *      Optionally defined in: LLD
- **/
-    int ioctl(struct scsi_device *sdp, int cmd, void *arg)
-
-
-/**
- *      proc_info - supports /proc/scsi/{driver_name}/{host_no}
- *      @buffer: anchor point to output to (0==writeto1_read0) or fetch from
- *               (1==writeto1_read0).
- *      @start: where "interesting" data is written to. Ignored when
- *              1==writeto1_read0.
- *      @offset: offset within buffer 0==writeto1_read0 is actually
- *               interested in. Ignored when 1==writeto1_read0 .
- *      @length: maximum (or actual) extent of buffer
- *      @host_no: host number of interest (struct Scsi_Host::host_no)
- *      @writeto1_read0: 1 -> data coming from user space towards driver
- *                            (e.g. "echo some_string > /proc/scsi/xyz/2")
- *                       0 -> user what data from this driver
- *                            (e.g. "cat /proc/scsi/xyz/2")
- *
- *      Returns length when 1==writeto1_read0. Otherwise number of chars
- *      output to buffer past offset.
- *
- *      Locks: none held
- *
- *      Calling context: process
- *
- *      Notes: Driven from scsi_proc.c which interfaces to proc_fs. proc_fs
- *      support can now be configured out of the scsi subsystem.
- *
- *      Optionally defined in: LLD
- **/
-    int proc_info(char * buffer, char ** start, off_t offset, 
-                  int length, int host_no, int writeto1_read0)
-
-
-/**
- *      queuecommand - queue scsi command, invoke scp->scsi_done on completion
- *      @shost: pointer to the scsi host object
- *      @scp: pointer to scsi command object
- *
- *      Returns 0 on success.
- *
- *      If there's a failure, return either:
- *
- *      SCSI_MLQUEUE_DEVICE_BUSY if the device queue is full, or
- *      SCSI_MLQUEUE_HOST_BUSY if the entire host queue is full
- *
- *      On both of these returns, the mid-layer will requeue the I/O
- *
- *      - if the return is SCSI_MLQUEUE_DEVICE_BUSY, only that particular
- *      device will be paused, and it will be unpaused when a command to
- *      the device returns (or after a brief delay if there are no more
- *      outstanding commands to it).  Commands to other devices continue
- *      to be processed normally.
- *
- *      - if the return is SCSI_MLQUEUE_HOST_BUSY, all I/O to the host
- *      is paused and will be unpaused when any command returns from
- *      the host (or after a brief delay if there are no outstanding
- *      commands to the host).
- *
- *      For compatibility with earlier versions of queuecommand, any
- *      other return value is treated the same as
- *      SCSI_MLQUEUE_HOST_BUSY.
- *
- *      Other types of errors that are detected immediately may be
- *      flagged by setting scp->result to an appropriate value,
- *      invoking the scp->scsi_done callback, and then returning 0
- *      from this function. If the command is not performed
- *      immediately (and the LLD is starting (or will start) the given
- *      command) then this function should place 0 in scp->result and
- *      return 0.
- *
- *      Command ownership.  If the driver returns zero, it owns the
- *      command and must take responsibility for ensuring the
- *      scp->scsi_done callback is executed.  Note: the driver may
- *      call scp->scsi_done before returning zero, but after it has
- *      called scp->scsi_done, it may not return any value other than
- *      zero.  If the driver makes a non-zero return, it must not
- *      execute the command's scsi_done callback at any time.
- *
- *      Locks: up to and including 2.6.36, struct Scsi_Host::host_lock
- *             held on entry (with "irqsave") and is expected to be
- *             held on return. From 2.6.37 onwards, queuecommand is
- *             called without any locks held.
- *
- *      Calling context: in interrupt (soft irq) or process context
- *
- *      Notes: This function should be relatively fast. Normally it
- *      will not wait for IO to complete. Hence the scp->scsi_done
- *      callback is invoked (often directly from an interrupt service
- *      routine) some time after this function has returned. In some
- *      cases (e.g. pseudo adapter drivers that manufacture the
- *      response to a SCSI INQUIRY) the scp->scsi_done callback may be
- *      invoked before this function returns.  If the scp->scsi_done
- *      callback is not invoked within a certain period the SCSI mid
- *      level will commence error processing.  If a status of CHECK
- *      CONDITION is placed in "result" when the scp->scsi_done
- *      callback is invoked, then the LLD driver should perform
- *      autosense and fill in the struct scsi_cmnd::sense_buffer
- *      array. The scsi_cmnd::sense_buffer array is zeroed prior to
- *      the mid level queuing a command to an LLD.
- *
- *      Defined in: LLD
- **/
-    int queuecommand(struct Scsi_Host *shost, struct scsi_cmnd * scp)
-
-
-/**
- *      slave_alloc -   prior to any commands being sent to a new device 
- *                      (i.e. just prior to scan) this call is made
- *      @sdp: pointer to new device (about to be scanned)
- *
- *      Returns 0 if ok. Any other return is assumed to be an error and
- *      the device is ignored.
- *
- *      Locks: none
- *
- *      Calling context: process
- *
- *      Notes: Allows the driver to allocate any resources for a device
- *      prior to its initial scan. The corresponding scsi device may not
- *      exist but the mid level is just about to scan for it (i.e. send
- *      and INQUIRY command plus ...). If a device is found then
- *      slave_configure() will be called while if a device is not found
- *      slave_destroy() is called.
- *      For more details see the include/scsi/scsi_host.h file.
- *
- *      Optionally defined in: LLD
- **/
-    int slave_alloc(struct scsi_device *sdp)
-
-
-/**
- *      slave_configure - driver fine tuning for given device just after it
- *                     has been first scanned (i.e. it responded to an
- *                     INQUIRY)
- *      @sdp: device that has just been attached
- *
- *      Returns 0 if ok. Any other return is assumed to be an error and
- *      the device is taken offline. [offline devices will _not_ have
- *      slave_destroy() called on them so clean up resources.]
- *
- *      Locks: none
- *
- *      Calling context: process
- *
- *      Notes: Allows the driver to inspect the response to the initial
- *      INQUIRY done by the scanning code and take appropriate action.
- *      For more details see the include/scsi/scsi_host.h file.
- *
- *      Optionally defined in: LLD
- **/
-    int slave_configure(struct scsi_device *sdp)
-
-
-/**
- *      slave_destroy - given device is about to be shut down. All
- *                      activity has ceased on this device.
- *      @sdp: device that is about to be shut down
- *
- *      Returns nothing
- *
- *      Locks: none
- *
- *      Calling context: process
- *
- *      Notes: Mid level structures for given device are still in place
- *      but are about to be torn down. Any per device resources allocated
- *      by this driver for given device should be freed now. No further
- *      commands will be sent for this sdp instance. [However the device
- *      could be re-attached in the future in which case a new instance
- *      of struct scsi_device would be supplied by future slave_alloc()
- *      and slave_configure() calls.]
- *
- *      Optionally defined in: LLD
- **/
-    void slave_destroy(struct scsi_device *sdp)
-
-
-
-Data Structures
-===============
-struct scsi_host_template
--------------------------
-There is one "struct scsi_host_template" instance per LLD ***. It is
-typically initialized as a file scope static in a driver's header file. That
-way members that are not explicitly initialized will be set to 0 or NULL.
-Member of interest:
-    name         - name of driver (may contain spaces, please limit to
-                   less than 80 characters)
-    proc_name    - name used in "/proc/scsi/<proc_name>/<host_no>" and
-                   by sysfs in one of its "drivers" directories. Hence
-                   "proc_name" should only contain characters acceptable
-                   to a Unix file name.
-   (*queuecommand)() - primary callback that the mid level uses to inject
-                   SCSI commands into an LLD.
-The structure is defined and commented in include/scsi/scsi_host.h
-
-*** In extreme situations a single driver may have several instances
-    if it controls several different classes of hardware (e.g. an LLD
-    that handles both ISA and PCI cards and has a separate instance of
-    struct scsi_host_template for each class).
-
-struct Scsi_Host
-----------------
-There is one struct Scsi_Host instance per host (HBA) that an LLD
-controls. The struct Scsi_Host structure has many members in common
-with "struct scsi_host_template". When a new struct Scsi_Host instance
-is created (in scsi_host_alloc() in hosts.c) those common members are
-initialized from the driver's struct scsi_host_template instance. Members
-of interest:
-    host_no      - system wide unique number that is used for identifying
-                   this host. Issued in ascending order from 0.
-    can_queue    - must be greater than 0; do not send more than can_queue
-                   commands to the adapter.
-    this_id      - scsi id of host (scsi initiator) or -1 if not known
-    sg_tablesize - maximum scatter gather elements allowed by host.
-                   Set this to SG_ALL or less to avoid chained SG lists.
-                   Must be at least 1.
-    max_sectors  - maximum number of sectors (usually 512 bytes) allowed
-                   in a single SCSI command. The default value of 0 leads
-                   to a setting of SCSI_DEFAULT_MAX_SECTORS (defined in
-                   scsi_host.h) which is currently set to 1024. So for a
-                   disk the maximum transfer size is 512 KB when max_sectors
-                   is not defined. Note that this size may not be sufficient
-                   for disk firmware uploads.
-    cmd_per_lun  - maximum number of commands that can be queued on devices
-                   controlled by the host. Overridden by LLD calls to
-                   scsi_change_queue_depth().
-    unchecked_isa_dma - 1=>only use bottom 16 MB of ram (ISA DMA addressing
-                   restriction), 0=>can use full 32 bit (or better) DMA
-                   address space
-    no_async_abort - 1=>Asynchronous aborts are not supported
-                     0=>Timed-out commands will be aborted asynchronously
-    hostt        - pointer to driver's struct scsi_host_template from which
-                   this struct Scsi_Host instance was spawned
-    hostt->proc_name  - name of LLD. This is the driver name that sysfs uses
-    transportt   - pointer to driver's struct scsi_transport_template instance
-                   (if any). FC and SPI transports currently supported.
-    sh_list      - a double linked list of pointers to all struct Scsi_Host
-                   instances (currently ordered by ascending host_no)
-    my_devices   - a double linked list of pointers to struct scsi_device 
-                   instances that belong to this host.
-    hostdata[0]  - area reserved for LLD at end of struct Scsi_Host. Size
-                   is set by the second argument (named 'xtr_bytes') to
-                   scsi_host_alloc() or scsi_register().
-    vendor_id    - a unique value that identifies the vendor supplying
-                   the LLD for the Scsi_Host.  Used most often in validating
-                   vendor-specific message requests.  Value consists of an
-                   identifier type and a vendor-specific value.
-                   See scsi_netlink.h for a description of valid formats.
-
-The scsi_host structure is defined in include/scsi/scsi_host.h
-
-struct scsi_device
-------------------
-Generally, there is one instance of this structure for each SCSI logical unit
-on a host. Scsi devices connected to a host are uniquely identified by a
-channel number, target id and logical unit number (lun).
-The structure is defined in include/scsi/scsi_device.h
-
-struct scsi_cmnd
-----------------
-Instances of this structure convey SCSI commands to the LLD and responses
-back to the mid level. The SCSI mid level will ensure that no more SCSI
-commands become queued against the LLD than are indicated by
-scsi_change_queue_depth() (or struct Scsi_Host::cmd_per_lun). There will
-be at least one instance of struct scsi_cmnd available for each SCSI device.
-Members of interest:
-    cmnd         - array containing SCSI command
-    cmnd_len     - length (in bytes) of SCSI command
-    sc_data_direction - direction of data transfer in data phase. See
-                "enum dma_data_direction" in include/linux/dma-mapping.h
-    request_bufflen - number of data bytes to transfer (0 if no data phase)
-    use_sg       - ==0 -> no scatter gather list, hence transfer data
-                          to/from request_buffer
-                 - >0 ->  scatter gather list (actually an array) in
-                          request_buffer with use_sg elements
-    request_buffer - either contains data buffer or scatter gather list
-                     depending on the setting of use_sg. Scatter gather
-                     elements are defined by 'struct scatterlist' found
-                     in include/linux/scatterlist.h .
-    done         - function pointer that should be invoked by LLD when the
-                   SCSI command is completed (successfully or otherwise).
-                   Should only be called by an LLD if the LLD has accepted
-                   the command (i.e. queuecommand() returned or will return
-                   0). The LLD may invoke 'done'  prior to queuecommand()
-                   finishing.
-    result       - should be set by LLD prior to calling 'done'. A value
-                   of 0 implies a successfully completed command (and all
-                   data (if any) has been transferred to or from the SCSI
-                   target device). 'result' is a 32 bit unsigned integer that
-                   can be viewed as 4 related bytes. The SCSI status value is
-                   in the LSB. See include/scsi/scsi.h status_byte(),
-                   msg_byte(), host_byte() and driver_byte() macros and
-                   related constants.
-    sense_buffer - an array (maximum size: SCSI_SENSE_BUFFERSIZE bytes) that
-                   should be written when the SCSI status (LSB of 'result')
-                   is set to CHECK_CONDITION (2). When CHECK_CONDITION is
-                   set, if the top nibble of sense_buffer[0] has the value 7
-                   then the mid level will assume the sense_buffer array
-                   contains a valid SCSI sense buffer; otherwise the mid
-                   level will issue a REQUEST_SENSE SCSI command to
-                   retrieve the sense buffer. The latter strategy is error
-                   prone in the presence of command queuing so the LLD should
-                   always "auto-sense".
-    device       - pointer to scsi_device object that this command is
-                   associated with.
-    resid        - an LLD should set this signed integer to the requested
-                   transfer length (i.e. 'request_bufflen') less the number
-                   of bytes that are actually transferred. 'resid' is
-                   preset to 0 so an LLD can ignore it if it cannot detect
-                   underruns (overruns should be rare). If possible an LLD
-                   should set 'resid' prior to invoking 'done'. The most
-                   interesting case is data transfers from a SCSI target
-                   device (e.g. READs) that underrun.
-    underflow    - LLD should place (DID_ERROR << 16) in 'result' if
-                   actual number of bytes transferred is less than this
-                   figure. Not many LLDs implement this check and some that
-                   do just output an error message to the log rather than
-                   report a DID_ERROR. Better for an LLD to implement
-                   'resid'.
-
-It is recommended that a LLD set 'resid' on data transfers from a SCSI
-target device (e.g. READs). It is especially important that 'resid' is set
-when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
-(and possibly RECOVERED ERROR). In these cases if a LLD is in doubt how much
-data has been received then the safest approach is to indicate no bytes have
-been received. For example: to indicate that no valid data has been received
-a LLD might use these helpers:
-    scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
-where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
-bytes blocks has been received 'resid' could be set like this:
-    scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
-
-The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
-
-
-Locks
-=====
-Each struct Scsi_Host instance has a spin_lock called struct 
-Scsi_Host::default_lock which is initialized in scsi_host_alloc() [found in 
-hosts.c]. Within the same function the struct Scsi_Host::host_lock pointer
-is initialized to point at default_lock.  Thereafter lock and unlock
-operations performed by the mid level use the struct Scsi_Host::host_lock
-pointer.  Previously drivers could override the host_lock pointer but
-this is not allowed anymore.
-
-
-Autosense
-=========
-Autosense (or auto-sense) is defined in the SAM-2 document as "the
-automatic return of sense data to the application client coincident
-with the completion of a SCSI command" when a status of CHECK CONDITION
-occurs. LLDs should perform autosense. This should be done when the LLD
-detects a CHECK CONDITION status by either: 
-    a) instructing the SCSI protocol (e.g. SCSI Parallel Interface (SPI))
-       to perform an extra data in phase on such responses
-    b) or, the LLD issuing a REQUEST SENSE command itself
-
-Either way, when a status of CHECK CONDITION is detected, the mid level
-decides whether the LLD has performed autosense by checking struct 
-scsi_cmnd::sense_buffer[0] . If this byte has an upper nibble of 7 (or 0xf)
-then autosense is assumed to have taken place. If it has another value (and
-this byte is initialized to 0 before each command) then the mid level will
-issue a REQUEST SENSE command.
-
-In the presence of queued commands the "nexus" that maintains sense
-buffer data from the command that failed until a following REQUEST SENSE
-may get out of synchronization. This is why it is best for the LLD
-to perform autosense.
-
-
-Changes since lk 2.4 series
-===========================
-io_request_lock has been replaced by several finer grained locks. The lock 
-relevant to LLDs is struct Scsi_Host::host_lock and there is
-one per SCSI host.
-
-The older error handling mechanism has been removed. This means the
-LLD interface functions abort() and reset() have been removed.
-The struct scsi_host_template::use_new_eh_code flag has been removed.
-
-In the 2.4 series the SCSI subsystem configuration descriptions were 
-aggregated with the configuration descriptions from all other Linux 
-subsystems in the Documentation/Configure.help file. In the 2.6 series, 
-the SCSI subsystem now has its own (much smaller) drivers/scsi/Kconfig
-file that contains both configuration and help information.
-
-struct SHT has been renamed to struct scsi_host_template.
-
-Addition of the "hotplug initialization model" and many extra functions
-to support it.
-
-
-Credits
-=======
-The following people have contributed to this document:
-        Mike Anderson <andmike at us dot ibm dot com>
-        James Bottomley <James dot Bottomley at hansenpartnership dot com>
-        Patrick Mansfield <patmans at us dot ibm dot com> 
-        Christoph Hellwig <hch at infradead dot org>
-        Doug Ledford <dledford at redhat dot com>
-        Andries Brouwer <Andries dot Brouwer at cwi dot nl>
-        Randy Dunlap <rdunlap at xenotime dot net>
-        Alan Stern <stern at rowland dot harvard dot edu>
-
-
-Douglas Gilbert
-dgilbert at interlog dot com
-21st September 2004