scsi: docs: convert scsi.txt to ReST

Link: https://lore.kernel.org/r/c617b37769a82901def0fed3d236a25995c4e160.1583136624.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>

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+.. SPDX-License-Identifier: GPL-2.0
+
+============================
+SCSI subsystem documentation
+============================
+
+The Linux Documentation Project (LDP) maintains a document describing
+the SCSI subsystem in the Linux kernel (lk) 2.4 series. See:
+http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO . The LDP has single
+and multiple page HTML renderings as well as postscript and pdf.
+It can also be found at:
+http://web.archive.org/web/%2E/http://www.torque.net/scsi/SCSI-2.4-HOWTO
+
+Notes on using modules in the SCSI subsystem
+============================================
+The scsi support in the linux kernel can be modularized in a number of
+different ways depending upon the needs of the end user.  To understand
+your options, we should first define a few terms.
+
+The scsi-core (also known as the "mid level") contains the core of scsi
+support.  Without it you can do nothing with any of the other scsi drivers.
+The scsi core support can be a module (scsi_mod.o), or it can be built into
+the kernel. If the core is a module, it must be the first scsi module
+loaded, and if you unload the modules, it will have to be the last one
+unloaded.  In practice the modprobe and rmmod commands (and "autoclean")
+will enforce the correct ordering of loading and unloading modules in
+the SCSI subsystem.
+
+The individual upper and lower level drivers can be loaded in any order
+once the scsi core is present in the kernel (either compiled in or loaded
+as a module).  The disk driver (sd_mod.o), cdrom driver (sr_mod.o),
+tape driver [1]_ (st.o) and scsi generics driver (sg.o) represent the upper
+level drivers to support the various assorted devices which can be
+controlled.  You can for example load the tape driver to use the tape drive,
+and then unload it once you have no further need for the driver (and release
+the associated memory).
+
+The lower level drivers are the ones that support the individual cards that
+are supported for the hardware platform that you are running under. Those
+individual cards are often called Host Bus Adapters (HBAs). For example the
+aic7xxx.o driver is used to control all recent SCSI controller cards from
+Adaptec. Almost all lower level drivers can be built either as modules or
+built into the kernel.
+
+.. [1] There is a variant of the st driver for controlling OnStream tape
+       devices. Its module name is osst.o .
+