docs: w1: convert to ReST and add to the kAPI group of docs

The 1wire documentation was written with w1 developers in
mind, so, it makes sense to add it together with the driver-api
set.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 202 insertions, 0 deletions

diff --git a/Documentation/w1/w1-netlink.rst b/Documentation/w1/w1-netlink.rst
new file mode 100644
index 000000000000..aaa13243a5e4
--- /dev/null
+++ b/Documentation/w1/w1-netlink.rst
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+===============================================
+Userspace communication protocol over connector
+===============================================
+
+Message types
+=============
+
+There are three types of messages between w1 core and userspace:
+
+1. Events. They are generated each time a new master or slave device
+   is found either due to automatic or requested search.
+2. Userspace commands.
+3. Replies to userspace commands.
+
+
+Protocol
+========
+
+::
+
+  [struct cn_msg] - connector header.
+	Its length field is equal to size of the attached data
+  [struct w1_netlink_msg] - w1 netlink header.
+	__u8 type 	- message type.
+			W1_LIST_MASTERS
+				list current bus masters
+			W1_SLAVE_ADD/W1_SLAVE_REMOVE
+				slave add/remove events
+			W1_MASTER_ADD/W1_MASTER_REMOVE
+				master add/remove events
+			W1_MASTER_CMD
+				userspace command for bus master
+				device (search/alarm search)
+			W1_SLAVE_CMD
+				userspace command for slave device
+				(read/write/touch)
+	__u8 status	- error indication from kernel
+	__u16 len	- size of data attached to this header data
+	union {
+		__u8 id[8];			 - slave unique device id
+		struct w1_mst {
+			__u32		id;	 - master's id
+			__u32		res;	 - reserved
+		} mst;
+	} id;
+
+  [struct w1_netlink_cmd] - command for given master or slave device.
+	__u8 cmd	- command opcode.
+			W1_CMD_READ 	- read command
+			W1_CMD_WRITE	- write command
+			W1_CMD_SEARCH	- search command
+			W1_CMD_ALARM_SEARCH - alarm search command
+			W1_CMD_TOUCH	- touch command
+				(write and sample data back to userspace)
+			W1_CMD_RESET	- send bus reset
+			W1_CMD_SLAVE_ADD	- add slave to kernel list
+			W1_CMD_SLAVE_REMOVE	- remove slave from kernel list
+			W1_CMD_LIST_SLAVES	- get slaves list from kernel
+	__u8 res	- reserved
+	__u16 len	- length of data for this command
+		For read command data must be allocated like for write command
+	__u8 data[0]	- data for this command
+
+
+Each connector message can include one or more w1_netlink_msg with
+zero or more attached w1_netlink_cmd messages.
+
+For event messages there are no w1_netlink_cmd embedded structures,
+only connector header and w1_netlink_msg strucutre with "len" field
+being zero and filled type (one of event types) and id:
+either 8 bytes of slave unique id in host order,
+or master's id, which is assigned to bus master device
+when it is added to w1 core.
+
+Currently replies to userspace commands are only generated for read
+command request. One reply is generated exactly for one w1_netlink_cmd
+read request. Replies are not combined when sent - i.e. typical reply
+messages looks like the following::
+
+  [cn_msg][w1_netlink_msg][w1_netlink_cmd]
+  cn_msg.len = sizeof(struct w1_netlink_msg) +
+	     sizeof(struct w1_netlink_cmd) +
+	     cmd->len;
+  w1_netlink_msg.len = sizeof(struct w1_netlink_cmd) + cmd->len;
+  w1_netlink_cmd.len = cmd->len;
+
+Replies to W1_LIST_MASTERS should send a message back to the userspace
+which will contain list of all registered master ids in the following
+format::
+
+	cn_msg (CN_W1_IDX.CN_W1_VAL as id, len is equal to sizeof(struct
+	w1_netlink_msg) plus number of masters multiplied by 4)
+	w1_netlink_msg (type: W1_LIST_MASTERS, len is equal to
+		number of masters multiplied by 4 (u32 size))
+	id0 ... idN
+
+Each message is at most 4k in size, so if number of master devices
+exceeds this, it will be split into several messages.
+
+W1 search and alarm search commands.
+
+request::
+
+  [cn_msg]
+    [w1_netlink_msg type = W1_MASTER_CMD
+	id is equal to the bus master id to use for searching]
+    [w1_netlink_cmd cmd = W1_CMD_SEARCH or W1_CMD_ALARM_SEARCH]
+
+reply::
+
+  [cn_msg, ack = 1 and increasing, 0 means the last message,
+	seq is equal to the request seq]
+  [w1_netlink_msg type = W1_MASTER_CMD]
+  [w1_netlink_cmd cmd = W1_CMD_SEARCH or W1_CMD_ALARM_SEARCH
+	len is equal to number of IDs multiplied by 8]
+  [64bit-id0 ... 64bit-idN]
+
+Length in each header corresponds to the size of the data behind it, so
+w1_netlink_cmd->len = N * 8; where N is number of IDs in this message.
+Can be zero.
+
+::
+
+  w1_netlink_msg->len = sizeof(struct w1_netlink_cmd) + N * 8;
+  cn_msg->len = sizeof(struct w1_netlink_msg) +
+	      sizeof(struct w1_netlink_cmd) +
+	      N*8;
+
+W1 reset command::
+
+  [cn_msg]
+    [w1_netlink_msg type = W1_MASTER_CMD
+	id is equal to the bus master id to use for searching]
+    [w1_netlink_cmd cmd = W1_CMD_RESET]
+
+
+Command status replies
+======================
+
+Each command (either root, master or slave with or without w1_netlink_cmd
+structure) will be 'acked' by the w1 core. Format of the reply is the same
+as request message except that length parameters do not account for data
+requested by the user, i.e. read/write/touch IO requests will not contain
+data, so w1_netlink_cmd.len will be 0, w1_netlink_msg.len will be size
+of the w1_netlink_cmd structure and cn_msg.len will be equal to the sum
+of the sizeof(struct w1_netlink_msg) and sizeof(struct w1_netlink_cmd).
+If reply is generated for master or root command (which do not have
+w1_netlink_cmd attached), reply will contain only cn_msg and w1_netlink_msg
+structures.
+
+w1_netlink_msg.status field will carry positive error value
+(EINVAL for example) or zero in case of success.
+
+All other fields in every structure will mirror the same parameters in the
+request message (except lengths as described above).
+
+Status reply is generated for every w1_netlink_cmd embedded in the
+w1_netlink_msg, if there are no w1_netlink_cmd structures,
+reply will be generated for the w1_netlink_msg.
+
+All w1_netlink_cmd command structures are handled in every w1_netlink_msg,
+even if there were errors, only length mismatch interrupts message processing.
+
+
+Operation steps in w1 core when new command is received
+=======================================================
+
+When new message (w1_netlink_msg) is received w1 core detects if it is
+master or slave request, according to w1_netlink_msg.type field.
+Then master or slave device is searched for.
+When found, master device (requested or those one on where slave device
+is found) is locked. If slave command is requested, then reset/select
+procedure is started to select given device.
+
+Then all requested in w1_netlink_msg operations are performed one by one.
+If command requires reply (like read command) it is sent on command completion.
+
+When all commands (w1_netlink_cmd) are processed master device is unlocked
+and next w1_netlink_msg header processing started.
+
+
+Connector [1] specific documentation
+====================================
+
+Each connector message includes two u32 fields as "address".
+w1 uses CN_W1_IDX and CN_W1_VAL defined in include/linux/connector.h header.
+Each message also includes sequence and acknowledge numbers.
+Sequence number for event messages is appropriate bus master sequence number
+increased with each event message sent "through" this master.
+Sequence number for userspace requests is set by userspace application.
+Sequence number for reply is the same as was in request, and
+acknowledge number is set to seq+1.
+
+
+Additional documentation, source code examples
+==============================================
+
+1. Documentation/driver-api/connector.rst
+2. http://www.ioremap.net/archive/w1
+
+   This archive includes userspace application w1d.c which uses
+   read/write/search commands for all master/slave devices found on the bus.