firmware: arm_scmi: Clarify raw per-channel ABI documentation

Explain more in detail how the per-channel <m> identifier is chosen.

Suggested-by: Vincent Guittot <[email protected]>
Signed-off-by: Cristian Marussi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sudeep Holla <[email protected]>

Documentation/ABI/testing/debugfs-scmi-raw

@@ -80,9 +80,13 @@ Description:	SCMI Raw synchronous message injection/snooping facility; write
 		Each write to the entry causes one command request to be built
 		and sent while the replies are read back one message at time
 		(receiving an EOF at each message boundary).
+		Channel identifier <m> matches the SCMI protocol number which
+		has been associated with this transport channel in the DT
+		description, with base protocol number 0x10 being the default
+		channel for this instance.
 		Note that these per-channel entries rooted at <..>/channels
 		exist only if the transport is configured to have more than
-		one channel.
+		one default channel.
 Users:		Debugging, any userspace test suite
 
 What:		/sys/kernel/debug/scmi/<n>/raw/channels/<m>/message_async
@@ -103,7 +107,11 @@ Description:	SCMI Raw asynchronous message injection/snooping facility; write
 		Each write to the entry causes one command request to be built
 		and sent while the replies are read back one message at time
 		(receiving an EOF at each message boundary).
+		Channel identifier <m> matches the SCMI protocol number which
+		has been associated with this transport channel in the DT
+		description, with base protocol number 0x10 being the default
+		channel for this instance.
 		Note that these per-channel entries rooted at <..>/channels
 		exist only if the transport is configured to have more than
-		one channel.
+		one default channel.
 Users:		Debugging, any userspace test suite