Documentation: soundwire: Add more documentation

This adds documentation for error handling, locking and streams.
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Sanyog Kale <sanyog.r.kale@intel.com>
Signed-off-by: Shreyas NC <shreyas.nc@intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/driver-api/soundwire/error_handling.rst

+========================
+SoundWire Error Handling
+========================
+
+The SoundWire PHY was designed with care and errors on the bus are going to
+be very unlikely, and if they happen it should be limited to single bit
+errors. Examples of this design can be found in the synchronization
+mechanism (sync loss after two errors) and short CRCs used for the Bulk
+Register Access.
+
+The errors can be detected with multiple mechanisms:
+
+1. Bus clash or parity errors: This mechanism relies on low-level detectors
+   that are independent of the payload and usages, and they cover both control
+   and audio data. The current implementation only logs such errors.
+   Improvements could be invalidating an entire programming sequence and
+   restarting from a known position. In the case of such errors outside of a
+   control/command sequence, there is no concealment or recovery for audio
+   data enabled by the SoundWire protocol, the location of the error will also
+   impact its audibility (most-significant bits will be more impacted in PCM),
+   and after a number of such errors are detected the bus might be reset. Note
+   that bus clashes due to programming errors (two streams using the same bit
+   slots) or electrical issues during the transmit/receive transition cannot
+   be distinguished, although a recurring bus clash when audio is enabled is a
+   indication of a bus allocation issue. The interrupt mechanism can also help
+   identify Slaves which detected a Bus Clash or a Parity Error, but they may
+   not be responsible for the errors so resetting them individually is not a
+   viable recovery strategy.
+
+2. Command status: Each command is associated with a status, which only
+   covers transmission of the data between devices. The ACK status indicates
+   that the command was received and will be executed by the end of the
+   current frame. A NAK indicates that the command was in error and will not
+   be applied. In case of a bad programming (command sent to non-existent
+   Slave or to a non-implemented register) or electrical issue, no response
+   signals the command was ignored. Some Master implementations allow for a
+   command to be retransmitted several times.  If the retransmission fails,
+   backtracking and restarting the entire programming sequence might be a
+   solution. Alternatively some implementations might directly issue a bus
+   reset and re-enumerate all devices.
+
+3. Timeouts: In a number of cases such as ChannelPrepare or
+   ClockStopPrepare, the bus driver is supposed to poll a register field until
+   it transitions to a NotFinished value of zero. The MIPI SoundWire spec 1.1
+   does not define timeouts but the MIPI SoundWire DisCo document adds
+   recommendation on timeouts. If such configurations do not complete, the
+   driver will return a -ETIMEOUT. Such timeouts are symptoms of a faulty
+   Slave device and are likely impossible to recover from.
+
+Errors during global reconfiguration sequences are extremely difficult to
+handle:
+
+1. BankSwitch: An error during the last command issuing a BankSwitch is
+   difficult to backtrack from. Retransmitting the Bank Switch command may be
+   possible in a single segment setup, but this can lead to synchronization
+   problems when enabling multiple bus segments (a command with side effects
+   such as frame reconfiguration would be handled at different times). A global
+   hard-reset might be the best solution.
+
+Note that SoundWire does not provide a mechanism to detect illegal values
+written in valid registers. In a number of cases the standard even mentions
+that the Slave might behave in implementation-defined ways. The bus
+implementation does not provide a recovery mechanism for such errors, Slave
+or Master driver implementers are responsible for writing valid values in
+valid registers and implement additional range checking if needed.

Documentation/driver-api/soundwire/index.rst

@@ -6,6 +6,9 @@ SoundWire Documentation
    :maxdepth: 1
 
    summary
+   stream
+   error_handling
+   locking
 
 .. only::  subproject
 

Documentation/driver-api/soundwire/locking.rst

+=================
+SoundWire Locking
+=================
+
+This document explains locking mechanism of the SoundWire Bus. Bus uses
+following locks in order to avoid race conditions in Bus operations on
+shared resources.
+
+  - Bus lock
+
+  - Message lock
+
+Bus lock
+========
+
+SoundWire Bus lock is a mutex and is part of Bus data structure
+(sdw_bus) which is used for every Bus instance. This lock is used to
+serialize each of the following operations(s) within SoundWire Bus instance.
+
+  - Addition and removal of Slave(s), changing Slave status.
+
+  - Prepare, Enable, Disable and De-prepare stream operations.
+
+  - Access of Stream data structure.
+
+Message lock
+============
+
+SoundWire message transfer lock. This mutex is part of
+Bus data structure (sdw_bus). This lock is used to serialize the message
+transfers (read/write) within a SoundWire Bus instance.
+
+Below examples show how locks are acquired.
+
+Example 1
+---------
+
+Message transfer.
+
+  1. For every message transfer
+
+     a. Acquire Message lock.
+
+     b. Transfer message (Read/Write) to Slave1 or broadcast message on
+        Bus in case of bank switch.
+
+     c. Release Message lock ::
+
+	+----------+                    +---------+
+	|          |                    |         |
+	|   Bus    |                    | Master  |
+	|          |                    | Driver  |
+	|          |                    |         |
+	+----+-----+                    +----+----+
+	     |                               |
+	     |     bus->ops->xfer_msg()      |
+	     <-------------------------------+   a. Acquire Message lock
+	     |                               |   b. Transfer message
+	     |                               |
+	     +------------------------------->   c. Release Message lock
+	     |    return success/error       |   d. Return success/error
+	     |                               |
+	     +                               +
+
+Example 2
+---------
+
+Prepare operation.
+
+  1. Acquire lock for Bus instance associated with Master 1.
+
+  2. For every message transfer in Prepare operation
+
+     a. Acquire Message lock.
+
+     b. Transfer message (Read/Write) to Slave1 or broadcast message on
+        Bus in case of bank switch.
+
+     c. Release Message lock.
+
+  3. Release lock for Bus instance associated with Master 1 ::
+
+	+----------+                    +---------+
+	|          |                    |         |
+	|   Bus    |                    | Master  |
+	|          |                    | Driver  |
+	|          |                    |         |
+	+----+-----+                    +----+----+
+	     |                               |
+	     |    sdw_prepare_stream()       |
+	     <-------------------------------+   1. Acquire bus lock
+	     |                               |   2. Perform stream prepare
+	     |                               |
+	     |                               |
+	     |     bus->ops->xfer_msg()      |
+	     <-------------------------------+   a. Acquire Message lock
+	     |                               |   b. Transfer message
+	     |                               |
+	     +------------------------------->   c. Release Message lock
+	     |    return success/error       |   d. Return success/error
+	     |                               |
+	     |                               |
+	     |    return success/error       |   3. Release bus lock
+	     +------------------------------->   4. Return success/error
+	     |                               |
+	     +                               +