src/kernel/linux/v4.19/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
	     |                               |
	     +                               +