=================
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
| |
+ +
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