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