cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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locking.rst (3463B)

      1=================
      2SoundWire Locking
      3=================
      4
      5This document explains locking mechanism of the SoundWire Bus. Bus uses
      6following locks in order to avoid race conditions in Bus operations on
      7shared resources.
      8
      9  - Bus lock
     10
     11  - Message lock
     12
     13Bus lock
     14========
     15
     16SoundWire Bus lock is a mutex and is part of Bus data structure
     17(sdw_bus) which is used for every Bus instance. This lock is used to
     18serialize each of the following operations(s) within SoundWire Bus instance.
     19
     20  - Addition and removal of Slave(s), changing Slave status.
     21
     22  - Prepare, Enable, Disable and De-prepare stream operations.
     23
     24  - Access of Stream data structure.
     25
     26Message lock
     27============
     28
     29SoundWire message transfer lock. This mutex is part of
     30Bus data structure (sdw_bus). This lock is used to serialize the message
     31transfers (read/write) within a SoundWire Bus instance.
     32
     33Below examples show how locks are acquired.
     34
     35Example 1
     36---------
     37
     38Message transfer.
     39
     40  1. For every message transfer
     41
     42     a. Acquire Message lock.
     43
     44     b. Transfer message (Read/Write) to Slave1 or broadcast message on
     45        Bus in case of bank switch.
     46
     47     c. Release Message lock
     48
     49     ::
     50
     51	+----------+                    +---------+
     52	|          |                    |         |
     53	|   Bus    |                    | Master  |
     54	|          |                    | Driver  |
     55	|          |                    |         |
     56	+----+-----+                    +----+----+
     57	     |                               |
     58	     |     bus->ops->xfer_msg()      |
     59	     <-------------------------------+   a. Acquire Message lock
     60	     |                               |   b. Transfer message
     61	     |                               |
     62	     +------------------------------->   c. Release Message lock
     63	     |    return success/error       |   d. Return success/error
     64	     |                               |
     65	     +                               +
     66
     67Example 2
     68---------
     69
     70Prepare operation.
     71
     72  1. Acquire lock for Bus instance associated with Master 1.
     73
     74  2. For every message transfer in Prepare operation
     75
     76     a. Acquire Message lock.
     77
     78     b. Transfer message (Read/Write) to Slave1 or broadcast message on
     79        Bus in case of bank switch.
     80
     81     c. Release Message lock.
     82
     83  3. Release lock for Bus instance associated with Master 1 ::
     84
     85	+----------+                    +---------+
     86	|          |                    |         |
     87	|   Bus    |                    | Master  |
     88	|          |                    | Driver  |
     89	|          |                    |         |
     90	+----+-----+                    +----+----+
     91	     |                               |
     92	     |    sdw_prepare_stream()       |
     93	     <-------------------------------+   1. Acquire bus lock
     94	     |                               |   2. Perform stream prepare
     95	     |                               |
     96	     |                               |
     97	     |     bus->ops->xfer_msg()      |
     98	     <-------------------------------+   a. Acquire Message lock
     99	     |                               |   b. Transfer message
    100	     |                               |
    101	     +------------------------------->   c. Release Message lock
    102	     |    return success/error       |   d. Return success/error
    103	     |                               |
    104	     |                               |
    105	     |    return success/error       |   3. Release bus lock
    106	     +------------------------------->   4. Return success/error
    107	     |                               |
    108	     +                               +