Documentation: soundwire: Add more documentation

========================

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.

   :maxdepth: 1

   summary

   stream

   error_handling

   locking

.. only::  subproject

=================

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

	     |                               |

	     +                               +

=========================

Audio Stream in SoundWire

=========================

An audio stream is a logical or virtual connection created between

  (1) System memory buffer(s) and Codec(s)

  (2) DSP memory buffer(s) and Codec(s)

  (3) FIFO(s) and Codec(s)

  (4) Codec(s) and Codec(s)

which is typically driven by a DMA(s) channel through the data link. An

audio stream contains one or more channels of data. All channels within

stream must have same sample rate and same sample size.

Assume a stream with two channels (Left & Right) is opened using SoundWire

interface. Below are some ways a stream can be represented in SoundWire.

Stream Sample in memory (System memory, DSP memory or FIFOs) ::

	-------------------------

	| L | R | L | R | L | R |

	-------------------------

Example 1: Stereo Stream with L and R channels is rendered from Master to

Slave. Both Master and Slave is using single port. ::

	+---------------+                    Clock Signal  +---------------+

	|    Master     +----------------------------------+     Slave     |

	|   Interface   |                                  |   Interface   |

	|               |                                  |       1       |

	|               |                     Data Signal  |               |

	|    L  +  R    +----------------------------------+    L  +  R    |

	|     (Data)    |     Data Direction               |     (Data)    |

	+---------------+  +----------------------->       +---------------+

Example 2: Stereo Stream with L and R channels is captured from Slave to

Master. Both Master and Slave is using single port. ::

	+---------------+                    Clock Signal  +---------------+

	|    Master     +----------------------------------+     Slave     |

	|   Interface   |                                  |   Interface   |

	|               |                                  |       1       |

	|               |                     Data Signal  |               |

	|    L  +  R    +----------------------------------+    L  +  R    |

	|     (Data)    |     Data Direction               |     (Data)    |

	+---------------+  <-----------------------+       +---------------+

Example 3: Stereo Stream with L and R channels is rendered by Master. Each

of the L and R channel is received by two different Slaves. Master and both

Slaves are using single port. ::

	+---------------+                    Clock Signal  +---------------+

	|    Master     +---------+------------------------+     Slave     |

	|   Interface   |         |                        |   Interface   |

	|               |         |                        |       1       |

	|               |         |           Data Signal  |               |

	|    L  +  R    +---+------------------------------+       L       |

	|     (Data)    |   |     |    Data Direction      |     (Data)    |

	+---------------+   |     |   +------------->      +---------------+

	                    |     |

	                    |     |

	                    |     |                        +---------------+

	                    |     +----------------------> |     Slave     |

	                    |                              |   Interface   |

	                    |                              |       2       |

	                    |                              |               |

	                    +----------------------------> |       R       |

	                                                   |     (Data)    |

	                                                   +---------------+

Example 4: Stereo Stream with L and R channel is rendered by two different

Ports of the Master and is received by only single Port of the Slave

interface. ::

	+--------------------+

	|                    |

	|     +--------------+                             +----------------+

	|     |             ||                             |                |

	|     |  Data Port  ||  L Channel                  |                |

	|     |      1      |------------+                 |                |

	|     |  L Channel  ||           |                 +-----+----+     |

	|     |   (Data)    ||           |   L + R Channel ||    Data |     |

	| Master  +----------+           | +---+---------> ||    Port |     |

	| Interface          |           |                 ||     1   |     |

	|     +--------------+           |                 ||         |     |

	|     |             ||           |                 +----------+     |

	|     |  Data Port  |------------+                 |                |

	|     |      2      ||  R Channel                  |     Slave      |

	|     |  R Channel  ||                             |   Interface    |

	|     |   (Data)    ||                             |       1        |

	|     +--------------+         Clock Signal        |     L  +  R    |

	|                    +---------------------------> |      (Data)    |

	+--------------------+                             |                |

							   +----------------+

SoundWire Stream Management flow

================================

Stream definitions

------------------

  (1) Current stream: This is classified as the stream on which operation has

      to be performed like prepare, enable, disable, de-prepare etc.

  (2) Active stream: This is classified as the stream which is already active

      on Bus other than current stream. There can be multiple active streams

      on the Bus.

SoundWire Bus manages stream operations for each stream getting

rendered/captured on the SoundWire Bus. This section explains Bus operations

done for each of the stream allocated/released on Bus. Following are the

stream states maintained by the Bus for each of the audio stream.

SoundWire stream states

-----------------------

Below shows the SoundWire stream states and state transition diagram. ::

	+-----------+     +------------+     +----------+     +----------+

	| ALLOCATED +---->| CONFIGURED +---->| PREPARED +---->| ENABLED  |

	|   STATE   |     |    STATE   |     |  STATE   |     |  STATE   |

	+-----------+     +------------+     +----------+     +----+-----+

	                                                           ^

	                                                           |

	                                                           |

	                                                           v

	         +----------+           +------------+        +----+-----+

	         | RELEASED |<----------+ DEPREPARED |<-------+ DISABLED |

	         |  STATE   |           |   STATE    |        |  STATE   |

	         +----------+           +------------+        +----------+

NOTE: State transition between prepare and deprepare is supported in Spec

but not in the software (subsystem)

NOTE2: Stream state transition checks need to be handled by caller

framework, for example ALSA/ASoC. No checks for stream transition exist in

SoundWire subsystem.

Stream State Operations

-----------------------

Below section explains the operations done by the Bus on Master(s) and

Slave(s) as part of stream state transitions.

SDW_STREAM_ALLOCATED

~~~~~~~~~~~~~~~~~~~~

Allocation state for stream. This is the entry state

of the stream. Operations performed before entering in this state:

  (1) A stream runtime is allocated for the stream. This stream

      runtime is used as a reference for all the operations performed

      on the stream.

  (2) The resources required for holding stream runtime information are

      allocated and initialized. This holds all stream related information

      such as stream type (PCM/PDM) and parameters, Master and Slave

      interface associated with the stream, stream state etc.

After all above operations are successful, stream state is set to

``SDW_STREAM_ALLOCATED``.

Bus implements below API for allocate a stream which needs to be called once

per stream. From ASoC DPCM framework, this stream state maybe linked to

.startup() operation.

  .. code-block:: c

  int sdw_alloc_stream(char * stream_name);

SDW_STREAM_CONFIGURED

~~~~~~~~~~~~~~~~~~~~~

Configuration state of stream. Operations performed before entering in

this state:

  (1) The resources allocated for stream information in SDW_STREAM_ALLOCATED

      state are updated here. This includes stream parameters, Master(s)

      and Slave(s) runtime information associated with current stream.

  (2) All the Master(s) and Slave(s) associated with current stream provide

      the port information to Bus which includes port numbers allocated by

      Master(s) and Slave(s) for current stream and their channel mask.

After all above operations are successful, stream state is set to

``SDW_STREAM_CONFIGURED``.

Bus implements below APIs for CONFIG state which needs to be called by

the respective Master(s) and Slave(s) associated with stream. These APIs can

only be invoked once by respective Master(s) and Slave(s). From ASoC DPCM

framework, this stream state is linked to .hw_params() operation.

  .. code-block:: c

  int sdw_stream_add_master(struct sdw_bus * bus,

		struct sdw_stream_config * stream_config,

		struct sdw_ports_config * ports_config,

		struct sdw_stream_runtime * stream);

  int sdw_stream_add_slave(struct sdw_slave * slave,

		struct sdw_stream_config * stream_config,

		struct sdw_ports_config * ports_config,

		struct sdw_stream_runtime * stream);

SDW_STREAM_PREPARED

~~~~~~~~~~~~~~~~~~~

Prepare state of stream. Operations performed before entering in this state:

  (1) Bus parameters such as bandwidth, frame shape, clock frequency,

      are computed based on current stream as well as already active

      stream(s) on Bus. Re-computation is required to accommodate current

      stream on the Bus.

  (2) Transport and port parameters of all Master(s) and Slave(s) port(s) are

      computed for the current as well as already active stream based on frame

      shape and clock frequency computed in step 1.

  (3) Computed Bus and transport parameters are programmed in Master(s) and

      Slave(s) registers. The banked registers programming is done on the

      alternate bank (bank currently unused). Port(s) are enabled for the

      already active stream(s) on the alternate bank (bank currently unused).

      This is done in order to not disrupt already active stream(s).

  (4) Once all the values are programmed, Bus initiates switch to alternate

      bank where all new values programmed gets into effect.

  (5) Ports of Master(s) and Slave(s) for current stream are prepared by

      programming PrepareCtrl register.

After all above operations are successful, stream state is set to

``SDW_STREAM_PREPARED``.

Bus implements below API for PREPARE state which needs to be called once per

stream. From ASoC DPCM framework, this stream state is linked to

.prepare() operation.

  .. code-block:: c

  int sdw_prepare_stream(struct sdw_stream_runtime * stream);

SDW_STREAM_ENABLED

~~~~~~~~~~~~~~~~~~

Enable state of stream. The data port(s) are enabled upon entering this state.

Operations performed before entering in this state:

  (1) All the values computed in SDW_STREAM_PREPARED state are programmed

      in alternate bank (bank currently unused). It includes programming of

      already active stream(s) as well.

  (2) All the Master(s) and Slave(s) port(s) for the current stream are

      enabled on alternate bank (bank currently unused) by programming

      ChannelEn register.

  (3) Once all the values are programmed, Bus initiates switch to alternate

      bank where all new values programmed gets into effect and port(s)

      associated with current stream are enabled.

After all above operations are successful, stream state is set to

``SDW_STREAM_ENABLED``.

Bus implements below API for ENABLE state which needs to be called once per

stream. From ASoC DPCM framework, this stream state is linked to

.trigger() start operation.

  .. code-block:: c

  int sdw_enable_stream(struct sdw_stream_runtime * stream);

SDW_STREAM_DISABLED

~~~~~~~~~~~~~~~~~~~

Disable state of stream. The data port(s) are disabled upon exiting this state.

Operations performed before entering in this state:

  (1) All the Master(s) and Slave(s) port(s) for the current stream are

      disabled on alternate bank (bank currently unused) by programming

      ChannelEn register.

  (2) All the current configuration of Bus and active stream(s) are programmed

      into alternate bank (bank currently unused).

  (3) Once all the values are programmed, Bus initiates switch to alternate

      bank where all new values programmed gets into effect and port(s) associated

      with current stream are disabled.

After all above operations are successful, stream state is set to

``SDW_STREAM_DISABLED``.

Bus implements below API for DISABLED state which needs to be called once

per stream. From ASoC DPCM framework, this stream state is linked to

.trigger() stop operation.

  .. code-block:: c

  int sdw_disable_stream(struct sdw_stream_runtime * stream);

SDW_STREAM_DEPREPARED

~~~~~~~~~~~~~~~~~~~~~

De-prepare state of stream. Operations performed before entering in this

state:

  (1) All the port(s) of Master(s) and Slave(s) for current stream are

      de-prepared by programming PrepareCtrl register.

  (2) The payload bandwidth of current stream is reduced from the total

      bandwidth requirement of bus and new parameters calculated and

      applied by performing bank switch etc.

After all above operations are successful, stream state is set to

``SDW_STREAM_DEPREPARED``.

Bus implements below API for DEPREPARED state which needs to be called once

per stream. From ASoC DPCM framework, this stream state is linked to

.trigger() stop operation.

  .. code-block:: c

  int sdw_deprepare_stream(struct sdw_stream_runtime * stream);

SDW_STREAM_RELEASED

~~~~~~~~~~~~~~~~~~~

Release state of stream. Operations performed before entering in this state:

  (1) Release port resources for all Master(s) and Slave(s) port(s)

      associated with current stream.

  (2) Release Master(s) and Slave(s) runtime resources associated with

      current stream.

  (3) Release stream runtime resources associated with current stream.

After all above operations are successful, stream state is set to

``SDW_STREAM_RELEASED``.

Bus implements below APIs for RELEASE state which needs to be called by

all the Master(s) and Slave(s) associated with stream. From ASoC DPCM

framework, this stream state is linked to .hw_free() operation.

  .. code-block:: c

  int sdw_stream_remove_master(struct sdw_bus * bus,

		struct sdw_stream_runtime * stream);

  int sdw_stream_remove_slave(struct sdw_slave * slave,

		struct sdw_stream_runtime * stream);

The .shutdown() ASoC DPCM operation calls below Bus API to release

stream assigned as part of ALLOCATED state.

In .shutdown() the data structure maintaining stream state are freed up.

  .. code-block:: c

  void sdw_release_stream(struct sdw_stream_runtime * stream);

Not Supported

=============

1. A single port with multiple channels supported cannot be used between two

streams or across stream. For example a port with 4 channels cannot be used

to handle 2 independent stereo streams even though it's possible in theory

in SoundWire.