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Documentation: soundwire: Add more documentation

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This adds documentation for error handling, locking and streams.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Sanyog Kale <sanyog.r.kale@intel.com>
Signed-off-by: Shreyas NC <shreyas.nc@intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
This commit is contained in:
Sanyog Kale 2018-04-26 18:38:02 +05:30 committed by Vinod Koul
parent fce45d1142
commit 89634f99a8
4 changed files with 546 additions and 0 deletions
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Documentation/driver-api/soundwire/error_handling.rst Normal file
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========================
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.

3
Documentation/driver-api/soundwire/index.rst
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@ -6,6 +6,9 @@ SoundWire Documentation
:maxdepth: 1 :maxdepth: 1
summary summary
stream
error_handling
locking
.. only:: subproject .. only:: subproject

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Documentation/driver-api/soundwire/locking.rst Normal file
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=================
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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Documentation/driver-api/soundwire/stream.rst Normal file
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=========================
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.