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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 channels is rendered by
+Master. Both of the L and R channels are received by two different
+Slaves. Master and both Slaves are using single port handling
+L+R. Each Slave device processes the L + R data locally, typically
+based on static configuration or dynamic orientation, and may drive
+one or more speakers. ::
+
+ +---------------+ Clock Signal +---------------+
+ | Master +---------+------------------------+ Slave |
+ | Interface | | | Interface |
+ | | | | 1 |
+ | | | Data Signal | |
+ | L + R +---+------------------------------+ L + R |
+ | (Data) | | | Data Direction | (Data) |
+ +---------------+ | | +-------------> +---------------+
+ | |
+ | |
+ | | +---------------+
+ | +----------------------> | Slave |
+ | | Interface |
+ | | 2 |
+ | | |
+ +----------------------------> | L + R |
+ | (Data) |
+ +---------------+
+
+Example 5: 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) |
+ +--------------------+ | |
+ +----------------+
+
+Example 6: Stereo Stream with L and R channel is rendered by 2 Masters, each
+rendering one channel, and is received by two different Slaves, each
+receiving one channel. Both Masters and both Slaves are using single port. ::
+
+ +---------------+ Clock Signal +---------------+
+ | Master +----------------------------------+ Slave |
+ | Interface | | Interface |
+ | 1 | | 1 |
+ | | Data Signal | |
+ | L +----------------------------------+ L |
+ | (Data) | Data Direction | (Data) |
+ +---------------+ +-----------------------> +---------------+
+
+ +---------------+ Clock Signal +---------------+
+ | Master +----------------------------------+ Slave |
+ | Interface | | Interface |
+ | 2 | | 2 |
+ | | Data Signal | |
+ | R +----------------------------------+ R |
+ | (Data) | Data Direction | (Data) |
+ +---------------+ +-----------------------> +---------------+
+
+Example 7: Stereo Stream with L and R channel is rendered by 2
+Masters, each rendering both channels. Each Slave receives L + R. This
+is the same application as Example 4 but with Slaves placed on
+separate links. ::
+
+ +---------------+ Clock Signal +---------------+
+ | Master +----------------------------------+ Slave |
+ | Interface | | Interface |
+ | 1 | | 1 |
+ | | Data Signal | |
+ | L + R +----------------------------------+ L + R |
+ | (Data) | Data Direction | (Data) |
+ +---------------+ +-----------------------> +---------------+
+
+ +---------------+ Clock Signal +---------------+
+ | Master +----------------------------------+ Slave |
+ | Interface | | Interface |
+ | 2 | | 2 |
+ | | Data Signal | |
+ | L + R +----------------------------------+ L + R |
+ | (Data) | Data Direction | (Data) |
+ +---------------+ +-----------------------> +---------------+
+
+Example 8: 4-channel Stream is rendered by 2 Masters, each rendering a
+2 channels. Each Slave receives 2 channels. ::
+
+ +---------------+ Clock Signal +---------------+
+ | Master +----------------------------------+ Slave |
+ | Interface | | Interface |
+ | 1 | | 1 |
+ | | Data Signal | |
+ | L1 + R1 +----------------------------------+ L1 + R1 |
+ | (Data) | Data Direction | (Data) |
+ +---------------+ +-----------------------> +---------------+
+
+ +---------------+ Clock Signal +---------------+
+ | Master +----------------------------------+ Slave |
+ | Interface | | Interface |
+ | 2 | | 2 |
+ | | Data Signal | |
+ | L2 + R2 +----------------------------------+ L2 + R2 |
+ | (Data) | Data Direction | (Data) |
+ +---------------+ +-----------------------> +---------------+
+
+Note1: In multi-link cases like above, to lock, one would acquire a global
+lock and then go on locking bus instances. But, in this case the caller
+framework(ASoC DPCM) guarantees that stream operations on a card are
+always serialized. So, there is no race condition and hence no need for
+global lock.
+
+Note2: A Slave device may be configured to receive all channels
+transmitted on a link for a given Stream (Example 4) or just a subset
+of the data (Example 3). The configuration of the Slave device is not
+handled by a SoundWire subsystem API, but instead by the
+snd_soc_dai_set_tdm_slot() API. The platform or machine driver will
+typically configure which of the slots are used. For Example 4, the
+same slots would be used by all Devices, while for Example 3 the Slave
+Device1 would use e.g. Slot 0 and Slave device2 slot 1.
+
+Note3: Multiple Sink ports can extract the same information for the
+same bitSlots in the SoundWire frame, however multiple Source ports
+shall be configured with different bitSlot configurations. This is the
+same limitation as with I2S/PCM TDM usages.
+
+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 | v
+ +----------+ +-----+------+ +-+--+-----+
+ | RELEASED |<----------+ DEPREPARED |<-------+ DISABLED |
+ | STATE | | STATE | | STATE |
+ +----------+ +------------+ +----------+
+
+NOTE: State transitions between ``SDW_STREAM_ENABLED`` and
+``SDW_STREAM_DISABLED`` are only relevant when then INFO_PAUSE flag is
+supported at the ALSA/ASoC level. Likewise the transition between
+``SDW_DISABLED_STATE`` and ``SDW_PREPARED_STATE`` depends on the
+INFO_RESUME flag.
+
+NOTE2: The framework implements basic state transition checks, but
+does not e.g. check if a transition from DISABLED to ENABLED is valid
+on a specific platform. Such tests need to be added at the ALSA/ASoC
+level.
+
+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);
+
+The SoundWire core provides a sdw_startup_stream() helper function,
+typically called during a dailink .startup() callback, which performs
+stream allocation and sets the stream pointer for all DAIs
+connected to a stream.
+
+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:
+
+ (0) Steps 1 and 2 are omitted in the case of a resume operation,
+ where the bus bandwidth is known.
+
+ (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. Since the .trigger() operations may not
+follow the .prepare(), a direct transition from
+``SDW_STREAM_PREPARED`` to ``SDW_STREAM_DEPREPARED`` is allowed.
+
+.. 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.
+
+When the INFO_PAUSE flag is supported, a direct transition to
+``SDW_STREAM_ENABLED`` is allowed.
+
+For resume operations where ASoC will use the .prepare() callback, the
+stream can transition from ``SDW_STREAM_DISABLED`` to
+``SDW_STREAM_PREPARED``, with all required settings restored but
+without updating the bandwidth and bit allocation.
+
+.. 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. ALSA/ASoC do not have a concept of 'deprepare', and
+the mapping from this stream state to ALSA/ASoC operation may be
+implementation specific.
+
+When the INFO_PAUSE flag is supported, the stream state is linked to
+the .hw_free() operation - the stream is not deprepared on a
+TRIGGER_STOP.
+
+Other implementations may transition to the ``SDW_STREAM_DEPREPARED``
+state on TRIGGER_STOP, should they require a transition through the
+``SDW_STREAM_PREPARED`` state.
+
+.. 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);
+
+The SoundWire core provides a sdw_shutdown_stream() helper function,
+typically called during a dailink .shutdown() callback, which clears
+the stream pointer for all DAIS connected to a stream and releases the
+memory allocated for the 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.
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