ALSA: pcm: add a documentation for tracepoints

In PCM interface/protocol for userspace, parameters of runtime for PCM
substream is decided by an interaction between applications and ALSA
PCM core. In former commits, some tracepoints were added to probe a part
of the interaction.

This commit adds a documentation about the interaction and the
tracepoints.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/designs/index.rst

@@ -9,6 +9,7 @@ Designs and Implementations
    compress-offload
    timestamping
    jack-controls
+   tracepoints
    procfile
    powersave
    oss-emulation

Documentation/sound/designs/tracepoints.rst

+===================
+Tracepoints in ALSA
+===================
+
+2017/07/02
+Takasahi Sakamoto
+
+Tracepoints in ALSA PCM core
+============================
+
+ALSA PCM core registers ``snd_pcm`` subsystem to kernel tracepoint system.
+This subsystem includes two categories of tracepoints; for state of PCM buffer
+and for processing of PCM hardware parameters. These tracepoints are available
+when corresponding kernel configurations are enabled. When ``CONFIG_SND_DEBUG``
+is enabled, the latter tracepoints are available. When additional
+``SND_PCM_XRUN_DEBUG`` is enabled too, the former trace points are enabled.
+
+Tracepoints for state of PCM buffer
+------------------------------------
+
+This category includes four tracepoints; ``hwptr``, ``applptr``, ``xrun`` and
+``hw_ptr_error``.
+
+Tracepoints for processing of PCM hardware parameters
+-----------------------------------------------------
+
+This category includes two tracepoints; ``hw_mask_param`` and
+``hw_interval_param``.
+
+In a design of ALSA PCM core, data transmission is abstracted as PCM substream.
+Applications manage PCM substream to maintain data transmission for PCM frames.
+Before starting the data transmission, applications need to configure PCM
+substream. In this procedure, PCM hardware parameters are decided by
+interaction between applications and ALSA PCM core. Once decided, runtime of
+the PCM substream keeps the parameters.
+
+The parameters are described in :c:type:`struct snd_pcm_hw_params`. This
+structure includes several types of parameters. Applications set preferable
+value to these parameters, then execute ioctl(2) with SNDRV_PCM_IOCTL_HW_REFINE
+or SNDRV_PCM_IOCTL_HW_PARAMS. The former is used just for refining available
+set of parameters. The latter is used for an actual decision of the parameters.
+
+The :c:type:`struct snd_pcm_hw_params` structure has below members:
+
+``flags``
+        Configurable. ALSA PCM core and some drivers handle this flag to select
+        convenient parameters or change their behaviour.
+``masks``
+        Configurable. This type of parameter is described in
+        :c:type:`struct snd_mask` and represent mask values. As of PCM protocol
+        v2.0.13, three types are defined.
+
+        - SNDRV_PCM_HW_PARAM_ACCESS
+        - SNDRV_PCM_HW_PARAM_FORMAT
+        - SNDRV_PCM_HW_PARAM_SUBFORMAT
+``intervals``
+        Configurable. This type of parameter is described in
+        :c:type:`struct snd_interval` and represent values with a range. As of
+        PCM protocol v2.0.13, twelve types are defined.
+
+        - SNDRV_PCM_HW_PARAM_SAMPLE_BITS
+        - SNDRV_PCM_HW_PARAM_FRAME_BITS
+        - SNDRV_PCM_HW_PARAM_CHANNELS
+        - SNDRV_PCM_HW_PARAM_RATE
+        - SNDRV_PCM_HW_PARAM_PERIOD_TIME
+        - SNDRV_PCM_HW_PARAM_PERIOD_SIZE
+        - SNDRV_PCM_HW_PARAM_PERIOD_BYTES
+        - SNDRV_PCM_HW_PARAM_PERIODS
+        - SNDRV_PCM_HW_PARAM_BUFFER_TIME
+        - SNDRV_PCM_HW_PARAM_BUFFER_SIZE
+        - SNDRV_PCM_HW_PARAM_BUFFER_BYTES
+        - SNDRV_PCM_HW_PARAM_TICK_TIME
+``rmask``
+        Configurable. This is evaluated at ioctl(2) with
+        SNDRV_PCM_IOCTL_HW_REFINE only. Applications can select which
+        mask/interval parameter can be changed by ALSA PCM core. For
+        SNDRV_PCM_IOCTL_HW_PARAMS, this mask is ignored and all of parameters
+        are going to be changed.
+``cmask``
+        Read-only. After returning from ioctl(2), buffer in user space for
+        :c:type:`struct snd_pcm_hw_params` includes result of each operation.
+        This mask represents which mask/interval parameter is actually changed.
+``info``
+        Read-only. This represents hardware/driver capabilities as bit flags
+        with SNDRV_PCM_INFO_XXX. Typically, applications execute ioctl(2) with
+        SNDRV_PCM_IOCTL_HW_REFINE to retrieve this flag, then decide candidates
+        of parameters and execute ioctl(2) with SNDRV_PCM_IOCTL_HW_PARAMS to
+        configure PCM substream.
+``msbits``
+        Read-only. This value represents available bit width in MSB side of
+        a PCM sample. When a parameter of SNDRV_PCM_HW_PARAM_SAMPLE_BITS was
+        decided as a fixed number, this value is also calculated according to
+        it. Else, zero. But this behaviour depends on implementations in driver
+        side.
+``rate_num``
+        Read-only. This value represents numerator of sampling rate in fraction
+        notation. Basically, when a parameter of SNDRV_PCM_HW_PARAM_RATE was
+        decided as a single value, this value is also calculated according to
+        it. Else, zero. But this behaviour depends on implementations in driver
+        side.
+``rate_den``
+        Read-only. This value represents denominator of sampling rate in
+        fraction notation. Basically, when a parameter of
+        SNDRV_PCM_HW_PARAM_RATE was decided as a single value, this value is
+        also calculated according to it. Else, zero. But this behaviour depends
+        on implementations in driver side.
+``fifo_size``
+        Read-only. This value represents the size of FIFO in serial sound
+        interface of hardware. Basically, each driver can assigns a proper
+        value to this parameter but some drivers intentionally set zero with
+        a care of hardware design or data transmission protocol.
+
+ALSA PCM core handles buffer of :c:type:`struct snd_pcm_hw_params` when
+applications execute ioctl(2) with SNDRV_PCM_HW_REFINE or SNDRV_PCM_HW_PARAMS.
+Parameters in the buffer are changed according to
+:c:type:`struct snd_pcm_hardware` and rules of constraints in the runtime. The
+structure describes capabilities of handled hardware. The rules describes
+dependencies on which a parameter is decided according to several parameters.
+A rule has a callback function, and drivers can register arbitrary functions
+to compute the target parameter. ALSA PCM core registers some rules to the
+runtime as a default.
+
+Each driver can join in the interaction as long as it prepared for two stuffs
+in a callback of :c:type:`struct snd_pcm_ops.open`.
+
+1. In the callback, drivers are expected to change a member of
+   :c:type:`struct snd_pcm_hardware` type in the runtime, according to
+   capacities of corresponding hardware.
+2. In the same callback, drivers are also expected to register additional rules
+   of constraints into the runtime when several parameters have dependencies
+   due to hardware design.
+
+The driver can refers to result of the interaction in a callback of
+:c:type:`struct snd_pcm_ops.hw_params`, however it should not change the
+content.
+
+Tracepoints in this category are designed to trace changes of the
+mask/interval parameters. When ALSA PCM core changes them, ``hw_mask_param`` or
+``hw_interval_param`` event is probed according to type of the changed parameter.
+
+ALSA PCM core also has a pretty print format for each of the tracepoints. Below
+is an example for ``hw_mask_param``.
+
+::
+
+    hw_mask_param: pcmC0D0p 001/023 FORMAT 00000000000000000000001000000044 00000000000000000000001000000044
+
+
+Below is an example for ``hw_interval_param``.
+
+::
+
+    hw_interval_param: pcmC0D0p 000/023 BUFFER_SIZE 0 0 [0 4294967295] 0 1 [0 4294967295]
+
+The first three fields are common. They represent name of ALSA PCM character
+device, rules of constraint and name of the changed parameter, in order. The
+field for rules of constraint consists of two sub-fields; index of applied rule
+and total number of rules added to the runtime. As an exception, the index 000
+means that the parameter is changed by ALSA PCM core, regardless of the rules.
+
+The rest of field represent state of the parameter before/after changing. These
+fields are different according to type of the parameter. For parameters of mask
+type, the fields represent hexadecimal dump of content of the parameter. For
+parameters of interval type, the fields represent values of each member of
+``empty``, ``integer``, ``openmin``, ``min``, ``max``, ``openmax`` in
+:c:type:`struct snd_interval` in this order.
+
+Tracepoints in drivers
+======================
+
+Some drivers have tracepoints for developers' convenience. For them, please
+refer to each documentation or implementation.