389 lines
13 KiB
ReStructuredText
389 lines
13 KiB
ReStructuredText
===========
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Dynamic PCM
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===========
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Description
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===========
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Dynamic PCM allows an ALSA PCM device to digitally route its PCM audio to
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various digital endpoints during the PCM stream runtime. e.g. PCM0 can route
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digital audio to I2S DAI0, I2S DAI1 or PDM DAI2. This is useful for on SoC DSP
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drivers that expose several ALSA PCMs and can route to multiple DAIs.
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The DPCM runtime routing is determined by the ALSA mixer settings in the same
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way as the analog signal is routed in an ASoC codec driver. DPCM uses a DAPM
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graph representing the DSP internal audio paths and uses the mixer settings to
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determine the path used by each ALSA PCM.
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DPCM re-uses all the existing component codec, platform and DAI drivers without
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any modifications.
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Phone Audio System with SoC based DSP
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-------------------------------------
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Consider the following phone audio subsystem. This will be used in this
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document for all examples :-
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::
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| Front End PCMs | SoC DSP | Back End DAIs | Audio devices |
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*************
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PCM0 <------------> * * <----DAI0-----> Codec Headset
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* *
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PCM1 <------------> * * <----DAI1-----> Codec Speakers
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* DSP *
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PCM2 <------------> * * <----DAI2-----> MODEM
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* *
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PCM3 <------------> * * <----DAI3-----> BT
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* *
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* * <----DAI4-----> DMIC
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* *
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* * <----DAI5-----> FM
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*************
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This diagram shows a simple smart phone audio subsystem. It supports Bluetooth,
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FM digital radio, Speakers, Headset Jack, digital microphones and cellular
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modem. This sound card exposes 4 DSP front end (FE) ALSA PCM devices and
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supports 6 back end (BE) DAIs. Each FE PCM can digitally route audio data to any
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of the BE DAIs. The FE PCM devices can also route audio to more than 1 BE DAI.
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Example - DPCM Switching playback from DAI0 to DAI1
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---------------------------------------------------
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Audio is being played to the Headset. After a while the user removes the headset
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and audio continues playing on the speakers.
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Playback on PCM0 to Headset would look like :-
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::
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*************
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PCM0 <============> * * <====DAI0=====> Codec Headset
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* *
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PCM1 <------------> * * <----DAI1-----> Codec Speakers
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* DSP *
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PCM2 <------------> * * <----DAI2-----> MODEM
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* *
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PCM3 <------------> * * <----DAI3-----> BT
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* *
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* * <----DAI4-----> DMIC
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* *
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* * <----DAI5-----> FM
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*************
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The headset is removed from the jack by user so the speakers must now be used :-
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::
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*************
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PCM0 <============> * * <----DAI0-----> Codec Headset
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* *
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PCM1 <------------> * * <====DAI1=====> Codec Speakers
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* DSP *
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PCM2 <------------> * * <----DAI2-----> MODEM
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* *
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PCM3 <------------> * * <----DAI3-----> BT
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* *
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* * <----DAI4-----> DMIC
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* *
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* * <----DAI5-----> FM
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*************
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The audio driver processes this as follows :-
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1. Machine driver receives Jack removal event.
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2. Machine driver OR audio HAL disables the Headset path.
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3. DPCM runs the PCM trigger(stop), hw_free(), shutdown() operations on DAI0
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for headset since the path is now disabled.
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4. Machine driver or audio HAL enables the speaker path.
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5. DPCM runs the PCM ops for startup(), hw_params(), prepare() and
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trigger(start) for DAI1 Speakers since the path is enabled.
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In this example, the machine driver or userspace audio HAL can alter the routing
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and then DPCM will take care of managing the DAI PCM operations to either bring
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the link up or down. Audio playback does not stop during this transition.
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DPCM machine driver
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===================
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The DPCM enabled ASoC machine driver is similar to normal machine drivers
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except that we also have to :-
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1. Define the FE and BE DAI links.
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2. Define any FE/BE PCM operations.
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3. Define widget graph connections.
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FE and BE DAI links
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-------------------
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::
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| Front End PCMs | SoC DSP | Back End DAIs | Audio devices |
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*************
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PCM0 <------------> * * <----DAI0-----> Codec Headset
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* *
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PCM1 <------------> * * <----DAI1-----> Codec Speakers
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* DSP *
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PCM2 <------------> * * <----DAI2-----> MODEM
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* *
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PCM3 <------------> * * <----DAI3-----> BT
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* *
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* * <----DAI4-----> DMIC
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* *
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* * <----DAI5-----> FM
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*************
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For the example above we have to define 4 FE DAI links and 6 BE DAI links. The
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FE DAI links are defined as follows :-
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::
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static struct snd_soc_dai_link machine_dais[] = {
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{
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.name = "PCM0 System",
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.stream_name = "System Playback",
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.cpu_dai_name = "System Pin",
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.platform_name = "dsp-audio",
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.codec_name = "snd-soc-dummy",
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.codec_dai_name = "snd-soc-dummy-dai",
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.dynamic = 1,
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.trigger = {SND_SOC_DPCM_TRIGGER_POST, SND_SOC_DPCM_TRIGGER_POST},
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.dpcm_playback = 1,
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},
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.....< other FE and BE DAI links here >
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};
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This FE DAI link is pretty similar to a regular DAI link except that we also
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set the DAI link to a DPCM FE with the ``dynamic = 1``. The supported FE stream
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directions should also be set with the ``dpcm_playback`` and ``dpcm_capture``
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flags. There is also an option to specify the ordering of the trigger call for
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each FE. This allows the ASoC core to trigger the DSP before or after the other
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components (as some DSPs have strong requirements for the ordering DAI/DSP
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start and stop sequences).
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The FE DAI above sets the codec and code DAIs to dummy devices since the BE is
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dynamic and will change depending on runtime config.
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The BE DAIs are configured as follows :-
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::
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static struct snd_soc_dai_link machine_dais[] = {
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.....< FE DAI links here >
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{
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.name = "Codec Headset",
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.cpu_dai_name = "ssp-dai.0",
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.platform_name = "snd-soc-dummy",
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.no_pcm = 1,
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.codec_name = "rt5640.0-001c",
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.codec_dai_name = "rt5640-aif1",
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.ignore_suspend = 1,
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.ignore_pmdown_time = 1,
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.be_hw_params_fixup = hswult_ssp0_fixup,
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.ops = &haswell_ops,
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.dpcm_playback = 1,
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.dpcm_capture = 1,
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},
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.....< other BE DAI links here >
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};
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This BE DAI link connects DAI0 to the codec (in this case RT5460 AIF1). It sets
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the ``no_pcm`` flag to mark it has a BE and sets flags for supported stream
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directions using ``dpcm_playback`` and ``dpcm_capture`` above.
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The BE has also flags set for ignoring suspend and PM down time. This allows
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the BE to work in a hostless mode where the host CPU is not transferring data
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like a BT phone call :-
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::
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*************
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PCM0 <------------> * * <----DAI0-----> Codec Headset
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* *
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PCM1 <------------> * * <----DAI1-----> Codec Speakers
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* DSP *
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PCM2 <------------> * * <====DAI2=====> MODEM
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* *
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PCM3 <------------> * * <====DAI3=====> BT
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* *
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* * <----DAI4-----> DMIC
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* *
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* * <----DAI5-----> FM
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*************
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This allows the host CPU to sleep while the DSP, MODEM DAI and the BT DAI are
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still in operation.
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A BE DAI link can also set the codec to a dummy device if the codec is a device
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that is managed externally.
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Likewise a BE DAI can also set a dummy cpu DAI if the CPU DAI is managed by the
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DSP firmware.
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FE/BE PCM operations
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--------------------
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The BE above also exports some PCM operations and a ``fixup`` callback. The fixup
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callback is used by the machine driver to (re)configure the DAI based upon the
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FE hw params. i.e. the DSP may perform SRC or ASRC from the FE to BE.
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e.g. DSP converts all FE hw params to run at fixed rate of 48k, 16bit, stereo for
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DAI0. This means all FE hw_params have to be fixed in the machine driver for
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DAI0 so that the DAI is running at desired configuration regardless of the FE
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configuration.
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::
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static int dai0_fixup(struct snd_soc_pcm_runtime *rtd,
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struct snd_pcm_hw_params *params)
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{
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struct snd_interval *rate = hw_param_interval(params,
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SNDRV_PCM_HW_PARAM_RATE);
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struct snd_interval *channels = hw_param_interval(params,
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SNDRV_PCM_HW_PARAM_CHANNELS);
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/* The DSP will convert the FE rate to 48k, stereo */
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rate->min = rate->max = 48000;
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channels->min = channels->max = 2;
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/* set DAI0 to 16 bit */
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params_set_format(params, SNDRV_PCM_FORMAT_S16_LE);
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return 0;
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}
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The other PCM operation are the same as for regular DAI links. Use as necessary.
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Widget graph connections
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------------------------
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The BE DAI links will normally be connected to the graph at initialisation time
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by the ASoC DAPM core. However, if the BE codec or BE DAI is a dummy then this
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has to be set explicitly in the driver :-
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::
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/* BE for codec Headset - DAI0 is dummy and managed by DSP FW */
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{"DAI0 CODEC IN", NULL, "AIF1 Capture"},
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{"AIF1 Playback", NULL, "DAI0 CODEC OUT"},
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Writing a DPCM DSP driver
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=========================
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The DPCM DSP driver looks much like a standard platform class ASoC driver
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combined with elements from a codec class driver. A DSP platform driver must
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implement :-
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1. Front End PCM DAIs - i.e. struct snd_soc_dai_driver.
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2. DAPM graph showing DSP audio routing from FE DAIs to BEs.
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3. DAPM widgets from DSP graph.
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4. Mixers for gains, routing, etc.
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5. DMA configuration.
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6. BE AIF widgets.
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Items 6 is important for routing the audio outside of the DSP. AIF need to be
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defined for each BE and each stream direction. e.g for BE DAI0 above we would
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have :-
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::
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SND_SOC_DAPM_AIF_IN("DAI0 RX", NULL, 0, SND_SOC_NOPM, 0, 0),
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SND_SOC_DAPM_AIF_OUT("DAI0 TX", NULL, 0, SND_SOC_NOPM, 0, 0),
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The BE AIF are used to connect the DSP graph to the graphs for the other
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component drivers (e.g. codec graph).
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Hostless PCM streams
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====================
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A hostless PCM stream is a stream that is not routed through the host CPU. An
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example of this would be a phone call from handset to modem.
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::
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*************
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PCM0 <------------> * * <----DAI0-----> Codec Headset
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* *
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PCM1 <------------> * * <====DAI1=====> Codec Speakers/Mic
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* DSP *
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PCM2 <------------> * * <====DAI2=====> MODEM
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* *
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PCM3 <------------> * * <----DAI3-----> BT
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* *
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* * <----DAI4-----> DMIC
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* *
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* * <----DAI5-----> FM
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*************
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In this case the PCM data is routed via the DSP. The host CPU in this use case
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is only used for control and can sleep during the runtime of the stream.
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The host can control the hostless link either by :-
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1. Configuring the link as a CODEC <-> CODEC style link. In this case the link
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is enabled or disabled by the state of the DAPM graph. This usually means
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there is a mixer control that can be used to connect or disconnect the path
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between both DAIs.
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2. Hostless FE. This FE has a virtual connection to the BE DAI links on the DAPM
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graph. Control is then carried out by the FE as regular PCM operations.
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This method gives more control over the DAI links, but requires much more
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userspace code to control the link. Its recommended to use CODEC<->CODEC
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unless your HW needs more fine grained sequencing of the PCM ops.
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CODEC <-> CODEC link
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--------------------
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This DAI link is enabled when DAPM detects a valid path within the DAPM graph.
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The machine driver sets some additional parameters to the DAI link i.e.
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::
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static const struct snd_soc_pcm_stream dai_params = {
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.formats = SNDRV_PCM_FMTBIT_S32_LE,
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.rate_min = 8000,
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.rate_max = 8000,
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.channels_min = 2,
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.channels_max = 2,
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};
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static struct snd_soc_dai_link dais[] = {
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< ... more DAI links above ... >
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{
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.name = "MODEM",
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.stream_name = "MODEM",
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.cpu_dai_name = "dai2",
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.codec_dai_name = "modem-aif1",
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.codec_name = "modem",
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.dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF
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| SND_SOC_DAIFMT_CBM_CFM,
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.params = &dai_params,
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}
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< ... more DAI links here ... >
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These parameters are used to configure the DAI hw_params() when DAPM detects a
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valid path and then calls the PCM operations to start the link. DAPM will also
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call the appropriate PCM operations to disable the DAI when the path is no
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longer valid.
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Hostless FE
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-----------
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The DAI link(s) are enabled by a FE that does not read or write any PCM data.
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This means creating a new FE that is connected with a virtual path to both
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DAI links. The DAI links will be started when the FE PCM is started and stopped
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when the FE PCM is stopped. Note that the FE PCM cannot read or write data in
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this configuration.
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