1===========
2Dynamic PCM
3===========
4
5Description
6===========
7
8Dynamic PCM allows an ALSA PCM device to digitally route its PCM audio to
9various digital endpoints during the PCM stream runtime. e.g. PCM0 can route
10digital audio to I2S DAI0, I2S DAI1 or PDM DAI2. This is useful for on SoC DSP
11drivers that expose several ALSA PCMs and can route to multiple DAIs.
12
13The DPCM runtime routing is determined by the ALSA mixer settings in the same
14way as the analog signal is routed in an ASoC codec driver. DPCM uses a DAPM
15graph representing the DSP internal audio paths and uses the mixer settings to
16determine the path used by each ALSA PCM.
17
18DPCM re-uses all the existing component codec, platform and DAI drivers without
19any modifications.
20
21
22Phone Audio System with SoC based DSP
23-------------------------------------
24
25Consider the following phone audio subsystem. This will be used in this
26document for all examples :-
27::
28
29  | Front End PCMs    |  SoC DSP  | Back End DAIs | Audio devices |
30
31                      *************
32  PCM0 <------------> *           * <----DAI0-----> Codec Headset
33                      *           *
34  PCM1 <------------> *           * <----DAI1-----> Codec Speakers
35                      *   DSP     *
36  PCM2 <------------> *           * <----DAI2-----> MODEM
37                      *           *
38  PCM3 <------------> *           * <----DAI3-----> BT
39                      *           *
40                      *           * <----DAI4-----> DMIC
41                      *           *
42                      *           * <----DAI5-----> FM
43                      *************
44
45This diagram shows a simple smart phone audio subsystem. It supports Bluetooth,
46FM digital radio, Speakers, Headset Jack, digital microphones and cellular
47modem. This sound card exposes 4 DSP front end (FE) ALSA PCM devices and
48supports 6 back end (BE) DAIs. Each FE PCM can digitally route audio data to any
49of the BE DAIs. The FE PCM devices can also route audio to more than 1 BE DAI.
50
51
52
53Example - DPCM Switching playback from DAI0 to DAI1
54---------------------------------------------------
55
56Audio is being played to the Headset. After a while the user removes the headset
57and audio continues playing on the speakers.
58
59Playback on PCM0 to Headset would look like :-
60::
61
62                      *************
63  PCM0 <============> *           * <====DAI0=====> Codec Headset
64                      *           *
65  PCM1 <------------> *           * <----DAI1-----> Codec Speakers
66                      *   DSP     *
67  PCM2 <------------> *           * <----DAI2-----> MODEM
68                      *           *
69  PCM3 <------------> *           * <----DAI3-----> BT
70                      *           *
71                      *           * <----DAI4-----> DMIC
72                      *           *
73                      *           * <----DAI5-----> FM
74                      *************
75
76The headset is removed from the jack by user so the speakers must now be used :-
77::
78
79                      *************
80  PCM0 <============> *           * <----DAI0-----> Codec Headset
81                      *           *
82  PCM1 <------------> *           * <====DAI1=====> Codec Speakers
83                      *   DSP     *
84  PCM2 <------------> *           * <----DAI2-----> MODEM
85                      *           *
86  PCM3 <------------> *           * <----DAI3-----> BT
87                      *           *
88                      *           * <----DAI4-----> DMIC
89                      *           *
90                      *           * <----DAI5-----> FM
91                      *************
92
93The audio driver processes this as follows :-
94
951. Machine driver receives Jack removal event.
96
972. Machine driver OR audio HAL disables the Headset path.
98
993. DPCM runs the PCM trigger(stop), hw_free(), shutdown() operations on DAI0
100   for headset since the path is now disabled.
101
1024. Machine driver or audio HAL enables the speaker path.
103
1045. DPCM runs the PCM ops for startup(), hw_params(), prepare() and
105   trigger(start) for DAI1 Speakers since the path is enabled.
106
107In this example, the machine driver or userspace audio HAL can alter the routing
108and then DPCM will take care of managing the DAI PCM operations to either bring
109the link up or down. Audio playback does not stop during this transition.
110
111
112
113DPCM machine driver
114===================
115
116The DPCM enabled ASoC machine driver is similar to normal machine drivers
117except that we also have to :-
118
1191. Define the FE and BE DAI links.
120
1212. Define any FE/BE PCM operations.
122
1233. Define widget graph connections.
124
125
126FE and BE DAI links
127-------------------
128::
129
130  | Front End PCMs    |  SoC DSP  | Back End DAIs | Audio devices |
131
132                      *************
133  PCM0 <------------> *           * <----DAI0-----> Codec Headset
134                      *           *
135  PCM1 <------------> *           * <----DAI1-----> Codec Speakers
136                      *   DSP     *
137  PCM2 <------------> *           * <----DAI2-----> MODEM
138                      *           *
139  PCM3 <------------> *           * <----DAI3-----> BT
140                      *           *
141                      *           * <----DAI4-----> DMIC
142                      *           *
143                      *           * <----DAI5-----> FM
144                      *************
145
146For the example above we have to define 4 FE DAI links and 6 BE DAI links. The
147FE DAI links are defined as follows :-
148::
149
150  static struct snd_soc_dai_link machine_dais[] = {
151	{
152		.name = "PCM0 System",
153		.stream_name = "System Playback",
154		.cpu_dai_name = "System Pin",
155		.platform_name = "dsp-audio",
156		.codec_name = "snd-soc-dummy",
157		.codec_dai_name = "snd-soc-dummy-dai",
158		.dynamic = 1,
159		.trigger = {SND_SOC_DPCM_TRIGGER_POST, SND_SOC_DPCM_TRIGGER_POST},
160		.dpcm_playback = 1,
161	},
162	.....< other FE and BE DAI links here >
163  };
164
165This FE DAI link is pretty similar to a regular DAI link except that we also
166set the DAI link to a DPCM FE with the ``dynamic = 1``. The supported FE stream
167directions should also be set with the ``dpcm_playback`` and ``dpcm_capture``
168flags. There is also an option to specify the ordering of the trigger call for
169each FE. This allows the ASoC core to trigger the DSP before or after the other
170components (as some DSPs have strong requirements for the ordering DAI/DSP
171start and stop sequences).
172
173The FE DAI above sets the codec and code DAIs to dummy devices since the BE is
174dynamic and will change depending on runtime config.
175
176The BE DAIs are configured as follows :-
177::
178
179  static struct snd_soc_dai_link machine_dais[] = {
180	.....< FE DAI links here >
181	{
182		.name = "Codec Headset",
183		.cpu_dai_name = "ssp-dai.0",
184		.platform_name = "snd-soc-dummy",
185		.no_pcm = 1,
186		.codec_name = "rt5640.0-001c",
187		.codec_dai_name = "rt5640-aif1",
188		.ignore_suspend = 1,
189		.ignore_pmdown_time = 1,
190		.be_hw_params_fixup = hswult_ssp0_fixup,
191		.ops = &haswell_ops,
192		.dpcm_playback = 1,
193		.dpcm_capture = 1,
194	},
195	.....< other BE DAI links here >
196  };
197
198This BE DAI link connects DAI0 to the codec (in this case RT5460 AIF1). It sets
199the ``no_pcm`` flag to mark it has a BE and sets flags for supported stream
200directions using ``dpcm_playback`` and ``dpcm_capture`` above.
201
202The BE has also flags set for ignoring suspend and PM down time. This allows
203the BE to work in a hostless mode where the host CPU is not transferring data
204like a BT phone call :-
205::
206
207                      *************
208  PCM0 <------------> *           * <----DAI0-----> Codec Headset
209                      *           *
210  PCM1 <------------> *           * <----DAI1-----> Codec Speakers
211                      *   DSP     *
212  PCM2 <------------> *           * <====DAI2=====> MODEM
213                      *           *
214  PCM3 <------------> *           * <====DAI3=====> BT
215                      *           *
216                      *           * <----DAI4-----> DMIC
217                      *           *
218                      *           * <----DAI5-----> FM
219                      *************
220
221This allows the host CPU to sleep while the DSP, MODEM DAI and the BT DAI are
222still in operation.
223
224A BE DAI link can also set the codec to a dummy device if the codec is a device
225that is managed externally.
226
227Likewise a BE DAI can also set a dummy cpu DAI if the CPU DAI is managed by the
228DSP firmware.
229
230
231FE/BE PCM operations
232--------------------
233
234The BE above also exports some PCM operations and a ``fixup`` callback. The fixup
235callback is used by the machine driver to (re)configure the DAI based upon the
236FE hw params. i.e. the DSP may perform SRC or ASRC from the FE to BE.
237
238e.g. DSP converts all FE hw params to run at fixed rate of 48k, 16bit, stereo for
239DAI0. This means all FE hw_params have to be fixed in the machine driver for
240DAI0 so that the DAI is running at desired configuration regardless of the FE
241configuration.
242::
243
244  static int dai0_fixup(struct snd_soc_pcm_runtime *rtd,
245			struct snd_pcm_hw_params *params)
246  {
247	struct snd_interval *rate = hw_param_interval(params,
248			SNDRV_PCM_HW_PARAM_RATE);
249	struct snd_interval *channels = hw_param_interval(params,
250						SNDRV_PCM_HW_PARAM_CHANNELS);
251
252	/* The DSP will convert the FE rate to 48k, stereo */
253	rate->min = rate->max = 48000;
254	channels->min = channels->max = 2;
255
256	/* set DAI0 to 16 bit */
257	params_set_format(params, SNDRV_PCM_FORMAT_S16_LE);
258	return 0;
259  }
260
261The other PCM operation are the same as for regular DAI links. Use as necessary.
262
263
264Widget graph connections
265------------------------
266
267The BE DAI links will normally be connected to the graph at initialisation time
268by the ASoC DAPM core. However, if the BE codec or BE DAI is a dummy then this
269has to be set explicitly in the driver :-
270::
271
272  /* BE for codec Headset -  DAI0 is dummy and managed by DSP FW */
273  {"DAI0 CODEC IN", NULL, "AIF1 Capture"},
274  {"AIF1 Playback", NULL, "DAI0 CODEC OUT"},
275
276
277Writing a DPCM DSP driver
278=========================
279
280The DPCM DSP driver looks much like a standard platform class ASoC driver
281combined with elements from a codec class driver. A DSP platform driver must
282implement :-
283
2841. Front End PCM DAIs - i.e. struct snd_soc_dai_driver.
285
2862. DAPM graph showing DSP audio routing from FE DAIs to BEs.
287
2883. DAPM widgets from DSP graph.
289
2904. Mixers for gains, routing, etc.
291
2925. DMA configuration.
293
2946. BE AIF widgets.
295
296Items 6 is important for routing the audio outside of the DSP. AIF need to be
297defined for each BE and each stream direction. e.g for BE DAI0 above we would
298have :-
299::
300
301  SND_SOC_DAPM_AIF_IN("DAI0 RX", NULL, 0, SND_SOC_NOPM, 0, 0),
302  SND_SOC_DAPM_AIF_OUT("DAI0 TX", NULL, 0, SND_SOC_NOPM, 0, 0),
303
304The BE AIF are used to connect the DSP graph to the graphs for the other
305component drivers (e.g. codec graph).
306
307
308Hostless PCM streams
309====================
310
311A hostless PCM stream is a stream that is not routed through the host CPU. An
312example of this would be a phone call from handset to modem.
313::
314
315                      *************
316  PCM0 <------------> *           * <----DAI0-----> Codec Headset
317                      *           *
318  PCM1 <------------> *           * <====DAI1=====> Codec Speakers/Mic
319                      *   DSP     *
320  PCM2 <------------> *           * <====DAI2=====> MODEM
321                      *           *
322  PCM3 <------------> *           * <----DAI3-----> BT
323                      *           *
324                      *           * <----DAI4-----> DMIC
325                      *           *
326                      *           * <----DAI5-----> FM
327                      *************
328
329In this case the PCM data is routed via the DSP. The host CPU in this use case
330is only used for control and can sleep during the runtime of the stream.
331
332The host can control the hostless link either by :-
333
334 1. Configuring the link as a CODEC <-> CODEC style link. In this case the link
335    is enabled or disabled by the state of the DAPM graph. This usually means
336    there is a mixer control that can be used to connect or disconnect the path
337    between both DAIs.
338
339 2. Hostless FE. This FE has a virtual connection to the BE DAI links on the DAPM
340    graph. Control is then carried out by the FE as regular PCM operations.
341    This method gives more control over the DAI links, but requires much more
342    userspace code to control the link. Its recommended to use CODEC<->CODEC
343    unless your HW needs more fine grained sequencing of the PCM ops.
344
345
346CODEC <-> CODEC link
347--------------------
348
349This DAI link is enabled when DAPM detects a valid path within the DAPM graph.
350The machine driver sets some additional parameters to the DAI link i.e.
351::
352
353  static const struct snd_soc_pcm_stream dai_params = {
354	.formats = SNDRV_PCM_FMTBIT_S32_LE,
355	.rate_min = 8000,
356	.rate_max = 8000,
357	.channels_min = 2,
358	.channels_max = 2,
359  };
360
361  static struct snd_soc_dai_link dais[] = {
362	< ... more DAI links above ... >
363	{
364		.name = "MODEM",
365		.stream_name = "MODEM",
366		.cpu_dai_name = "dai2",
367		.codec_dai_name = "modem-aif1",
368		.codec_name = "modem",
369		.dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF
370				| SND_SOC_DAIFMT_CBM_CFM,
371		.params = &dai_params,
372	}
373	< ... more DAI links here ... >
374
375These parameters are used to configure the DAI hw_params() when DAPM detects a
376valid path and then calls the PCM operations to start the link. DAPM will also
377call the appropriate PCM operations to disable the DAI when the path is no
378longer valid.
379
380
381Hostless FE
382-----------
383
384The DAI link(s) are enabled by a FE that does not read or write any PCM data.
385This means creating a new FE that is connected with a virtual path to both
386DAI links. The DAI links will be started when the FE PCM is started and stopped
387when the FE PCM is stopped. Note that the FE PCM cannot read or write data in
388this configuration.
389