1 ===========
2 Dynamic PCM
3 ===========
4
5 Description
6 ===========
7
8 Dynamic PCM allows an ALSA PCM device to digitally route its PCM audio to
9 various digital endpoints during the PCM stream runtime. e.g. PCM0 can route
10 digital audio to I2S DAI0, I2S DAI1 or PDM DAI2. This is useful for on SoC DSP
11 drivers that expose several ALSA PCMs and can route to multiple DAIs.
12
13 The DPCM runtime routing is determined by the ALSA mixer settings in the same
14 way as the analog signal is routed in an ASoC codec driver. DPCM uses a DAPM
15 graph representing the DSP internal audio paths and uses the mixer settings to
16 determine the path used by each ALSA PCM.
17
18 DPCM re-uses all the existing component codec, platform and DAI drivers without
19 any modifications.
20
21
22 Phone Audio System with SoC based DSP
23 -------------------------------------
24
25 Consider the following phone audio subsystem. This will be used in this
26 document 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
45 This diagram shows a simple smart phone audio subsystem. It supports Bluetooth,
46 FM digital radio, Speakers, Headset Jack, digital microphones and cellular
47 modem. This sound card exposes 4 DSP front end (FE) ALSA PCM devices and
48 supports 6 back end (BE) DAIs. Each FE PCM can digitally route audio data to any
49 of the BE DAIs. The FE PCM devices can also route audio to more than 1 BE DAI.
50
51
52
53 Example - DPCM Switching playback from DAI0 to DAI1
54 ---------------------------------------------------
55
56 Audio is being played to the Headset. After a while the user removes the headset
57 and audio continues playing on the speakers.
58
59 Playback 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
76 The 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
93 The audio driver processes this as follows :-
94
95 1. Machine driver receives Jack removal event.
96
97 2. Machine driver OR audio HAL disables the Headset path.
98
99 3. DPCM runs the PCM trigger(stop), hw_free(), shutdown() operations on DAI0
100 for headset since the path is now disabled.
101
102 4. Machine driver or audio HAL enables the speaker path.
103
104 5. DPCM runs the PCM ops for startup(), hw_params(), prepare() and
105 trigger(start) for DAI1 Speakers since the path is enabled.
106
107 In this example, the machine driver or userspace audio HAL can alter the routing
108 and then DPCM will take care of managing the DAI PCM operations to either bring
109 the link up or down. Audio playback does not stop during this transition.
110
111
112
113 DPCM machine driver
114 ===================
115
116 The DPCM enabled ASoC machine driver is similar to normal machine drivers
117 except that we also have to :-
118
119 1. Define the FE and BE DAI links.
120
121 2. Define any FE/BE PCM operations.
122
123 3. Define widget graph connections.
124
125
126 FE 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
146 For the example above we have to define 4 FE DAI links and 6 BE DAI links. The
147 FE 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
165 This FE DAI link is pretty similar to a regular DAI link except that we also
166 set the DAI link to a DPCM FE with the ``dynamic = 1``. The supported FE stream
167 directions should also be set with the ``dpcm_playback`` and ``dpcm_capture``
168 flags. There is also an option to specify the ordering of the trigger call for
169 each FE. This allows the ASoC core to trigger the DSP before or after the other
170 components (as some DSPs have strong requirements for the ordering DAI/DSP
171 start and stop sequences).
172
173 The FE DAI above sets the codec and code DAIs to dummy devices since the BE is
174 dynamic and will change depending on runtime config.
175
176 The 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
198 This BE DAI link connects DAI0 to the codec (in this case RT5460 AIF1). It sets
199 the ``no_pcm`` flag to mark it has a BE and sets flags for supported stream
200 directions using ``dpcm_playback`` and ``dpcm_capture`` above.
201
202 The BE has also flags set for ignoring suspend and PM down time. This allows
203 the BE to work in a hostless mode where the host CPU is not transferring data
204 like 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
221 This allows the host CPU to sleep while the DSP, MODEM DAI and the BT DAI are
222 still in operation.
223
224 A BE DAI link can also set the codec to a dummy device if the codec is a device
225 that is managed externally.
226
227 Likewise a BE DAI can also set a dummy cpu DAI if the CPU DAI is managed by the
228 DSP firmware.
229
230
231 FE/BE PCM operations
232 --------------------
233
234 The BE above also exports some PCM operations and a ``fixup`` callback. The fixup
235 callback is used by the machine driver to (re)configure the DAI based upon the
236 FE hw params. i.e. the DSP may perform SRC or ASRC from the FE to BE.
237
238 e.g. DSP converts all FE hw params to run at fixed rate of 48k, 16bit, stereo for
239 DAI0. This means all FE hw_params have to be fixed in the machine driver for
240 DAI0 so that the DAI is running at desired configuration regardless of the FE
241 configuration.
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
261 The other PCM operation are the same as for regular DAI links. Use as necessary.
262
263
264 Widget graph connections
265 ------------------------
266
267 The BE DAI links will normally be connected to the graph at initialisation time
268 by the ASoC DAPM core. However, if the BE codec or BE DAI is a dummy then this
269 has 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
277 Writing a DPCM DSP driver
278 =========================
279
280 The DPCM DSP driver looks much like a standard platform class ASoC driver
281 combined with elements from a codec class driver. A DSP platform driver must
282 implement :-
283
284 1. Front End PCM DAIs - i.e. struct snd_soc_dai_driver.
285
286 2. DAPM graph showing DSP audio routing from FE DAIs to BEs.
287
288 3. DAPM widgets from DSP graph.
289
290 4. Mixers for gains, routing, etc.
291
292 5. DMA configuration.
293
294 6. BE AIF widgets.
295
296 Items 6 is important for routing the audio outside of the DSP. AIF need to be
297 defined for each BE and each stream direction. e.g for BE DAI0 above we would
298 have :-
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
304 The BE AIF are used to connect the DSP graph to the graphs for the other
305 component drivers (e.g. codec graph).
306
307
308 Hostless PCM streams
309 ====================
310
311 A hostless PCM stream is a stream that is not routed through the host CPU. An
312 example 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
329 In this case the PCM data is routed via the DSP. The host CPU in this use case
330 is only used for control and can sleep during the runtime of the stream.
331
332 The 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
346 CODEC <-> CODEC link
347 --------------------
348
349 This DAI link is enabled when DAPM detects a valid path within the DAPM graph.
350 The 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 .c2c_params = &dai_params,
372 .num_c2c_params = 1,
373 }
374 < ... more DAI links here ... >
375
376 These parameters are used to configure the DAI hw_params() when DAPM detects a
377 valid path and then calls the PCM operations to start the link. DAPM will also
378 call the appropriate PCM operations to disable the DAI when the path is no
379 longer valid.
380
381
382 Hostless FE
383 -----------
384
385 The DAI link(s) are enabled by a FE that does not read or write any PCM data.
386 This means creating a new FE that is connected with a virtual path to both
387 DAI links. The DAI links will be started when the FE PCM is started and stopped
388 when the FE PCM is stopped. Note that the FE PCM cannot read or write data in
389 this configuration.
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