1 =========== 1 ===========
2 Dynamic PCM 2 Dynamic PCM
3 =========== 3 ===========
4 4
5 Description 5 Description
6 =========== 6 ===========
7 7
8 Dynamic PCM allows an ALSA PCM device to digit 8 Dynamic PCM allows an ALSA PCM device to digitally route its PCM audio to
9 various digital endpoints during the PCM strea 9 various digital endpoints during the PCM stream runtime. e.g. PCM0 can route
10 digital audio to I2S DAI0, I2S DAI1 or PDM DAI 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 11 drivers that expose several ALSA PCMs and can route to multiple DAIs.
12 12
13 The DPCM runtime routing is determined by the 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 14 way as the analog signal is routed in an ASoC codec driver. DPCM uses a DAPM
15 graph representing the DSP internal audio path 15 graph representing the DSP internal audio paths and uses the mixer settings to
16 determine the path used by each ALSA PCM. !! 16 determine the patch used by each ALSA PCM.
17 17
18 DPCM re-uses all the existing component codec, 18 DPCM re-uses all the existing component codec, platform and DAI drivers without
19 any modifications. 19 any modifications.
20 20
21 21
22 Phone Audio System with SoC based DSP 22 Phone Audio System with SoC based DSP
23 ------------------------------------- 23 -------------------------------------
24 24
25 Consider the following phone audio subsystem. 25 Consider the following phone audio subsystem. This will be used in this
26 document for all examples :- 26 document for all examples :-
27 :: 27 ::
28 28
29 | Front End PCMs | SoC DSP | Back End D 29 | Front End PCMs | SoC DSP | Back End DAIs | Audio devices |
30 30
31 ************* 31 *************
32 PCM0 <------------> * * <----DAI0- 32 PCM0 <------------> * * <----DAI0-----> Codec Headset
33 * * 33 * *
34 PCM1 <------------> * * <----DAI1- 34 PCM1 <------------> * * <----DAI1-----> Codec Speakers
35 * DSP * 35 * DSP *
36 PCM2 <------------> * * <----DAI2- 36 PCM2 <------------> * * <----DAI2-----> MODEM
37 * * 37 * *
38 PCM3 <------------> * * <----DAI3- 38 PCM3 <------------> * * <----DAI3-----> BT
39 * * 39 * *
40 * * <----DAI4- 40 * * <----DAI4-----> DMIC
41 * * 41 * *
42 * * <----DAI5- 42 * * <----DAI5-----> FM
43 ************* 43 *************
44 44
45 This diagram shows a simple smart phone audio 45 This diagram shows a simple smart phone audio subsystem. It supports Bluetooth,
46 FM digital radio, Speakers, Headset Jack, digi 46 FM digital radio, Speakers, Headset Jack, digital microphones and cellular
47 modem. This sound card exposes 4 DSP front end 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 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 ro 49 of the BE DAIs. The FE PCM devices can also route audio to more than 1 BE DAI.
50 50
51 51
52 52
53 Example - DPCM Switching playback from DAI0 to 53 Example - DPCM Switching playback from DAI0 to DAI1
54 ---------------------------------------------- 54 ---------------------------------------------------
55 55
56 Audio is being played to the Headset. After a 56 Audio is being played to the Headset. After a while the user removes the headset
57 and audio continues playing on the speakers. 57 and audio continues playing on the speakers.
58 58
59 Playback on PCM0 to Headset would look like :- 59 Playback on PCM0 to Headset would look like :-
60 :: 60 ::
61 61
62 ************* 62 *************
63 PCM0 <============> * * <====DAI0= 63 PCM0 <============> * * <====DAI0=====> Codec Headset
64 * * 64 * *
65 PCM1 <------------> * * <----DAI1- 65 PCM1 <------------> * * <----DAI1-----> Codec Speakers
66 * DSP * 66 * DSP *
67 PCM2 <------------> * * <----DAI2- 67 PCM2 <------------> * * <----DAI2-----> MODEM
68 * * 68 * *
69 PCM3 <------------> * * <----DAI3- 69 PCM3 <------------> * * <----DAI3-----> BT
70 * * 70 * *
71 * * <----DAI4- 71 * * <----DAI4-----> DMIC
72 * * 72 * *
73 * * <----DAI5- 73 * * <----DAI5-----> FM
74 ************* 74 *************
75 75
76 The headset is removed from the jack by user s 76 The headset is removed from the jack by user so the speakers must now be used :-
77 :: 77 ::
78 78
79 ************* 79 *************
80 PCM0 <============> * * <----DAI0- 80 PCM0 <============> * * <----DAI0-----> Codec Headset
81 * * 81 * *
82 PCM1 <------------> * * <====DAI1= 82 PCM1 <------------> * * <====DAI1=====> Codec Speakers
83 * DSP * 83 * DSP *
84 PCM2 <------------> * * <----DAI2- 84 PCM2 <------------> * * <----DAI2-----> MODEM
85 * * 85 * *
86 PCM3 <------------> * * <----DAI3- 86 PCM3 <------------> * * <----DAI3-----> BT
87 * * 87 * *
88 * * <----DAI4- 88 * * <----DAI4-----> DMIC
89 * * 89 * *
90 * * <----DAI5- 90 * * <----DAI5-----> FM
91 ************* 91 *************
92 92
93 The audio driver processes this as follows :- 93 The audio driver processes this as follows :-
94 94
95 1. Machine driver receives Jack removal event. 95 1. Machine driver receives Jack removal event.
96 96
97 2. Machine driver OR audio HAL disables the He 97 2. Machine driver OR audio HAL disables the Headset path.
98 98
99 3. DPCM runs the PCM trigger(stop), hw_free(), 99 3. DPCM runs the PCM trigger(stop), hw_free(), shutdown() operations on DAI0
100 for headset since the path is now disabled. 100 for headset since the path is now disabled.
101 101
102 4. Machine driver or audio HAL enables the spe 102 4. Machine driver or audio HAL enables the speaker path.
103 103
104 5. DPCM runs the PCM ops for startup(), hw_par !! 104 5. DPCM runs the PCM ops for startup(), hw_params(), prepapre() and
105 trigger(start) for DAI1 Speakers since the 105 trigger(start) for DAI1 Speakers since the path is enabled.
106 106
107 In this example, the machine driver or userspa 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 D 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 s 109 the link up or down. Audio playback does not stop during this transition.
110 110
111 111
112 112
113 DPCM machine driver 113 DPCM machine driver
114 =================== 114 ===================
115 115
116 The DPCM enabled ASoC machine driver is simila 116 The DPCM enabled ASoC machine driver is similar to normal machine drivers
117 except that we also have to :- 117 except that we also have to :-
118 118
119 1. Define the FE and BE DAI links. 119 1. Define the FE and BE DAI links.
120 120
121 2. Define any FE/BE PCM operations. 121 2. Define any FE/BE PCM operations.
122 122
123 3. Define widget graph connections. 123 3. Define widget graph connections.
124 124
125 125
126 FE and BE DAI links 126 FE and BE DAI links
127 ------------------- 127 -------------------
128 :: 128 ::
129 129
130 | Front End PCMs | SoC DSP | Back End D 130 | Front End PCMs | SoC DSP | Back End DAIs | Audio devices |
131 131
132 ************* 132 *************
133 PCM0 <------------> * * <----DAI0- 133 PCM0 <------------> * * <----DAI0-----> Codec Headset
134 * * 134 * *
135 PCM1 <------------> * * <----DAI1- 135 PCM1 <------------> * * <----DAI1-----> Codec Speakers
136 * DSP * 136 * DSP *
137 PCM2 <------------> * * <----DAI2- 137 PCM2 <------------> * * <----DAI2-----> MODEM
138 * * 138 * *
139 PCM3 <------------> * * <----DAI3- 139 PCM3 <------------> * * <----DAI3-----> BT
140 * * 140 * *
141 * * <----DAI4- 141 * * <----DAI4-----> DMIC
142 * * 142 * *
143 * * <----DAI5- 143 * * <----DAI5-----> FM
144 ************* 144 *************
145 145
146 For the example above we have to define 4 FE D 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 :- 147 FE DAI links are defined as follows :-
148 :: 148 ::
149 149
150 static struct snd_soc_dai_link machine_dais[ 150 static struct snd_soc_dai_link machine_dais[] = {
151 { 151 {
152 .name = "PCM0 System", 152 .name = "PCM0 System",
153 .stream_name = "System Playbac 153 .stream_name = "System Playback",
154 .cpu_dai_name = "System Pin", 154 .cpu_dai_name = "System Pin",
155 .platform_name = "dsp-audio", 155 .platform_name = "dsp-audio",
156 .codec_name = "snd-soc-dummy", 156 .codec_name = "snd-soc-dummy",
157 .codec_dai_name = "snd-soc-dum 157 .codec_dai_name = "snd-soc-dummy-dai",
158 .dynamic = 1, 158 .dynamic = 1,
159 .trigger = {SND_SOC_DPCM_TRIGG 159 .trigger = {SND_SOC_DPCM_TRIGGER_POST, SND_SOC_DPCM_TRIGGER_POST},
160 .dpcm_playback = 1, 160 .dpcm_playback = 1,
161 }, 161 },
162 .....< other FE and BE DAI links here 162 .....< other FE and BE DAI links here >
163 }; 163 };
164 164
165 This FE DAI link is pretty similar to a regula 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 ``dynam 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_ 167 directions should also be set with the ``dpcm_playback`` and ``dpcm_capture``
168 flags. There is also an option to specify the 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 169 each FE. This allows the ASoC core to trigger the DSP before or after the other
170 components (as some DSPs have strong requireme 170 components (as some DSPs have strong requirements for the ordering DAI/DSP
171 start and stop sequences). 171 start and stop sequences).
172 172
173 The FE DAI above sets the codec and code DAIs 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 c 174 dynamic and will change depending on runtime config.
175 175
176 The BE DAIs are configured as follows :- 176 The BE DAIs are configured as follows :-
177 :: 177 ::
178 178
179 static struct snd_soc_dai_link machine_dais[ 179 static struct snd_soc_dai_link machine_dais[] = {
180 .....< FE DAI links here > 180 .....< FE DAI links here >
181 { 181 {
182 .name = "Codec Headset", 182 .name = "Codec Headset",
183 .cpu_dai_name = "ssp-dai.0", 183 .cpu_dai_name = "ssp-dai.0",
184 .platform_name = "snd-soc-dumm 184 .platform_name = "snd-soc-dummy",
185 .no_pcm = 1, 185 .no_pcm = 1,
186 .codec_name = "rt5640.0-001c", 186 .codec_name = "rt5640.0-001c",
187 .codec_dai_name = "rt5640-aif1 187 .codec_dai_name = "rt5640-aif1",
188 .ignore_suspend = 1, 188 .ignore_suspend = 1,
189 .ignore_pmdown_time = 1, 189 .ignore_pmdown_time = 1,
190 .be_hw_params_fixup = hswult_s 190 .be_hw_params_fixup = hswult_ssp0_fixup,
191 .ops = &haswell_ops, 191 .ops = &haswell_ops,
192 .dpcm_playback = 1, 192 .dpcm_playback = 1,
193 .dpcm_capture = 1, 193 .dpcm_capture = 1,
194 }, 194 },
195 .....< other BE DAI links here > 195 .....< other BE DAI links here >
196 }; 196 };
197 197
198 This BE DAI link connects DAI0 to the codec (i 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 se 199 the ``no_pcm`` flag to mark it has a BE and sets flags for supported stream
200 directions using ``dpcm_playback`` and ``dpcm_ 200 directions using ``dpcm_playback`` and ``dpcm_capture`` above.
201 201
202 The BE has also flags set for ignoring suspend 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 ho 203 the BE to work in a hostless mode where the host CPU is not transferring data
204 like a BT phone call :- 204 like a BT phone call :-
205 :: 205 ::
206 206
207 ************* 207 *************
208 PCM0 <------------> * * <----DAI0- 208 PCM0 <------------> * * <----DAI0-----> Codec Headset
209 * * 209 * *
210 PCM1 <------------> * * <----DAI1- 210 PCM1 <------------> * * <----DAI1-----> Codec Speakers
211 * DSP * 211 * DSP *
212 PCM2 <------------> * * <====DAI2= 212 PCM2 <------------> * * <====DAI2=====> MODEM
213 * * 213 * *
214 PCM3 <------------> * * <====DAI3= 214 PCM3 <------------> * * <====DAI3=====> BT
215 * * 215 * *
216 * * <----DAI4- 216 * * <----DAI4-----> DMIC
217 * * 217 * *
218 * * <----DAI5- 218 * * <----DAI5-----> FM
219 ************* 219 *************
220 220
221 This allows the host CPU to sleep while the DS !! 221 This allows the host CPU to sleep whilst the DSP, MODEM DAI and the BT DAI are
222 still in operation. 222 still in operation.
223 223
224 A BE DAI link can also set the codec to a dumm !! 224 A BE DAI link can also set the codec to a dummy device if the code is a device
225 that is managed externally. 225 that is managed externally.
226 226
227 Likewise a BE DAI can also set a dummy cpu DAI 227 Likewise a BE DAI can also set a dummy cpu DAI if the CPU DAI is managed by the
228 DSP firmware. 228 DSP firmware.
229 229
230 230
231 FE/BE PCM operations 231 FE/BE PCM operations
232 -------------------- 232 --------------------
233 233
234 The BE above also exports some PCM operations 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) 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 236 FE hw params. i.e. the DSP may perform SRC or ASRC from the FE to BE.
237 237
238 e.g. DSP converts all FE hw params to run at f 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 f 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 con 240 DAI0 so that the DAI is running at desired configuration regardless of the FE
241 configuration. 241 configuration.
242 :: 242 ::
243 243
244 static int dai0_fixup(struct snd_soc_pcm_run 244 static int dai0_fixup(struct snd_soc_pcm_runtime *rtd,
245 struct snd_pcm_hw_para 245 struct snd_pcm_hw_params *params)
246 { 246 {
247 struct snd_interval *rate = hw_param_i 247 struct snd_interval *rate = hw_param_interval(params,
248 SNDRV_PCM_HW_PARAM_RAT 248 SNDRV_PCM_HW_PARAM_RATE);
249 struct snd_interval *channels = hw_par 249 struct snd_interval *channels = hw_param_interval(params,
250 250 SNDRV_PCM_HW_PARAM_CHANNELS);
251 251
252 /* The DSP will convert the FE rate to !! 252 /* The DSP will covert the FE rate to 48k, stereo */
253 rate->min = rate->max = 48000; 253 rate->min = rate->max = 48000;
254 channels->min = channels->max = 2; 254 channels->min = channels->max = 2;
255 255
256 /* set DAI0 to 16 bit */ 256 /* set DAI0 to 16 bit */
257 params_set_format(params, SNDRV_PCM_FO !! 257 snd_mask_set(¶ms->masks[SNDRV_PCM_HW_PARAM_FORMAT -
>> 258 SNDRV_PCM_HW_PARAM_FIRST_MASK],
>> 259 SNDRV_PCM_FORMAT_S16_LE);
258 return 0; 260 return 0;
259 } 261 }
260 262
261 The other PCM operation are the same as for re 263 The other PCM operation are the same as for regular DAI links. Use as necessary.
262 264
263 265
264 Widget graph connections 266 Widget graph connections
265 ------------------------ 267 ------------------------
266 268
267 The BE DAI links will normally be connected to 269 The BE DAI links will normally be connected to the graph at initialisation time
268 by the ASoC DAPM core. However, if the BE code 270 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 :- 271 has to be set explicitly in the driver :-
270 :: 272 ::
271 273
272 /* BE for codec Headset - DAI0 is dummy and 274 /* BE for codec Headset - DAI0 is dummy and managed by DSP FW */
273 {"DAI0 CODEC IN", NULL, "AIF1 Capture"}, 275 {"DAI0 CODEC IN", NULL, "AIF1 Capture"},
274 {"AIF1 Playback", NULL, "DAI0 CODEC OUT"}, 276 {"AIF1 Playback", NULL, "DAI0 CODEC OUT"},
275 277
276 278
277 Writing a DPCM DSP driver 279 Writing a DPCM DSP driver
278 ========================= 280 =========================
279 281
280 The DPCM DSP driver looks much like a standard 282 The DPCM DSP driver looks much like a standard platform class ASoC driver
281 combined with elements from a codec class driv 283 combined with elements from a codec class driver. A DSP platform driver must
282 implement :- 284 implement :-
283 285
284 1. Front End PCM DAIs - i.e. struct snd_soc_da 286 1. Front End PCM DAIs - i.e. struct snd_soc_dai_driver.
285 287
286 2. DAPM graph showing DSP audio routing from F 288 2. DAPM graph showing DSP audio routing from FE DAIs to BEs.
287 289
288 3. DAPM widgets from DSP graph. 290 3. DAPM widgets from DSP graph.
289 291
290 4. Mixers for gains, routing, etc. 292 4. Mixers for gains, routing, etc.
291 293
292 5. DMA configuration. 294 5. DMA configuration.
293 295
294 6. BE AIF widgets. 296 6. BE AIF widgets.
295 297
296 Items 6 is important for routing the audio out 298 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. 299 defined for each BE and each stream direction. e.g for BE DAI0 above we would
298 have :- 300 have :-
299 :: 301 ::
300 302
301 SND_SOC_DAPM_AIF_IN("DAI0 RX", NULL, 0, SND_ 303 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 304 SND_SOC_DAPM_AIF_OUT("DAI0 TX", NULL, 0, SND_SOC_NOPM, 0, 0),
303 305
304 The BE AIF are used to connect the DSP graph t 306 The BE AIF are used to connect the DSP graph to the graphs for the other
305 component drivers (e.g. codec graph). 307 component drivers (e.g. codec graph).
306 308
307 309
308 Hostless PCM streams 310 Hostless PCM streams
309 ==================== 311 ====================
310 312
311 A hostless PCM stream is a stream that is not 313 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 han 314 example of this would be a phone call from handset to modem.
313 :: 315 ::
314 316
315 ************* 317 *************
316 PCM0 <------------> * * <----DAI0- 318 PCM0 <------------> * * <----DAI0-----> Codec Headset
317 * * 319 * *
318 PCM1 <------------> * * <====DAI1= 320 PCM1 <------------> * * <====DAI1=====> Codec Speakers/Mic
319 * DSP * 321 * DSP *
320 PCM2 <------------> * * <====DAI2= 322 PCM2 <------------> * * <====DAI2=====> MODEM
321 * * 323 * *
322 PCM3 <------------> * * <----DAI3- 324 PCM3 <------------> * * <----DAI3-----> BT
323 * * 325 * *
324 * * <----DAI4- 326 * * <----DAI4-----> DMIC
325 * * 327 * *
326 * * <----DAI5- 328 * * <----DAI5-----> FM
327 ************* 329 *************
328 330
329 In this case the PCM data is routed via the DS 331 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 332 is only used for control and can sleep during the runtime of the stream.
331 333
332 The host can control the hostless link either 334 The host can control the hostless link either by :-
333 335
334 1. Configuring the link as a CODEC <-> CODEC 336 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 337 is enabled or disabled by the state of the DAPM graph. This usually means
336 there is a mixer control that can be used 338 there is a mixer control that can be used to connect or disconnect the path
337 between both DAIs. 339 between both DAIs.
338 340
339 2. Hostless FE. This FE has a virtual connect 341 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 342 graph. Control is then carried out by the FE as regular PCM operations.
341 This method gives more control over the DA 343 This method gives more control over the DAI links, but requires much more
342 userspace code to control the link. Its re 344 userspace code to control the link. Its recommended to use CODEC<->CODEC
343 unless your HW needs more fine grained seq 345 unless your HW needs more fine grained sequencing of the PCM ops.
344 346
345 347
346 CODEC <-> CODEC link 348 CODEC <-> CODEC link
347 -------------------- 349 --------------------
348 350
349 This DAI link is enabled when DAPM detects a v 351 This DAI link is enabled when DAPM detects a valid path within the DAPM graph.
350 The machine driver sets some additional parame 352 The machine driver sets some additional parameters to the DAI link i.e.
351 :: 353 ::
352 354
353 static const struct snd_soc_pcm_stream dai_p 355 static const struct snd_soc_pcm_stream dai_params = {
354 .formats = SNDRV_PCM_FMTBIT_S32_LE, 356 .formats = SNDRV_PCM_FMTBIT_S32_LE,
355 .rate_min = 8000, 357 .rate_min = 8000,
356 .rate_max = 8000, 358 .rate_max = 8000,
357 .channels_min = 2, 359 .channels_min = 2,
358 .channels_max = 2, 360 .channels_max = 2,
359 }; 361 };
360 362
361 static struct snd_soc_dai_link dais[] = { 363 static struct snd_soc_dai_link dais[] = {
362 < ... more DAI links above ... > 364 < ... more DAI links above ... >
363 { 365 {
364 .name = "MODEM", 366 .name = "MODEM",
365 .stream_name = "MODEM", 367 .stream_name = "MODEM",
366 .cpu_dai_name = "dai2", 368 .cpu_dai_name = "dai2",
367 .codec_dai_name = "modem-aif1" 369 .codec_dai_name = "modem-aif1",
368 .codec_name = "modem", 370 .codec_name = "modem",
369 .dai_fmt = SND_SOC_DAIFMT_I2S 371 .dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF
370 | SND_SOC_DAIF 372 | SND_SOC_DAIFMT_CBM_CFM,
371 .c2c_params = &dai_params, !! 373 .params = &dai_params,
372 .num_c2c_params = 1, <<
373 } 374 }
374 < ... more DAI links here ... > 375 < ... more DAI links here ... >
375 376
376 These parameters are used to configure the DAI 377 These parameters are used to configure the DAI hw_params() when DAPM detects a
377 valid path and then calls the PCM operations t 378 valid path and then calls the PCM operations to start the link. DAPM will also
378 call the appropriate PCM operations to disable 379 call the appropriate PCM operations to disable the DAI when the path is no
379 longer valid. 380 longer valid.
380 381
381 382
382 Hostless FE 383 Hostless FE
383 ----------- 384 -----------
384 385
385 The DAI link(s) are enabled by a FE that does 386 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 387 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 388 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 P 389 when the FE PCM is stopped. Note that the FE PCM cannot read or write data in
389 this configuration. 390 this configuration.
>> 391
>> 392
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