dpcm.rst (13641B)
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.