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 }, 161 .....< other FE and BE DAI links here > 162 }; 163 164This FE DAI link is pretty similar to a regular DAI link except that we also 165set the DAI link to a DPCM FE with the ``dynamic = 1``. 166There is also an option to specify the ordering of the trigger call for 167each FE. This allows the ASoC core to trigger the DSP before or after the other 168components (as some DSPs have strong requirements for the ordering DAI/DSP 169start and stop sequences). 170 171The FE DAI above sets the codec and code DAIs to dummy devices since the BE is 172dynamic and will change depending on runtime config. 173 174The BE DAIs are configured as follows :- 175:: 176 177 static struct snd_soc_dai_link machine_dais[] = { 178 .....< FE DAI links here > 179 { 180 .name = "Codec Headset", 181 .cpu_dai_name = "ssp-dai.0", 182 .platform_name = "snd-soc-dummy", 183 .no_pcm = 1, 184 .codec_name = "rt5640.0-001c", 185 .codec_dai_name = "rt5640-aif1", 186 .ignore_suspend = 1, 187 .ignore_pmdown_time = 1, 188 .be_hw_params_fixup = hswult_ssp0_fixup, 189 .ops = &haswell_ops, 190 }, 191 .....< other BE DAI links here > 192 }; 193 194This BE DAI link connects DAI0 to the codec (in this case RT5460 AIF1). It sets 195the ``no_pcm`` flag to mark it has a BE. 196 197The BE has also flags set for ignoring suspend and PM down time. This allows 198the BE to work in a hostless mode where the host CPU is not transferring data 199like a BT phone call :- 200:: 201 202 ************* 203 PCM0 <------------> * * <----DAI0-----> Codec Headset 204 * * 205 PCM1 <------------> * * <----DAI1-----> Codec Speakers 206 * DSP * 207 PCM2 <------------> * * <====DAI2=====> MODEM 208 * * 209 PCM3 <------------> * * <====DAI3=====> BT 210 * * 211 * * <----DAI4-----> DMIC 212 * * 213 * * <----DAI5-----> FM 214 ************* 215 216This allows the host CPU to sleep while the DSP, MODEM DAI and the BT DAI are 217still in operation. 218 219A BE DAI link can also set the codec to a dummy device if the codec is a device 220that is managed externally. 221 222Likewise a BE DAI can also set a dummy cpu DAI if the CPU DAI is managed by the 223DSP firmware. 224 225 226FE/BE PCM operations 227-------------------- 228 229The BE above also exports some PCM operations and a ``fixup`` callback. The fixup 230callback is used by the machine driver to (re)configure the DAI based upon the 231FE hw params. i.e. the DSP may perform SRC or ASRC from the FE to BE. 232 233e.g. DSP converts all FE hw params to run at fixed rate of 48k, 16bit, stereo for 234DAI0. This means all FE hw_params have to be fixed in the machine driver for 235DAI0 so that the DAI is running at desired configuration regardless of the FE 236configuration. 237:: 238 239 static int dai0_fixup(struct snd_soc_pcm_runtime *rtd, 240 struct snd_pcm_hw_params *params) 241 { 242 struct snd_interval *rate = hw_param_interval(params, 243 SNDRV_PCM_HW_PARAM_RATE); 244 struct snd_interval *channels = hw_param_interval(params, 245 SNDRV_PCM_HW_PARAM_CHANNELS); 246 247 /* The DSP will convert the FE rate to 48k, stereo */ 248 rate->min = rate->max = 48000; 249 channels->min = channels->max = 2; 250 251 /* set DAI0 to 16 bit */ 252 params_set_format(params, SNDRV_PCM_FORMAT_S16_LE); 253 return 0; 254 } 255 256The other PCM operation are the same as for regular DAI links. Use as necessary. 257 258 259Widget graph connections 260------------------------ 261 262The BE DAI links will normally be connected to the graph at initialisation time 263by the ASoC DAPM core. However, if the BE codec or BE DAI is a dummy then this 264has to be set explicitly in the driver :- 265:: 266 267 /* BE for codec Headset - DAI0 is dummy and managed by DSP FW */ 268 {"DAI0 CODEC IN", NULL, "AIF1 Capture"}, 269 {"AIF1 Playback", NULL, "DAI0 CODEC OUT"}, 270 271 272Writing a DPCM DSP driver 273========================= 274 275The DPCM DSP driver looks much like a standard platform class ASoC driver 276combined with elements from a codec class driver. A DSP platform driver must 277implement :- 278 2791. Front End PCM DAIs - i.e. struct snd_soc_dai_driver. 280 2812. DAPM graph showing DSP audio routing from FE DAIs to BEs. 282 2833. DAPM widgets from DSP graph. 284 2854. Mixers for gains, routing, etc. 286 2875. DMA configuration. 288 2896. BE AIF widgets. 290 291Items 6 is important for routing the audio outside of the DSP. AIF need to be 292defined for each BE and each stream direction. e.g for BE DAI0 above we would 293have :- 294:: 295 296 SND_SOC_DAPM_AIF_IN("DAI0 RX", NULL, 0, SND_SOC_NOPM, 0, 0), 297 SND_SOC_DAPM_AIF_OUT("DAI0 TX", NULL, 0, SND_SOC_NOPM, 0, 0), 298 299The BE AIF are used to connect the DSP graph to the graphs for the other 300component drivers (e.g. codec graph). 301 302 303Hostless PCM streams 304==================== 305 306A hostless PCM stream is a stream that is not routed through the host CPU. An 307example of this would be a phone call from handset to modem. 308:: 309 310 ************* 311 PCM0 <------------> * * <----DAI0-----> Codec Headset 312 * * 313 PCM1 <------------> * * <====DAI1=====> Codec Speakers/Mic 314 * DSP * 315 PCM2 <------------> * * <====DAI2=====> MODEM 316 * * 317 PCM3 <------------> * * <----DAI3-----> BT 318 * * 319 * * <----DAI4-----> DMIC 320 * * 321 * * <----DAI5-----> FM 322 ************* 323 324In this case the PCM data is routed via the DSP. The host CPU in this use case 325is only used for control and can sleep during the runtime of the stream. 326 327The host can control the hostless link either by :- 328 329 1. Configuring the link as a CODEC <-> CODEC style link. In this case the link 330 is enabled or disabled by the state of the DAPM graph. This usually means 331 there is a mixer control that can be used to connect or disconnect the path 332 between both DAIs. 333 334 2. Hostless FE. This FE has a virtual connection to the BE DAI links on the DAPM 335 graph. Control is then carried out by the FE as regular PCM operations. 336 This method gives more control over the DAI links, but requires much more 337 userspace code to control the link. Its recommended to use CODEC<->CODEC 338 unless your HW needs more fine grained sequencing of the PCM ops. 339 340 341CODEC <-> CODEC link 342-------------------- 343 344This DAI link is enabled when DAPM detects a valid path within the DAPM graph. 345The machine driver sets some additional parameters to the DAI link i.e. 346:: 347 348 static const struct snd_soc_pcm_stream dai_params = { 349 .formats = SNDRV_PCM_FMTBIT_S32_LE, 350 .rate_min = 8000, 351 .rate_max = 8000, 352 .channels_min = 2, 353 .channels_max = 2, 354 }; 355 356 static struct snd_soc_dai_link dais[] = { 357 < ... more DAI links above ... > 358 { 359 .name = "MODEM", 360 .stream_name = "MODEM", 361 .cpu_dai_name = "dai2", 362 .codec_dai_name = "modem-aif1", 363 .codec_name = "modem", 364 .dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF 365 | SND_SOC_DAIFMT_CBM_CFM, 366 .c2c_params = &dai_params, 367 .num_c2c_params = 1, 368 } 369 < ... more DAI links here ... > 370 371These parameters are used to configure the DAI hw_params() when DAPM detects a 372valid path and then calls the PCM operations to start the link. DAPM will also 373call the appropriate PCM operations to disable the DAI when the path is no 374longer valid. 375 376 377Hostless FE 378----------- 379 380The DAI link(s) are enabled by a FE that does not read or write any PCM data. 381This means creating a new FE that is connected with a virtual path to both 382DAI links. The DAI links will be started when the FE PCM is started and stopped 383when the FE PCM is stopped. Note that the FE PCM cannot read or write data in 384this configuration. 385