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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	Dynamic PCM
2	===========
3	
4	1. Description
5	==============
6	
7	Dynamic PCM allows an ALSA PCM device to digitally route its PCM audio to
8	various digital endpoints during the PCM stream runtime. e.g. PCM0 can route
9	digital audio to I2S DAI0, I2S DAI1 or PDM DAI2. This is useful for on SoC DSP
10	drivers that expose several ALSA PCMs and can route to multiple DAIs.
11	
12	The DPCM runtime routing is determined by the ALSA mixer settings in the same
13	way as the analog signal is routed in an ASoC codec driver. DPCM uses a DAPM
14	graph representing the DSP internal audio paths and uses the mixer settings to
15	determine the patch used by each ALSA PCM.
16	
17	DPCM re-uses all the existing component codec, platform and DAI drivers without
18	any modifications.
19	
20	
21	Phone Audio System with SoC based DSP
22	-------------------------------------
23	
24	Consider the following phone audio subsystem. This will be used in this
25	document for all examples :-
26	
27	| Front End PCMs    |  SoC DSP  | Back End DAIs | Audio devices |
28	
29	                    *************
30	PCM0 <------------> *           * <----DAI0-----> Codec Headset
31	                    *           *
32	PCM1 <------------> *           * <----DAI1-----> Codec Speakers
33	                    *   DSP     *
34	PCM2 <------------> *           * <----DAI2-----> MODEM
35	                    *           *
36	PCM3 <------------> *           * <----DAI3-----> BT
37	                    *           *
38	                    *           * <----DAI4-----> DMIC
39	                    *           *
40	                    *           * <----DAI5-----> FM
41	                    *************
42	
43	This diagram shows a simple smart phone audio subsystem. It supports Bluetooth,
44	FM digital radio, Speakers, Headset Jack, digital microphones and cellular
45	modem. This sound card exposes 4 DSP front end (FE) ALSA PCM devices and
46	supports 6 back end (BE) DAIs. Each FE PCM can digitally route audio data to any
47	of the BE DAIs. The FE PCM devices can also route audio to more than 1 BE DAI.
48	
49	
50	
51	Example - DPCM Switching playback from DAI0 to DAI1
52	---------------------------------------------------
53	
54	Audio is being played to the Headset. After a while the user removes the headset
55	and audio continues playing on the speakers.
56	
57	Playback on PCM0 to Headset would look like :-
58	
59	                    *************
60	PCM0 <============> *           * <====DAI0=====> Codec Headset
61	                    *           *
62	PCM1 <------------> *           * <----DAI1-----> Codec Speakers
63	                    *   DSP     *
64	PCM2 <------------> *           * <----DAI2-----> MODEM
65	                    *           *
66	PCM3 <------------> *           * <----DAI3-----> BT
67	                    *           *
68	                    *           * <----DAI4-----> DMIC
69	                    *           *
70	                    *           * <----DAI5-----> FM
71	                    *************
72	
73	The headset is removed from the jack by user so the speakers must now be used :-
74	
75	                    *************
76	PCM0 <============> *           * <----DAI0-----> Codec Headset
77	                    *           *
78	PCM1 <------------> *           * <====DAI1=====> Codec Speakers
79	                    *   DSP     *
80	PCM2 <------------> *           * <----DAI2-----> MODEM
81	                    *           *
82	PCM3 <------------> *           * <----DAI3-----> BT
83	                    *           *
84	                    *           * <----DAI4-----> DMIC
85	                    *           *
86	                    *           * <----DAI5-----> FM
87	                    *************
88	
89	The audio driver processes this as follows :-
90	
91	 1) Machine driver receives Jack removal event.
92	
93	 2) Machine driver OR audio HAL disables the Headset path.
94	
95	 3) DPCM runs the PCM trigger(stop), hw_free(), shutdown() operations on DAI0
96	    for headset since the path is now disabled.
97	
98	 4) Machine driver or audio HAL enables the speaker path.
99	
100	 5) DPCM runs the PCM ops for startup(), hw_params(), prepapre() and
101	    trigger(start) for DAI1 Speakers since the path is enabled.
102	
103	In this example, the machine driver or userspace audio HAL can alter the routing
104	and then DPCM will take care of managing the DAI PCM operations to either bring
105	the link up or down. Audio playback does not stop during this transition.
106	
107	
108	
109	DPCM machine driver
110	===================
111	
112	The DPCM enabled ASoC machine driver is similar to normal machine drivers
113	except that we also have to :-
114	
115	 1) Define the FE and BE DAI links.
116	
117	 2) Define any FE/BE PCM operations.
118	
119	 3) Define widget graph connections.
120	
121	
122	1 FE and BE DAI links
123	---------------------
124	
125	| Front End PCMs    |  SoC DSP  | Back End DAIs | Audio devices |
126	
127	                    *************
128	PCM0 <------------> *           * <----DAI0-----> Codec Headset
129	                    *           *
130	PCM1 <------------> *           * <----DAI1-----> Codec Speakers
131	                    *   DSP     *
132	PCM2 <------------> *           * <----DAI2-----> MODEM
133	                    *           *
134	PCM3 <------------> *           * <----DAI3-----> BT
135	                    *           *
136	                    *           * <----DAI4-----> DMIC
137	                    *           *
138	                    *           * <----DAI5-----> FM
139	                    *************
140	
141	For the example above we have to define 4 FE DAI links and 6 BE DAI links. The
142	FE DAI links are defined as follows :-
143	
144	static struct snd_soc_dai_link machine_dais[] = {
145		{
146			.name = "PCM0 System",
147			.stream_name = "System Playback",
148			.cpu_dai_name = "System Pin",
149			.platform_name = "dsp-audio",
150			.codec_name = "snd-soc-dummy",
151			.codec_dai_name = "snd-soc-dummy-dai",
152			.dynamic = 1,
153			.trigger = {SND_SOC_DPCM_TRIGGER_POST, SND_SOC_DPCM_TRIGGER_POST},
154			.dpcm_playback = 1,
155		},
156		.....< other FE and BE DAI links here >
157	};
158	
159	This FE DAI link is pretty similar to a regular DAI link except that we also
160	set the DAI link to a DPCM FE with the "dynamic = 1". The supported FE stream
161	directions should also be set with the "dpcm_playback" and "dpcm_capture"
162	flags. There is also an option to specify the ordering of the trigger call for
163	each FE. This allows the ASoC core to trigger the DSP before or after the other
164	components (as some DSPs have strong requirements for the ordering DAI/DSP
165	start and stop sequences).
166	
167	The FE DAI above sets the codec and code DAIs to dummy devices since the BE is
168	dynamic and will change depending on runtime config.
169	
170	The BE DAIs are configured as follows :-
171	
172	static struct snd_soc_dai_link machine_dais[] = {
173		.....< FE DAI links here >
174		{
175			.name = "Codec Headset",
176			.cpu_dai_name = "ssp-dai.0",
177			.platform_name = "snd-soc-dummy",
178			.no_pcm = 1,
179			.codec_name = "rt5640.0-001c",
180			.codec_dai_name = "rt5640-aif1",
181			.ignore_suspend = 1,
182			.ignore_pmdown_time = 1,
183			.be_hw_params_fixup = hswult_ssp0_fixup,
184			.ops = &haswell_ops,
185			.dpcm_playback = 1,
186			.dpcm_capture = 1,
187		},
188		.....< other BE DAI links here >
189	};
190	
191	This BE DAI link connects DAI0 to the codec (in this case RT5460 AIF1). It sets
192	the "no_pcm" flag to mark it has a BE and sets flags for supported stream
193	directions using "dpcm_playback" and "dpcm_capture" above.
194	
195	The BE has also flags set for ignoring suspend and PM down time. This allows
196	the BE to work in a hostless mode where the host CPU is not transferring data
197	like a BT phone call :-
198	
199	                    *************
200	PCM0 <------------> *           * <----DAI0-----> Codec Headset
201	                    *           *
202	PCM1 <------------> *           * <----DAI1-----> Codec Speakers
203	                    *   DSP     *
204	PCM2 <------------> *           * <====DAI2=====> MODEM
205	                    *           *
206	PCM3 <------------> *           * <====DAI3=====> BT
207	                    *           *
208	                    *           * <----DAI4-----> DMIC
209	                    *           *
210	                    *           * <----DAI5-----> FM
211	                    *************
212	
213	This allows the host CPU to sleep whilst the DSP, MODEM DAI and the BT DAI are
214	still in operation.
215	
216	A BE DAI link can also set the codec to a dummy device if the code is a device
217	that is managed externally.
218	
219	Likewise a BE DAI can also set a dummy cpu DAI if the CPU DAI is managed by the
220	DSP firmware.
221	
222	
223	2 FE/BE PCM operations
224	----------------------
225	
226	The BE above also exports some PCM operations and a "fixup" callback. The fixup
227	callback is used by the machine driver to (re)configure the DAI based upon the
228	FE hw params. i.e. the DSP may perform SRC or ASRC from the FE to BE.
229	
230	e.g. DSP converts all FE hw params to run at fixed rate of 48k, 16bit, stereo for
231	DAI0. This means all FE hw_params have to be fixed in the machine driver for
232	DAI0 so that the DAI is running at desired configuration regardless of the FE
233	configuration.
234	
235	static int dai0_fixup(struct snd_soc_pcm_runtime *rtd,
236				struct snd_pcm_hw_params *params)
237	{
238		struct snd_interval *rate = hw_param_interval(params,
239				SNDRV_PCM_HW_PARAM_RATE);
240		struct snd_interval *channels = hw_param_interval(params,
241							SNDRV_PCM_HW_PARAM_CHANNELS);
242	
243		/* The DSP will covert the FE rate to 48k, stereo */
244		rate->min = rate->max = 48000;
245		channels->min = channels->max = 2;
246	
247		/* set DAI0 to 16 bit */
248		snd_mask_set(&params->masks[SNDRV_PCM_HW_PARAM_FORMAT -
249					    SNDRV_PCM_HW_PARAM_FIRST_MASK],
250					    SNDRV_PCM_FORMAT_S16_LE);
251		return 0;
252	}
253	
254	The other PCM operation are the same as for regular DAI links. Use as necessary.
255	
256	
257	3 Widget graph connections
258	--------------------------
259	
260	The BE DAI links will normally be connected to the graph at initialisation time
261	by the ASoC DAPM core. However, if the BE codec or BE DAI is a dummy then this
262	has to be set explicitly in the driver :-
263	
264	/* BE for codec Headset -  DAI0 is dummy and managed by DSP FW */
265	{"DAI0 CODEC IN", NULL, "AIF1 Capture"},
266	{"AIF1 Playback", NULL, "DAI0 CODEC OUT"},
267	
268	
269	Writing a DPCM DSP driver
270	=========================
271	
272	The DPCM DSP driver looks much like a standard platform class ASoC driver
273	combined with elements from a codec class driver. A DSP platform driver must
274	implement :-
275	
276	 1) Front End PCM DAIs - i.e. struct snd_soc_dai_driver.
277	
278	 2) DAPM graph showing DSP audio routing from FE DAIs to BEs.
279	
280	 3) DAPM widgets from DSP graph.
281	
282	 4) Mixers for gains, routing, etc.
283	
284	 5) DMA configuration.
285	
286	 6) BE AIF widgets.
287	
288	Items 6 is important for routing the audio outside of the DSP. AIF need to be
289	defined for each BE and each stream direction. e.g for BE DAI0 above we would
290	have :-
291	
292	SND_SOC_DAPM_AIF_IN("DAI0 RX", NULL, 0, SND_SOC_NOPM, 0, 0),
293	SND_SOC_DAPM_AIF_OUT("DAI0 TX", NULL, 0, SND_SOC_NOPM, 0, 0),
294	
295	The BE AIF are used to connect the DSP graph to the graphs for the other
296	component drivers (e.g. codec graph).
297	
298	
299	Hostless PCM streams
300	====================
301	
302	A hostless PCM stream is a stream that is not routed through the host CPU. An
303	example of this would be a phone call from handset to modem.
304	
305	
306	                    *************
307	PCM0 <------------> *           * <----DAI0-----> Codec Headset
308	                    *           *
309	PCM1 <------------> *           * <====DAI1=====> Codec Speakers/Mic
310	                    *   DSP     *
311	PCM2 <------------> *           * <====DAI2=====> MODEM
312	                    *           *
313	PCM3 <------------> *           * <----DAI3-----> BT
314	                    *           *
315	                    *           * <----DAI4-----> DMIC
316	                    *           *
317	                    *           * <----DAI5-----> FM
318	                    *************
319	
320	In this case the PCM data is routed via the DSP. The host CPU in this use case
321	is only used for control and can sleep during the runtime of the stream.
322	
323	The host can control the hostless link either by :-
324	
325	 1) Configuring the link as a CODEC <-> CODEC style link. In this case the link
326	    is enabled or disabled by the state of the DAPM graph. This usually means
327	    there is a mixer control that can be used to connect or disconnect the path
328	    between both DAIs.
329	
330	 2) Hostless FE. This FE has a virtual connection to the BE DAI links on the DAPM
331	    graph. Control is then carried out by the FE as regular PCM operations.
332	    This method gives more control over the DAI links, but requires much more
333	    userspace code to control the link. Its recommended to use CODEC<->CODEC
334	    unless your HW needs more fine grained sequencing of the PCM ops.
335	
336	
337	CODEC <-> CODEC link
338	--------------------
339	
340	This DAI link is enabled when DAPM detects a valid path within the DAPM graph.
341	The machine driver sets some additional parameters to the DAI link i.e.
342	
343	static const struct snd_soc_pcm_stream dai_params = {
344		.formats = SNDRV_PCM_FMTBIT_S32_LE,
345		.rate_min = 8000,
346		.rate_max = 8000,
347		.channels_min = 2,
348		.channels_max = 2,
349	};
350	
351	static struct snd_soc_dai_link dais[] = {
352		< ... more DAI links above ... >
353		{
354			.name = "MODEM",
355			.stream_name = "MODEM",
356			.cpu_dai_name = "dai2",
357			.codec_dai_name = "modem-aif1",
358			.codec_name = "modem",
359			.dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF
360					| SND_SOC_DAIFMT_CBM_CFM,
361			.params = &dai_params,
362		}
363		< ... more DAI links here ... >
364	
365	These parameters are used to configure the DAI hw_params() when DAPM detects a
366	valid path and then calls the PCM operations to start the link. DAPM will also
367	call the appropriate PCM operations to disable the DAI when the path is no
368	longer valid.
369	
370	
371	Hostless FE
372	-----------
373	
374	The DAI link(s) are enabled by a FE that does not read or write any PCM data.
375	This means creating a new FE that is connected with a virtual path to both
376	DAI links. The DAI links will be started when the FE PCM is started and stopped
377	when the FE PCM is stopped. Note that the FE PCM cannot read or write data in
378	this configuration.
379	
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