1==============================================
2Creating codec to codec dai link for ALSA dapm
3==============================================
4
5Mostly the flow of audio is always from CPU to codec so your system
6will look as below:
7::
8
9   ---------          ---------
10  |         |  dai   |         |
11      CPU    ------->    codec
12  |         |        |         |
13   ---------          ---------
14
15In case your system looks as below:
16::
17
18                       ---------
19                      |         |
20                        codec-2
21                      |         |
22                      ---------
23                           |
24                         dai-2
25                           |
26   ----------          ---------
27  |          |  dai-1 |         |
28      CPU     ------->  codec-1
29  |          |        |         |
30   ----------          ---------
31                           |
32                         dai-3
33                           |
34                       ---------
35                      |         |
36                        codec-3
37                      |         |
38                       ---------
39
40Suppose codec-2 is a bluetooth chip and codec-3 is connected to
41a speaker and you have a below scenario:
42codec-2 will receive the audio data and the user wants to play that
43audio through codec-3 without involving the CPU.This
44aforementioned case is the ideal case when codec to codec
45connection should be used.
46
47Your dai_link should appear as below in your machine
48file:
49::
50
51 /*
52  * this pcm stream only supports 24 bit, 2 channel and
53  * 48k sampling rate.
54  */
55 static const struct snd_soc_pcm_stream dsp_codec_params = {
56        .formats = SNDRV_PCM_FMTBIT_S24_LE,
57        .rate_min = 48000,
58        .rate_max = 48000,
59        .channels_min = 2,
60        .channels_max = 2,
61 };
62
63 {
64    .name = "CPU-DSP",
65    .stream_name = "CPU-DSP",
66    .cpu_dai_name = "samsung-i2s.0",
67    .codec_name = "codec-2,
68    .codec_dai_name = "codec-2-dai_name",
69    .platform_name = "samsung-i2s.0",
70    .dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF
71            | SND_SOC_DAIFMT_CBM_CFM,
72    .ignore_suspend = 1,
73    .c2c_params = &dsp_codec_params,
74    .num_c2c_params = 1,
75 },
76 {
77    .name = "DSP-CODEC",
78    .stream_name = "DSP-CODEC",
79    .cpu_dai_name = "wm0010-sdi2",
80    .codec_name = "codec-3,
81    .codec_dai_name = "codec-3-dai_name",
82    .dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF
83            | SND_SOC_DAIFMT_CBM_CFM,
84    .ignore_suspend = 1,
85    .c2c_params = &dsp_codec_params,
86    .num_c2c_params = 1,
87 },
88
89Above code snippet is motivated from sound/soc/samsung/speyside.c.
90
91Note the "c2c_params" callback which lets the dapm know that this
92dai_link is a codec to codec connection.
93
94In dapm core a route is created between cpu_dai playback widget
95and codec_dai capture widget for playback path and vice-versa is
96true for capture path. In order for this aforementioned route to get
97triggered, DAPM needs to find a valid endpoint which could be either
98a sink or source widget corresponding to playback and capture path
99respectively.
100
101In order to trigger this dai_link widget, a thin codec driver for
102the speaker amp can be created as demonstrated in wm8727.c file, it
103sets appropriate constraints for the device even if it needs no control.
104
105Make sure to name your corresponding cpu and codec playback and capture
106dai names ending with "Playback" and "Capture" respectively as dapm core
107will link and power those dais based on the name.
108
109A dai_link in a "simple-audio-card" will automatically be detected as
110codec to codec when all DAIs on the link belong to codec components.
111The dai_link will be initialized with the subset of stream parameters
112(channels, format, sample rate) supported by all DAIs on the link. Since
113there is no way to provide these parameters in the device tree, this is
114mostly useful for communication with simple fixed-function codecs, such
115as a Bluetooth controller or cellular modem.
116