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    .params = &dsp_codec_params,
74 },
75 {
76    .name = "DSP-CODEC",
77    .stream_name = "DSP-CODEC",
78    .cpu_dai_name = "wm0010-sdi2",
79    .codec_name = "codec-3,
80    .codec_dai_name = "codec-3-dai_name",
81    .dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF
82            | SND_SOC_DAIFMT_CBM_CFM,
83    .ignore_suspend = 1,
84    .params = &dsp_codec_params,
85 },
86
87Above code snippet is motivated from sound/soc/samsung/speyside.c.
88
89Note the "params" callback which lets the dapm know that this
90dai_link is a codec to codec connection.
91
92In dapm core a route is created between cpu_dai playback widget
93and codec_dai capture widget for playback path and vice-versa is
94true for capture path. In order for this aforementioned route to get
95triggered, DAPM needs to find a valid endpoint which could be either
96a sink or source widget corresponding to playback and capture path
97respectively.
98
99In order to trigger this dai_link widget, a thin codec driver for
100the speaker amp can be created as demonstrated in wm8727.c file, it
101sets appropriate constraints for the device even if it needs no control.
102
103Make sure to name your corresponding cpu and codec playback and capture
104dai names ending with "Playback" and "Capture" respectively as dapm core
105will link and power those dais based on the name.
106
107Note that in current device tree there is no way to mark a dai_link
108as codec to codec. However, it may change in future.
109