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 107A dai_link in a "simple-audio-card" will automatically be detected as 108codec to codec when all DAIs on the link belong to codec components. 109The dai_link will be initialized with the subset of stream parameters 110(channels, format, sample rate) supported by all DAIs on the link. Since 111there is no way to provide these parameters in the device tree, this is 112mostly useful for communication with simple fixed-function codecs, such 113as a Bluetooth controller or cellular modem. 114