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