Recently, the project encountered a problem that Bluetooth and voice simultaneously occupy a recording device.

The error report is printed as follows:
audio_hw_primary: cannot open pcm_cap: cannot open device '/dev/snd/pcmC0D0c': Device or resource busy

View in system / dev/snd:

# ls /dev/snd/ -l
total 0
crw-rw---- 1 system audio 116,   0 2019-04-19 17:50 controlC0
crw-rw---- 1 system audio 116,  32 2019-04-19 17:50 controlC1
crw-rw---- 1 system audio 116,  24 2019-04-19 17:50 pcmC0D0c
crw-rw---- 1 system audio 116,  16 2019-04-19 17:50 pcmC0D0p
crw-rw---- 1 system audio 116,  25 2019-04-19 17:50 pcmC0D1c
crw-rw---- 1 system audio 116,  17 2019-04-19 17:50 pcmC0D1p
crw-rw---- 1 system audio 116,  56 2019-04-19 17:50 pcmC1D0c
crw-rw---- 1 system audio 116,  48 2019-04-19 17:50 pcmC1D0p
crw-rw---- 1 system audio 116,  33 2019-04-19 17:50 timer

You can see all the sound card devices:
Control C0, Sound Card Control Equipment
pcmC0D0c, recording equipment
pcmC0D0p, playback device
timer, timing

C0D0c stands for Card 0 Device 0, that is, device 0 under sound card No. 0. C stands for capture, which means recording device; p stands for playback, which means playback device.

Playback: Description of playback capability, such as the number of channels supported by playback equipment, sampling rate, audio format;
capture: Capability description, such as channel number, sampling rate and audio format supported by recording equipment;

tinymix

By inputting tinymix, we can see the configuration parameters related to the audio channel:

# tinymix
Mixer name: 'saf775d-audio'
Number of controls: 22
ctl     type    num     name                                     value

0       INT     2       DAC1 Playback Volume                     -1 -1
1       INT     2       DAC2 Playback Volume                     -1 -1
2       INT     2       DAC3 Playback Volume                     -1 -1
3       INT     2       DAC4 Playback Volume                     -1 -1
4       INT     2       ADC1 Capture Volume                      -1 -1
5       INT     2       ADC2 Capture Volume                      -1 -1
6       BOOL    2       DAC1 Invert Switch                       On On
7       BOOL    2       DAC2 Invert Switch                       On On
8       BOOL    2       DAC3 Invert Switch                       On On
9       BOOL    2       DAC4 Invert Switch                       On On
10      BOOL    2       ADC1 Invert Switch                       On On
11      BOOL    2       ADC2 Invert Switch                       On On
12      BOOL    1       ADC High-Pass Filter Switch              On
13      BOOL    1       DAC De-emphasis Switch                   On
14      ENUM    1       ADC1 Single Ended Mode Switch            (null)
15      ENUM    1       ADC2 Single Ended Mode Switch            (null)
16      INT     1       DAC Single Volume Control Switch         -1
17      ENUM    1       DAC Soft Ramp & Zero Cross Control Switch (null)
18      BOOL    1       DAC Auto Mute Switch                     On
19      BOOL    1       Mute ADC Serial Port Switch              On
20      INT     1       ADC Single Volume Control Switch         -1
21      ENUM    1       ADC Soft Ramp & Zero Cross Control Switch (null)

DAC, Digital-to-analog converter, DAC
ADC, Analog-to-digital converter, ADC

DAC1 Playback Volume, maximum audio control value, corresponding to the corresponding registers, can modify the maximum volume output voltage;
ADC1 Capture Volume, maximum audio recording, corresponding to the corresponding registers, can modify the maximum volume output voltage;