ALSA sound card 12_Add volume control from scratch

Collect

1. The process of calling the application when setting the volume

（1）strace分析： amixer cset numid=1 30 (设置音量)
/dev/snd/controlC0
open
SNDRV_CTL_IOCTL_CARD_INFO
SNDRV_CTL_IOCTL_PVERSION
SNDRV_CEL_CTL_CTL_CTL_CTL_V READ
SNDRV_CTL_IOCTL_ELEM_WRITE
: snd_ctl_elem_write_user

(2) When the application calls SNDRV_CTL_IOCTL_ELEM_WRITE, the driver calls the snd_ctl_elem_write_user function, which copies some parameters from user space and then calls the snd_ctl_elem_write function.

(3) Function snd_ctl_elem_write

Find a snd_kcontrol structure and then call the put function of the snd_control structure.

(4) Who provides this snd_kcontrol structure?

ASOC drivers are divided into three parts (machine, codec, platform). The codec part should be provided because it is closely related to the sound card. When adjusting the volume, you must adjust it.

2. Write the program (uda1341.c (codec))

(1)Structure snd_kcontrol_new

static const struct snd_kcontrol_new uda1341_vol_control =
{ .iface = SNDRV_CTL_ELEM_IFACE_MIXER, //Indicates which type of device the snd_kcontrol structure is used for (indicates parameter setting)
.name = "Master Playback Volume", //Volume control. The snd_kcontrol of each sound card driver is different. Why can applications adjust its volume? For some commonly used properties, they all have fixed names. The application finds its snd_kcontrol item according to the name and calls the put function inside.
.info = uda1341_info_vol, //Get some information, such as the volume range.get
= uda1341_get_vol, //Get the current volume value.put
= uda1341_put_vol, //Set the volume
};

(2) Obtain volume information, such as minimum and maximum values

/*
* Get volume information, such as minimum and maximum values
*/
int uda1341_info_vol(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_info *uinfo)
{
uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER; //The type of volume value is integer
uinfo->count = 2; //The number of channels is dual-channel
uinfo->value.integer.min = 0; //Minimum integer,
uinfo->value.integer.max = 63; //Maximum integer,
return 0;
}

Because the volume control of uda1341 is 6 bits (0 means maximum volume, 63 means minimum volume), and in the application, 0 means minimum volume, and the larger the value, the higher the volume.

(3) Get the current volume value

/*
* Get the current volume value
*/
int uda1341_get_vol(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *ucontrol)
{
struct snd_soc_codec *codec = snd_kcontrol_chip(kcontrol);

ucontrol->value.integer.value[1] = //Here it means ucontrol->value.integer.value[0] is equal to ucontrol->value.integer.value[1], because it is dual channel
ucontrol->value.integer.value[0] = 63 - snd_soc_read(codec, UDA1341_DATA00); //Read the value of register DAT00, because the value of the driver and the value of the application are opposite in size, uda1341 does not support register read operation, to get the value of a register, it is to read a cache (this cache saves the value of the setting register)
return 0;
}

(4) Set the current volume value

/*
* Set the current volume value
*/
int uda1341_put_vol(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *ucontrol)
{
struct snd_soc_codec *codec = snd_kcontrol_chip(kcontrol);
unsigned int val;

val = 63 - ucontrol->value.integer.value[0];//The value passed by the application needs to be reversed when written to the register

snd_soc_write(codec, UDA1341_DATA00, val);//Write the value val to register DATA00 return

0;
}

3. Connection between structure snd_kcontrol_new and kernel part

(1) probe function

static int uda1341_soc_probe(struct snd_soc_codec *codec)
{
int ret;
uda1341_init_regs(codec);

ret = snd_soc_add_codec_controls(codec, &uda1341_vol_control, 1);
return ret;
}

4. Testing

amixer controlsView controls

amixer cget numid=1 means check the current volume

amixer cset numid=1 30 Set the volume

5、Input Mux

Because different boards have different microphone channels (in uda1341), with the same driver, the application should set the input mux option when recording.