- 采样率：采集频率越大，波形失真度越小，存储数据越多。
- 采样精度：记录电压位数越多，精度越高，恢复波形时失真度越小，存储数据越多。
头信息：采样率、精度、通道数
Android音频系统框架
APP通过AudioTrack使用AudioFilnger服务
# ls -l /dev/snd/
total 0
crw-rw---- 1 system audio 116, 3 2017-08-04 17:00 controlC0
crw-rw---- 1 system audio 116, 6 2017-08-04 17:00 controlC1
crw-rw---- 1 system audio 116, 8 2017-08-04 17:00 controlC2
crw-rw---- 1 system audio 116, 2 2017-08-04 17:00 pcmC0D0p
crw-rw---- 1 system audio 116, 5 2017-08-04 17:00 pcmC1D0c
crw-rw---- 1 system audio 116, 4 2017-08-04 17:00 pcmC1D0p
crw-rw---- 1 system audio 116, 7 2017-08-04 17:00 pcmC2D0c
crw-rw---- 1 system audio 116, 33 2017-08-04 17:00 timer
controlC0：起控制作用，C0表示card 0 pcmC0D0p：card 0，device 0，playback 播放 pcmC1D0c：card 1，device 0，capture 录音
一个声卡，可以有多个device 一个device，有播放、录音通道
每一个设备节点对应一个file_operations，主设备号116，对应同一个file_operations
sound.c
static const struct file_operations snd_fops =
{
.owner = THIS_MODULE,
.open = snd_open,
.llseek = noop_llseek,
};
static int __init alsa_sound_init(void)
{
snd_major = major;
snd_ecards_limit = cards_limit;
if (register_chrdev(major, "alsa", &snd_fops)) {
pr_err("ALSA core: unable to register native major device number %d\n", major);
return -EIO;
}
if (snd_info_init() < 0) {
unregister_chrdev(major, "alsa");
return -ENOMEM;
}
#ifndef MODULE
pr_info("Advanced Linux Sound Architecture Driver Initialized.\n");
#endif
return 0;
}
在sound.c中，首先注册了一个主设备号为116名称为alsa的字符设备。其file_operations只有open函数，没有read、write，说明它起中转作用，会根据次设备号找到更加具体的file_operations结构体。
static int snd_open(struct inode *inode, struct file *file)
{
unsigned int minor = iminor(inode);
struct snd_minor *mptr = NULL;
const struct file_operations *new_fops;
int err = 0;
if (minor >= ARRAY_SIZE(snd_minors))
return -ENODEV;
mutex_lock(&sound_mutex);
mptr = snd_minors[minor];
if (mptr == NULL) {
mptr = autoload_device(minor);
if (!mptr) {
mutex_unlock(&sound_mutex);
return -ENODEV;
}
}
new_fops = fops_get(mptr->f_ops);
mutex_unlock(&sound_mutex);
if (!new_fops)
return -ENODEV;
replace_fops(file, new_fops);
if (file->f_op->open)
err = file->f_op->open(inode, file);
return err;
}
这里可以看到，通过snd_minors找到具体的设备，而snd_minors赋值是在snd_register_device中完成。
/**
* snd_register_device - Register the ALSA device file for the card
* @type: the device type, SNDRV_DEVICE_TYPE_XXX
* @card: the card instance
* @dev: the device index
* @f_ops: the file operations
* @private_data: user pointer for f_ops->open()
* @device: the device to register
*
* Registers an ALSA device file for the given card.
* The operators have to be set in reg parameter.
*
* Return: Zero if successful, or a negative error code on failure.
*/
int snd_register_device(int type, struct snd_card *card, int dev,
const struct file_operations *f_ops,
void *private_data, struct device *device)
{
int minor;
int err = 0;
struct snd_minor *preg;
if (snd_BUG_ON(!device))
return -EINVAL;
preg = kmalloc(sizeof *preg, GFP_KERNEL);
if (preg == NULL)
return -ENOMEM;
preg->type = type;
preg->card = card ? card->number : -1;
preg->device = dev;
preg->f_ops = f_ops;
preg->private_data = private_data;
preg->card_ptr = card;
mutex_lock(&sound_mutex);
minor = snd_find_free_minor(type, card, dev);
if (minor < 0) {
err = minor;
goto error;
}
preg->dev = device;
device->devt = MKDEV(major, minor);
err = device_add(device);
if (err < 0)
goto error;
snd_minors[minor] = preg;
error:
mutex_unlock(&sound_mutex);
if (err < 0)
kfree(preg);
return err;
}
其中device_add方法，会创建对应的设备节点。
我们查找snd_register_device被调用的地方：
可以发现control.c、pcm.c、timer.c调用了这个方法，而根据之前ls /dev/snd/的结果，就可以对应起来。
alsa规范：实现定义好file_operations 顶层：sound.c： snd_fops 下层：control.c：control节点：snd_ctl_f_ops pcmC0D0p：
确定好file_operations，App就可以使用确定的接口访问声卡
硬件相关：分配、设置 snd_card结构体——>通过参数，设置参数，传输数据
一般流程：
//init.c
snd_card_new 创建声卡
snd_ctl_create(card);//创建控制接口
snd_device_new(card, SNDRV_DEV_CONTROL, card, &ops);
......
snd_ctl_dev_register
snd_register_device
/* Setup sound device */
snd_device_new
....
snd_card_register(card); //注册声卡
sound.c：register_chrdev(116, "alsa", &snd_fops)
control.c：
/*
* create control core:
* called from init.c
*/
int snd_ctl_create(struct snd_card *card)
{
static struct snd_device_ops ops = {
.dev_free = snd_ctl_dev_free,
.dev_register = snd_ctl_dev_register,
.dev_disconnect = snd_ctl_dev_disconnect,
};
int err;
if (snd_BUG_ON(!card))
return -ENXIO;
if (snd_BUG_ON(card->number < 0 || card->number >= SNDRV_CARDS))
return -ENXIO;
snd_device_initialize(&card->ctl_dev, card);
dev_set_name(&card->ctl_dev, "controlC%d", card->number);
err = snd_device_new(card, SNDRV_DEV_CONTROL, card, &ops);
if (err < 0)
put_device(&card->ctl_dev);
return err;
}
snd_device_new中，会创建一个snd_device，并将其添加到card->devices中，并且snd_device_ops会被设置到dev->ops中。
当调用到dev->ops->dev_register时，会调用snd_ctl_dev_register，此时就回到开头的内容，在声卡中注册一个名为controlC*的设备节点，并且其file_operations会被设置为snd_ctl_f_ops。
static const struct file_operations snd_ctl_f_ops =
{
.owner = THIS_MODULE,
.read = snd_ctl_read,
.open = snd_ctl_open,
.release = snd_ctl_release,
.llseek = no_llseek,
.poll = snd_ctl_poll,
.unlocked_ioctl = snd_ctl_ioctl,
.compat_ioctl = snd_ctl_ioctl_compat,
.fasync = snd_ctl_fasync,
};
/*
* registration of the control device
*/
static int snd_ctl_dev_register(struct snd_device *device)
{
struct snd_card *card = device->device_data;
return snd_register_device(SNDRV_DEVICE_TYPE_CONTROL, card, -1,
&snd_ctl_f_ops, card, &card->ctl_dev);
}
录音节点： sound/soc/soc-core.c
snd_soc_instantiate_card
snd_card_new
soc_probe_link_dais
snd_pcm_new
_snd_pcm_new
sound/core/pcm.c
static int _snd_pcm_new(struct snd_card *card, const char *id, int device,
int playback_count, int capture_count, bool internal,
struct snd_pcm **rpcm)
{
struct snd_pcm *pcm;
int err;
static struct snd_device_ops ops = {
.dev_free = snd_pcm_dev_free,
.dev_register = snd_pcm_dev_register,
.dev_disconnect = snd_pcm_dev_disconnect,
};
static struct snd_device_ops internal_ops = {
.dev_free = snd_pcm_dev_free,
};
if (snd_BUG_ON(!card))
return -ENXIO;
if (rpcm)
*rpcm = NULL;
pcm = kzalloc(sizeof(*pcm), GFP_KERNEL);
if (!pcm)
return -ENOMEM;
pcm->card = card;
pcm->device = device;
pcm->internal = internal;
mutex_init(&pcm->open_mutex);
init_waitqueue_head(&pcm->open_wait);
INIT_LIST_HEAD(&pcm->list);
if (id)
strlcpy(pcm->id, id, sizeof(pcm->id));
err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_PLAYBACK,
playback_count);
if (err < 0)
goto free_pcm;
err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_CAPTURE, capture_count);
if (err < 0)
goto free_pcm;
err = snd_device_new(card, SNDRV_DEV_PCM, pcm,
internal ? &internal_ops : &ops);
if (err < 0)
goto free_pcm;
if (rpcm)
*rpcm = pcm;
return 0;
free_pcm:
snd_pcm_free(pcm);
return err;
}
/**
* snd_pcm_new_stream - create a new PCM stream
* @pcm: the pcm instance
* @stream: the stream direction, SNDRV_PCM_STREAM_XXX
* @substream_count: the number of substreams
*
* Creates a new stream for the pcm.
* The corresponding stream on the pcm must have been empty before
* calling this, i.e. zero must be given to the argument of
* snd_pcm_new().
*
* Return: Zero if successful, or a negative error code on failure.
*/
int snd_pcm_new_stream(struct snd_pcm *pcm, int stream, int substream_count)
{
int idx, err;
struct snd_pcm_str *pstr = &pcm->streams[stream];
struct snd_pcm_substream *substream, *prev;
#if IS_ENABLED(CONFIG_SND_PCM_OSS)
mutex_init(&pstr->oss.setup_mutex);
#endif
pstr->stream = stream;
pstr->pcm = pcm;
pstr->substream_count = substream_count;
if (!substream_count)
return 0;
snd_device_initialize(&pstr->dev, pcm->card);
pstr->dev.groups = pcm_dev_attr_groups;
dev_set_name(&pstr->dev, "pcmC%iD%i%c", pcm->card->number, pcm->device,
stream == SNDRV_PCM_STREAM_PLAYBACK ? 'p' : 'c');
if (!pcm->internal) {
err = snd_pcm_stream_proc_init(pstr);
if (err < 0) {
pcm_err(pcm, "Error in snd_pcm_stream_proc_init\n");
return err;
}
}
prev = NULL;
for (idx = 0, prev = NULL; idx < substream_count; idx++) {
substream = kzalloc(sizeof(*substream), GFP_KERNEL);
if (!substream)
return -ENOMEM;
substream->pcm = pcm;
substream->pstr = pstr;
substream->number = idx;
substream->stream = stream;
sprintf(substream->name, "subdevice #%i", idx);
substream->buffer_bytes_max = UINT_MAX;
if (prev == NULL)
pstr->substream = substream;
else
prev->next = substream;
if (!pcm->internal) {
err = snd_pcm_substream_proc_init(substream);
if (err < 0) {
pcm_err(pcm,
"Error in snd_pcm_stream_proc_init\n");
if (prev == NULL)
pstr->substream = NULL;
else
prev->next = NULL;
kfree(substream);
return err;
}
}
substream->group = &substream->self_group;
spin_lock_init(&substream->self_group.lock);
mutex_init(&substream->self_group.mutex);
INIT_LIST_HEAD(&substream->self_group.substreams);
list_add_tail(&substream->link_list, &substream->self_group.substreams);
atomic_set(&substream->mmap_count, 0);
prev = substream;
}
return 0;
}
const struct file_operations snd_pcm_f_ops[2] = {
{
.owner = THIS_MODULE,
.write = snd_pcm_write,
.write_iter = snd_pcm_writev,
.open = snd_pcm_playback_open,
.release = snd_pcm_release,
.llseek = no_llseek,
.poll = snd_pcm_poll,
.unlocked_ioctl = snd_pcm_ioctl,
.compat_ioctl = snd_pcm_ioctl_compat,
.mmap = snd_pcm_mmap,
.fasync = snd_pcm_fasync,
.get_unmapped_area = snd_pcm_get_unmapped_area,
},
{
.owner = THIS_MODULE,
.read = snd_pcm_read,
.read_iter = snd_pcm_readv,
.open = snd_pcm_capture_open,
.release = snd_pcm_release,
.llseek = no_llseek,
.poll = snd_pcm_poll,
.unlocked_ioctl = snd_pcm_ioctl,
.compat_ioctl = snd_pcm_ioctl_compat,
.mmap = snd_pcm_mmap,
.fasync = snd_pcm_fasync,
.get_unmapped_area = snd_pcm_get_unmapped_area,
}
};
static int snd_pcm_dev_register(struct snd_device *device)
{
int cidx, err;
struct snd_pcm_substream *substream;
struct snd_pcm *pcm;
if (snd_BUG_ON(!device || !device->device_data))
return -ENXIO;
pcm = device->device_data;
mutex_lock(®ister_mutex);
err = snd_pcm_add(pcm);
if (err)
goto unlock;
for (cidx = 0; cidx < 2; cidx++) {
int devtype = -1;
if (pcm->streams[cidx].substream == NULL)
continue;
switch (cidx) {
case SNDRV_PCM_STREAM_PLAYBACK:
devtype = SNDRV_DEVICE_TYPE_PCM_PLAYBACK;
break;
case SNDRV_PCM_STREAM_CAPTURE:
devtype = SNDRV_DEVICE_TYPE_PCM_CAPTURE;
break;
}
/* register pcm */
err = snd_register_device(devtype, pcm->card, pcm->device,
&snd_pcm_f_ops[cidx], pcm,
&pcm->streams[cidx].dev);
if (err < 0) {
list_del_init(&pcm->list);
goto unlock;
}
for (substream = pcm->streams[cidx].substream; substream; substream = substream->next)
snd_pcm_timer_init(substream);
}
pcm_call_notify(pcm, n_register);
unlock:
mutex_unlock(®ister_mutex);
return err;
}
ASOC ASoc 包括什么？
Platform drivers，提供了配置/使能 SoC audio interface (或称 CPU DAI) 的能力; Codec drivers，提供了配置/使能 Codec 的能力; Machine drivers，描述了应该如何控制 CPU DAI 和 Codec，使他们互相配合在一起工作;
Linux ALSA 音频系统架构大致如下：
+--------+ +--------+ +--------+
|tinyplay| |tinycap | |tinymix |
+--------+ +--------+ +--------+
| ^ ^
V | V
+--------------------------------+
| ALSA Library API |
| (tinyalsa, alsa-lib) |
+--------------------------------+
user space ^
-------------------------------|---------------------
kernel space V
+--------------------------------+
| ALSA CORE |
| +-------+ +-------+ +------+ |
| | PCM | |CONTROL| | MIDI |...|
| +-------+ +-------+ +------+ |
+--------------------------------+
|
+--------------------------------+
| ASoC CORE |
+--------------------------------+
|
+--------------------------------+
| hardware driver |
| +-------+ +--------+ +-----+ |
| |Machine| |Platform| |Codec| |
| +-------+ +--------+ +-----+ |
+--------------------------------+
————————————————
一个声卡有多个kcontrol
一个kcontrol对应一个功能：比如：音量，开/关录音
kcontrol中有函数来设置功能。
