alsa声卡dev_snd_pcmC0D0p的open打开流程

将获得

lib

库中

_s nd_pcm_empty_open

函数

所以

open_func

将等于

_snd_pcm_empty_open

_snd_pcm_empty_open

_snd_pcm_asym_open

_snd_pcm_plug_open

_snd_pcm_softvol_open

_snd_pcm_dmix_open

_snd_pcm_hw_open

==> snd_pcm_hw_open(pcmp, name, card, device, subdevice, stream,

mode | (nonblock ? SND_PCM_NONBLOCK : 0),

0, sync_ptr_ioctl);

==> snd_ctl_hw_open

filename

等于

==> snd_open_device(filename, fmode);

ctl->ops = &snd_ctl_hw_ops;

ctl->private_data = hw;

ctl->poll_fd = fd;

*handle = ctl;

filename

==> fd = snd_open_device(filename, fmode);

==> return snd_pcm_hw_open_fd(pcmp, name, fd, 0, sync_ptr_ioctl);

==> snd_pcm_new(&pcm, SND_PCM_TYPE_HW, name, , mode);

pcm->ops = &snd_pcm_hw_ops;

pcm->fast_ops = &snd_pcm_hw_fast_ops;

static int snd_pcm_hw_mmap_control(snd_pcm_t *pcm)

{

snd_pcm_hw_t *hw = pcm->private_data;

void *ptr;

int err;

if (hw->sync_ptr == NULL) { //

如果还没有

mmap,

那么执行

mmap

映射内核空间驱动使用的声音缓冲区

ptr = mmap(NULL, page_align(sizeof(struct sndrv_pcm_mmap_control)),

PROT_READ|PROT_WRITE, MAP_FILE|MAP_SHARED,

hw->fd, SNDRV_PCM_MMAP_OFFSET_CONTROL);

if (ptr == MAP_FAILED || ptr == NULL) {

err = -errno;

SYSMSG(

return err;

}

hw->mmap_control = ptr; //

声卡驱动头部填充了一个结构体

sndrv_pcm_m map_control,

类似

qvfb

显

示原理

.

// struct sndrv_pcm_mmap_control {

//

sndrv_pcm_uframes_t appl_ptr;

/* RW: appl ptr (0...boundary-1) */

//

sndrv_pcm_uframes_t avail_min;

/* RW: min available frames for wakeup */

// };

} else {

hw->mmap_control->avail_min = 1;

}

snd_pcm_set_appl_ptr(pcm, &hw->mmap_control->appl_ptr, hw->fd,

SNDRV_PCM_MMAP_OFFSET_CONTROL);

return 0;

}

snd_pcm_mmap

switch (i->type) {

case SND_PCM_AREA_MMAP: //

表 示为数据区分配驱动内存

,

在

snd_ pcm_hw_channel_info

中设置

了

ptr = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_FILE|MAP_SHARED, i->,

i->);

/*

mmap

==> snd_pcm_mmap_data

==> snd_pcm_default_mmap

// mmap the DMA buffer on RAM

static int snd_pcm_default_mmap(struct snd_pcm_substream *substream,

struct vm_area_struct *area)

{

area->vm_ops = &snd_pcm_vm_ops_data; // vma

操作函数

,

当应用程序向该

area

读写不存在的内存数据

时

,

area->vm_private_data = substream;

//

将执行

snd_pcm_vm_ ops_data

中的

fault

//

函数

snd_pcm_mmap _data_fault

进一步以页为单位申请内存空间

,

所以如果用户程序需要

64k,

那么将

执行

16

次

,

每次申请

4k

空间

[ttp].

area->vm_flags |= VM_RESERVED;

atomic_inc(&substream->mmap_count);

return 0;

}

*/

if (ptr == MAP_FAILED) {

SYSERR(

return -errno;

}

i->addr = ptr;

==> snd_pcm_mmap_control

static int snd_pcm_mmap_control(struct snd_pcm_substream *substream, struct file *file,

struct vm_area_struct *area)

{

struct snd_pcm_runtime *runtime;

long size;

if (!(area->vm_flags & VM_READ))

return -EINVAL;

runtime = substream->runtime;

size = area->vm_end - area->vm_start;

if (size != PAGE_ALIGN(sizeof(struct snd_pcm_mmap_control)))

return -EINVAL;

area->vm_ops = &snd_pcm_vm_ops_control; //

当对

( area->vm_start,area-> vm_end)

之间空间操作

,

发

生

area->vm_private_data = substream;

//

缺页时

,

内核将调用该

vm_ops

方法来处理

fault

异常

,

area->vm_flags |= VM_RESERVED;

//

进而执行

snd_pcm_mm ap_control_fault

申请

1

个

page

空

间

< br>

return 0;

}

==> writei_func = snd_pcm_writei;

==> playback(argv[optind++]);

==> playback_go(fd, dtawave, pbrec_count, FORMAT_WAVE, name);

==> pcm_write(audiobuf, l);

==> writei_func(handle, data, count);

就是调用上面的

snd_pcm_writei

==> snd_pcm_writei

==> _snd_pcm_writei

==> pcm->fast_ops->writei(pcm->fast_op_arg, buffer, size);

==> snd_pcm_plugin_writei

==> snd_pcm_write_areas(pcm, areas, 0, size,

snd_pcm_plugin_write_areas);

==> avail = snd_pcm_avail_update(pcm); //

获取可用缓冲区位置偏移索引值

==> func()

就是

snd_p cm_plugin_write_areas

函数发送

102 4

帧音频数据

,

一帧对应一次完整采样

,

比如

stereo

< br>立体声

,24bits

量化< /p>

,

那么这里一帧对应

3*2

字节数据

,

即一次完整采样所需空间

[ttp].

==> plugin->write(pcm, areas, offset, frames,

slave_areas, slave_offset, &slave_frames);

即调用

snd_pcm_l inear_write_areas

函数将

areas

中的

frames

频数据拷贝到

slave_areas

内存区

==> pcm->fast_ops->mmap_commit(pcm->fast_op_arg, offset, frames);

==> snd_pcm_dmix_mmap_commit

==> snd_pcm_dmix_sync_area

/*

*

synchronize shm ring buffer with hardware

*/

static void snd_pcm_dmix_sync_area(snd_pcm_t *pcm)

==> /* add sample areas here */

src_areas = snd_pcm_mmap_areas(pcm);

dst_areas = snd_pcm_mmap_areas(dmix->spcm); //

添加

==> mix_areas(dmix, src_areas, dst_areas, appl_ptr, slave_appl_ptr, transfer);

if (dmix->interleaved) { //

可以将缓冲中的音频数据填充到硬件中

[ttp]

/*

* process all areas in one loop

* it optimizes the memory accesses for this case

*/

do_mix_areas(size * channels,

(unsigned char *)dst_areas[0].addr + sample_size * dst_ofs * channels,

(unsigned char *)src_areas[0].addr + sample_size * src_ofs * channels,

dmix->_buffer + dst_ofs * channels,

sample_size,

sample_size,

sizeof(signed int));

return;

}

==> do_mix_areas(size * channels,

(unsigned char *)dst_areas[0].addr + sample_size * dst_ofs * channels,

(unsigned char *)src_areas[0].addr + sample_size * src_ofs * channels,

dmix->_buffer + dst_ofs * channels,

sample_size,

sample_size,

sizeof(signed int));

这里的

do_mix_areas

在

i386

中

,

使用下 面完全用汇编实现的拷贝函数

MIX_AREAS_32

完成数 据从

src

到

dst

< br>的快速拷贝

,

每拷贝一次

,< /p>

声卡就会发出一点声音

[ttp]

/*

*

for plain i386, 32-bit version (24-bit resolution)

*/

static void MIX_AREAS_32(unsigned int size,

volatile signed int *dst, signed int *src,

volatile signed int *sum, size_t dst_step,

size_t src_step, size_t sum_step)

_snd_pcm_asym_open

_snd_pcm_dmix_open

snd_pcm_plugin_avail_update

==> snd_pcm_avail_update(slave);

==> pcm->fast_ops->avail_update(pcm->fast_op_arg);

==> snd_pcm_dmix_avail_update

==> snd_pcm_mmap_playback_avail(pcm);

alsa_sound_init

#define CONFIG_SND_MAJOR

116

/* standard configuration */

static int major = CONFIG_SND_MAJOR;

module_init(alsa_sound_init)

alsa_sound_init

==> register_chrdev(major,

//

主设 备号为

116

的所有设备都为

alsa

设备

,

节点

方 法集为

snd_fops

static const struct file_operations snd_fops =

// alsa

的设备名为

pcmC0 D1c

或

pcmC0D1p

等