在介绍PCM 之前，我们先给出创建PCM实例的框架。
#include <sound/pcm.h>
....
/* hardware definition */
static struct snd_pcm_hardware snd_mychip_playback_hw = {
.info = (SNDRV_PCM_INFO_MMAP |
SNDRV_PCM_INFO_INTERLEAVED |
SNDRV_PCM_INFO_BLOCK_TRANSFER |
SNDRV_PCM_INFO_MMAP_VALID),
.formats = SNDRV_PCM_FMTBIT_S16_LE,
.rates = SNDRV_PCM_RATE_8000_48000,
.rate_min = 8000,
.rate_max = 48000,
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = 32768,
.period_bytes_min = 4096,
.period_bytes_max = 32768,
.periods_min = 1,
.periods_max = 1024,
};
/* hardware definition */
static struct snd_pcm_hardware snd_mychip_capture_hw = {
.info = (SNDRV_PCM_INFO_MMAP |
SNDRV_PCM_INFO_INTERLEAVED |
SNDRV_PCM_INFO_BLOCK_TRANSFER |
SNDRV_PCM_INFO_MMAP_VALID),
.formats = SNDRV_PCM_FMTBIT_S16_LE,
.rates = SNDRV_PCM_RATE_8000_48000,
.rate_min = 8000,
.rate_max = 48000,
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = 32768,
.period_bytes_min = 4096,
.period_bytes_max = 32768,
.periods_min = 1,
.periods_max = 1024,
};
/* open callback */
static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
struct snd_pcm_runtime *runtime = substream->runtime;
runtime->hw = snd_mychip_playback_hw;
/* more hardware-initialization will be done here */
....
return 0;
}
/* close callback */
static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
/* the hardware-specific codes will be here */
....
return 0;
}
/* open callback */
static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
struct snd_pcm_runtime *runtime = substream->runtime;
runtime->hw = snd_mychip_capture_hw;
/* more hardware-initialization will be done here */
....
return 0;
}
/* close callback */
static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
/* the hardware-specific codes will be here */
....
return 0;
}
/* hw_params callback */
static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
struct snd_pcm_hw_params *hw_params)
{
return snd_pcm_lib_malloc_pages(substream,
params_buffer_bytes(hw_params));
}
/* hw_free callback */
static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
{
return snd_pcm_lib_free_pages(substream);
}
/* prepare callback */
static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
struct snd_pcm_runtime *runtime = substream->runtime;
/* set up the hardware with the current configuration
* for example...
*/
mychip_set_sample_format(chip, runtime->format);
mychip_set_sample_rate(chip, runtime->rate);
mychip_set_channels(chip, runtime->channels);
mychip_set_dma_setup(chip, runtime->dma_addr,
chip->buffer_size,
chip->period_size);
return 0;
}
/* trigger callback */
static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
int cmd)
{
switch (cmd) {
case SNDRV_PCM_TRIGGER_START:
/* do something to start the PCM engine */
....
break;
case SNDRV_PCM_TRIGGER_STOP:
/* do something to stop the PCM engine */
....
break;
default:
return -EINVAL;
}
}
/* pointer callback */
static snd_pcm_uframes_t
snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
unsigned int current_ptr;
/* get the current hardware pointer */
current_ptr = mychip_get_hw_pointer(chip);
return current_ptr;
}
/* operators */
static struct snd_pcm_ops snd_mychip_playback_ops = {
.open = snd_mychip_playback_open,
.close = snd_mychip_playback_close,
.ioctl = snd_pcm_lib_ioctl,
.hw_params = snd_mychip_pcm_hw_params,
.hw_free = snd_mychip_pcm_hw_free,
.prepare = snd_mychip_pcm_prepare,
.trigger = snd_mychip_pcm_trigger,
.pointer = snd_mychip_pcm_pointer,
};
/* operators */
static struct snd_pcm_ops snd_mychip_capture_ops = {
.open = snd_mychip_capture_open,
.close = snd_mychip_capture_close,
.ioctl = snd_pcm_lib_ioctl,
.hw_params = snd_mychip_pcm_hw_params,
.hw_free = snd_mychip_pcm_hw_free,
.prepare = snd_mychip_pcm_prepare,
.trigger = snd_mychip_pcm_trigger,
.pointer = snd_mychip_pcm_pointer,
};
/*
* definitions of capture are omitted here...
*/
/* create a pcm device */
static int snd_mychip_new_pcm(struct mychip *chip)
{
struct snd_pcm *pcm;
int err;
err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
if (err < 0)
return err;
pcm->private_data = chip;
strcpy(pcm->name, "My Chip");
chip->pcm = pcm;
/* set operators */
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
&snd_mychip_playback_ops);
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
&snd_mychip_capture_ops);
/* pre-allocation of buffers */
/* NOTE: this may fail */
snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
snd_dma_pci_data(chip->pci),
64*1024, 64*1024);
return 0;
}
1.创建pcm实例
ALSA driver为我们提供接口snd_pcm_new来创建PCM实例。但是我们最好是写一个如上述snd_mychip_new_pcm的函数来来对构建pcm实例的过程进行封装。
/**
* snd_pcm_new - create a new PCM instance
* @card: the card instance
* @id: the id string
* @device: the device index (zero based)
* @playback_count: the number of substreams for playback
* @capture_count: the number of substreams for capture
* @rpcm: the pointer to store the new pcm instance
*/
int snd_pcm_new(struct snd_card *card, const char *id, int device, int playback_count, int capture_count, struct snd_pcm **rpcm)
第三个参数表示新创建的PCM实例的index(0,1,2,3).可以在一个card上创建多个PCM 实例。每一个PCM又可以包含多个substream.如果芯片支持多路播放，那么将有多个substream.每次open/close都作用于某个substream.在创建PCM的substream时就指定了number（0~playback_count）.当App在调用alsa lib API:snd_pcm_open时，alsa core通过snd_pcm_attach_substream函数来open一个空闲的substream.
2.设置PCM的操作函数
创建完PCM函数后，就可设置PCM 的操作函数。
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
&snd_mychip_playback_ops);
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
&snd_mychip_capture_ops);
操作函数即我们写driver时需要实现的功能，以供alsa-core调用。ALSA PCM的操作函数包括：
static struct snd_pcm_ops snd_mychip_playback_ops = {
.open = snd_mychip_pcm_open,
.close = snd_mychip_pcm_close,
.ioctl = snd_pcm_lib_ioctl,
.hw_params = snd_mychip_pcm_hw_params,
.hw_free = snd_mychip_pcm_hw_free,
.prepare = snd_mychip_pcm_prepare,
.trigger = snd_mychip_pcm_trigger,
.pointer = snd_mychip_pcm_pointer,
};
每个函数都包含一个snd_pcm_substream 的指针，指向当前操作的substream.
在上面的例子中，每个操作函数里面都包含如下宏调用：
其中返回的是substream->private_data,sustream的private_data是pcm->private_data的一份拷贝。拷贝动作是在snd_pcm_open时调用的snd_pcm_attach_substream中进行。一般来说pcm的private_data是芯片专用数据，当然我们也可以overwrite以保存别的数据。
2.1 open
当open PCM的一路substream时，open函数就会被调用。
static int snd_xxx_open(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
struct snd_pcm_runtime *runtime = substream->runtime;
runtime->hw = snd_mychip_playback_hw;
return 0;
}
在open函数内，至少应该初始化此substream的runtime->hw结构体.snd_mychip_playback_hw是预先定义的硬件描述。
也可以在open函数里为substream分配private_data.如下：
data = kmalloc(sizeof(*data), GFP_KERNEL); substream->runtime->private_data = data;
如果芯片所支持的sample rate,samples等硬件配置有限制，也可以在open函数内设置限制。
2.2 close
当PCM的substream close时就会调用到close 函数。
如果有在open函数内分配了runtime的private_data, 那private data在close函数释放。
static int snd_xxx_close(struct snd_pcm_substream *substream)
{
....
kfree(substream->runtime->private_data);
....
}
2.4 hw_params
当App在设置substream的buffer size, the period size, the format等硬件参数时，将会调用到hw_params函数。
在hw_params函数中可以设置许多的硬件参数，包括buffer的分配。buffer分配：
snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
2.5 hw_free
hw_free用来释放在hw_params中释放的资源，如buffer.hw_free总是在close之前调用。
2.6 prepare
当app在调用alsa lib API：snd_pcm_prepare时，prepare函数将被调用，在此函数中可以设置format type, sample rate等参数。与在hw_params中设置参数不同的是每次app调用snd_pcm_prepare时都会去设置参数，而snd_pcm_prepare可能是在recovery undrrun时调用。
prepare函数并非原子操作，因此必须使用 schedule-related functions保证安全性。
2.7 trigger
当PCM在start，stop，pause时，会调用到trigger函数。
switch (cmd) {
case SNDRV_PCM_TRIGGER_START:
/* do something to start the PCM engine */
break;
case SNDRV_PCM_TRIGGER_STOP:
/* do something to stop the PCM engine */
break;
default:
return -EINVAL;
}
SNDRV_PCM_TRIGGER_XXX 定义在 <sound/pcm.h>. 至少我们应该在trigger函数中实现 START 和STOP commands。
当pcm支持pause操作时，必须实现 PAUSE_PUSH and PAUSE_RELEASE commands，PAUSE_PUSH用来pause pcm，PAUSE_RELEASE用来restart pcm.
trigger函数是atomic 的，因此在其中的操作越少越好，通常只用来trigger DMA.
2.8 pointer
当PCM middleware 层（alsa-core）需要获取当前的硬件指针(hardware position）时，就会调用pointer函数。pointer函数需要返回以frame为单位的hardware position(0~buffersize-1).
pointer通常在buffer-update 过程中调用，由中断函数中的snd_pcm_period_elapsed触发。即每次硬件中断，就会调用snd_pcm_period_elapsed函数来通知alsa-core来读取当前的hardware position,计算buffer中空余空间，唤醒sleep的polling thread.