Linux内核4.14版本——alsa框架分析(3)-PCM设备的创建

目录

​1. pcm设备的创建

1.1 snd_pcm_new

1.2 _snd_pcm_new

1.3 snd_pcm_dev_register

---

      ALSA已经为我们实现了功能强劲的PCM中间层，自己的驱动中只要实现一些底层的需要访问硬件的函数即可。

      要访问PCM的中间层代码，你首先要包含头文件，另外，如果需要访问一些与 hw_param相关的函数，可能也要包含。

      每个声卡最多可以包含4个pcm的实例，每个pcm实例对应一个pcm设备文件。pcm实例数量的这种限制源于linux设备号所占用的位大小，如果以后使用64位的设备号，我们将可以创建更多的pcm实例。不过大多数情况下，在嵌入式设备中，一个pcm实例已经足够了。

      一个pcm实例由一个playback stream和一个capture stream组成，这两个stream又分别有一个或多个substreams组成。

      下面一张图列出了pcm中间层几个重要的结构，他可以让我们从uml的角度看一看这列结构的关系，理清他们之间的关系，对我们理解pcm中间层的实现方式。

1. pcm设备的创建

snd_pcm_new->_snd_pcm_new->snd_pcm_dev_register->snd_register_device

 

1.1 snd_pcm_new

/**
 * 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
 *
 * Creates a new PCM instance.
 *
 * The pcm operators have to be set afterwards to the new instance
 * via snd_pcm_set_ops().
 *
 * Return: Zero if successful, or a negative error code on failure.
 */
int snd_pcm_new(struct snd_card *card, const char *id, int device,
		int playback_count, int capture_count, struct snd_pcm **rpcm)
{
	return _snd_pcm_new(card, id, device, playback_count, capture_count,
			false, rpcm);
}

1.2 _snd_pcm_new

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,
	};

........
	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, &ops);
	if (err < 0)
		goto free_pcm;

	if (rpcm)
		*rpcm = pcm;
	return 0;

free_pcm:
	snd_pcm_free(pcm);
	return err;
}

先创建playback和capture两个substream，最终创建pcm设备，ops最终为pcm的ops。

1.3 snd_pcm_dev_register

static int snd_pcm_dev_register(struct snd_device *device)
{
..............
	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_call_notify(pcm, n_register);

 unlock:
	mutex_unlock(®ister_mutex);
	return err;
}

这里注意snd_pcm_f_ops结构体，如下所示。sound\core\pcm_native.c

/*
 *  Register section
 */

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_playback_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_capture_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,
	}
};

该操作函数集被注册在snd_minors结构体的操作函数集中，所以上层应用最终的open,compat_ioctl等函数，最终都会调用到这里。