第4篇 sound.c分析（应用层如何调用到内核层的）

原文链接：https://blog.csdn.net/yangguoyu8023/article/details/122114620

1. alsa_sound_init

alsa的核心入口函数是sound\core\sound.c中的alsa_sound_init

static struct snd_minor *snd_minors[SNDRV_OS_MINORS];
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;
	mptr = snd_minors[minor];
    .......
	new_fops = fops_get(mptr->f_ops);
    .....
	if (file->f_op->open)
		err = file->f_op->open(inode, file);
}

static const struct file_operations snd_fops =
{
	.open =		snd_open,
	.llseek =	noop_llseek,
};

static int major = CONFIG_SND_MAJOR;
static int __init alsa_sound_init(void)
{
	snd_major = major;
	snd_ecards_limit = cards_limit;
	if (register_chrdev(major, "alsa", &snd_fops)) 
}

subsys_initcall(alsa_sound_init);

1.1 snd_minors

最终会通过次设备号来匹配snd_minors，并且调用它的open函数。我们看看全局变量snd_minors在哪注册的，它是静态变量，所以在sound.c文件中搜索，发现其注册为snd_register_device。

 
/**
 * snd_register_device - Register the ALSA device file for the card
 */
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;
	struct snd_minor *preg;

	preg = kmalloc(sizeof *preg, GFP_KERNEL);
	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;
	minor = snd_find_free_minor(type, card, dev);

	device->devt = MKDEV(major, minor);
	err = device_add(device);
	snd_minors[minor] = preg;
}

1.2 snd_register_device

搜索snd_register_device，看谁调用了该函数，发现有如下函数调用了。
snd_pcm_dev_register
snd_ctl_dev_register