ALSA的ioctl - hwdep

hwdep模块简述


字符设备驱动中,ioctl是一个很常见的IO设备操作函数,可以自定义cmd命令字并实现对应的设备IO控制。

音频设备的控制有所不同:驱动层大部分控制操作定义各种snd_kcontrol_new,然后注册到SNDRV_DEV_CONTROL模块中(sound\core\control.c),详见snd_kcontrol探究;而上层调用alsa-lib的snd_ctl_open/snd_mixer_open来打开底层的SNDRV_DEV_CONTROL模块,详见DAPM之二:audio paths与dapm kcontrol。这方法常见于mixer-control,如音量调整、部件开关、通路连接等等。

除此之外,alsa还是可以实现类似于ioctl的函数的,只不过它封装成一个设备模块SNDRV_DEV_HWDEP,代码sound\core\ hwdep.c。该模块实现了read/write/ioctl/llseek/poll/mmap等接口。hwdep是Hardware Dependant Interface的简称。


题外话:如果想看自己板上的alsa有什么类型的设备可以cat /proc/asound/devices,如

~ # cat /proc/asound/devices 
  0: [ 0]   : control
  4: [ 0- 0]: hardware dependent
 16: [ 0- 0]: digital audio playback
 24: [ 0- 0]: digital audio capture
 33:        : timer
设备节点号minor=0是control,=4是hwdep,=16是pcm-playback,=24是pcm-capture,=33是timer。


如下简单分析ioctl:

//套接字接口函数集
static const struct file_operations snd_hwdep_f_ops =
{
	.owner = 	THIS_MODULE,
	.llseek =	snd_hwdep_llseek,
	.read = 	snd_hwdep_read,
	.write =	snd_hwdep_write,
	.open =		snd_hwdep_open,
	.release =	snd_hwdep_release,
	.poll =		snd_hwdep_poll,
	.unlocked_ioctl =	snd_hwdep_ioctl,
	.compat_ioctl =	snd_hwdep_ioctl_compat,
	.mmap =		snd_hwdep_mmap,
};

static long snd_hwdep_ioctl(struct file * file, unsigned int cmd,
			    unsigned long arg)
{
	struct snd_hwdep *hw = file->private_data;
	void __user *argp = (void __user *)arg;
	switch (cmd) {
	case SNDRV_HWDEP_IOCTL_PVERSION:
		return put_user(SNDRV_HWDEP_VERSION, (int __user *)argp);
	case SNDRV_HWDEP_IOCTL_INFO:
		return snd_hwdep_info(hw, argp);
	case SNDRV_HWDEP_IOCTL_DSP_STATUS:
		return snd_hwdep_dsp_status(hw, argp);
	case SNDRV_HWDEP_IOCTL_DSP_LOAD:
		return snd_hwdep_dsp_load(hw, argp);
	}
	if (hw->ops.ioctl)
		return hw->ops.ioctl(hw, file, cmd, arg);
	return -ENOTTY;
}

从snd_hwdep_ioctl可以看出,系统默认只有4个cmd,功能主要是download dsp image。从return hw->ops.ioctl(hw, file, cmd, arg)语句可以看出,我们可自定义cmd和ioctl函数。


实现自定义的hwdep操作函数


1、 首先实现需要的操作函数:

static int my_hwdep_open(struct snd_hwdep * hw, struct file *file)
{
	printk(KERN_INFO "my_hwdep_open\n");
	return 0;
}

static int my_hwdep_ioctl(struct snd_hwdep * hw, struct file *file, unsigned int cmd, unsigned long arg)
{
#define MY_SOC_IOCTL_SET_CALL_PATH   _IOWR('H', 0x10, int)

	switch (cmd) {
	case MY_SOC_IOCTL_SET_CALL_PATH:
		//设置电话语音通路
		return 0;
		break;
	//......
	}
	
	err("Not supported ioctl for MY-HWDEP");
	return -ENOIOCTLCMD;
}

2、 注册操作函数到hwdep模块:
struct snd_hwdep *hwdep;

if (snd_hwdep_new(codec->card, "MY-HWDEP", 0, &hwdep) < 0) {
	printk(KERN_ERR "create MY-HWDEP fail");
	return;
}

sprintf(hwdep->name, "MY-HWDEP %d", 0);

hwdep->iface = SNDRV_HWDEP_IFACE_WMT;
hwdep->ops.open = wmt_hwdep_open;
hwdep->ops.ioctl = wmt_hwdep_ioctl;
这里摘录snd_hwdep_new的代码注释,让大家更明白上面的注册过程:

/**
 * snd_hwdep_new - create a new hwdep instance
 * @card: the card instance
 * @id: the id string
 * @device: the device index (zero-based)
 * @rhwdep: the pointer to store the new hwdep instance
 *
 * Creates a new hwdep instance with the given index on the card.
 * The callbacks (hwdep->ops) must be set on the returned instance
 * after this call manually by the caller.
 *
 * Returns zero if successful, or a negative error code on failure.
 */
按照以上实现hwdep ioctl后,上层可以通过alsa-lib的相关接口来调用。


上层调用范例


#include <fcntl.h>
#include <sys/ioctl.h>
#include <alsa/hwdep.h>
#include <alsa/error.h>
#include <stdio.h>

#define MY_SOC_IOCTL_SET_CALL_PATH   _IOWR('H', 0x10, int)

int main()
{
	const char *devicename = "hw:0,0";
	snd_hwdep_t *hwdep;
	int err;
	int enable = 1;
	
	if ((err = snd_hwdep_open(&hwdep, devicename, O_RDWR)) < 0) {
		printf("hwdep interface open error: %s \n", snd_strerror(err));
		return -1;
	}
	
	if ((err = snd_hwdep_ioctl(hwdep, MY_SOC_IOCTL_SET_CALL_PATH, &enable)) < 0) {
		printf("hwdep ioctl error: %s \n", snd_strerror(err));
	}
	
	snd_hwdep_close(hwdep);
	
	return 0;
}


总结


可以看出hwdep的本意主要是用于download dsp image,但通过它也可实现类似于其他字符设备的ioctl。我说过音频大多控制是通过snd_kcontrol,但有些功能如果使用这种方式会比较繁琐且模块太过耦合。

举个例子:电话语音通路,它不同于音乐回放通路,通话时才需要打开。如果用snd_kcontrol,则上层需要调用多个control.set,并且更换CODEC芯片的话,上层也要跟着修改control name;如果使用hwdep ioctl的话,就没有这个问题,只需要保证命令字cmd一致,底层如何管理通话通路的一系列部件开关,上层都不需要关心。

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