hwdep模塊簡述
字符設備驅動中，ioctl是一個很常見的IO設備操作函數，可以自定義cmd命令字並實現對應的設備IO控制。
音頻設備的控制有所不同：驅動層大部分控制操作定義各種snd_kcontrol_new，然後註冊到SNDRV_DEV_CONTROL模塊中（sound\core\control.c），詳見snd_kcontrol探究；而上層調用alsa-lib的snd_ctl_open/snd_mixer_open來打開底層的SNDRV_DEV_CONTROL模塊，詳見DAPM之二：audio paths與dapm kcontrol。這方法常見於mixer-control，如音量調整、部件開關、通路連接等等。
除此之外，alsa還是可以實現類似於ioctl的函數的，只不過它封裝成一個設備模塊SNDRV_DEV_HWDEP，代碼sound\core\ hwdep.c。該模塊實現了read/write/ioctl/llseek/poll/mmap等接口。hwdep是Hardware Dependant Interface的簡稱。
題外話：如果想看自己板上的alsa有什麼類型的設備可以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如下簡單分析ioctl：
//套接字接口函數集
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;
}從snd_hwdep_ioctl可以看出，系統默認只有4個cmd，功能主要是download dsp image。從return hw->ops.ioctl(hw, file, cmd, arg)語句可以看出，我們可自定義cmd和ioctl函數。
實現自定義的hwdep操作函數
1、 首先實現需要的操作函數：
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:
//設置電話語音通路
return 0;
break;
//......
}
err("Not supported ioctl for MY-HWDEP");
return -ENOIOCTLCMD;
}2、 註冊操作函數到hwdep模塊：
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;/**
* 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.
*/上層調用範例
#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;
}總結
可以看出hwdep的本意主要是用於download dsp image，但通過它也可實現類似於其他字符設備的ioctl。我說過音頻大多控制是通過snd_kcontrol，但有些功能如果使用這種方式會比較繁瑣且模塊太過耦合。
舉個例子：電話語音通路，它不同於音樂回放通路，通話時才需要打開。如果用snd_kcontrol，則上層需要調用多個control.set，並且更換CODEC芯片的話，上層也要跟着修改control name；如果使用hwdep ioctl的話，就沒有這個問題，只需要保證命令字cmd一致，底層如何管理通話通路的一系列部件開關，上層都不需要關心。
