Android Audio系统分析（三）写驱动

这里直接从hardware层开始分析

hardware/rockchip/audio/legacy_hal/AudioHardware.cpp

1 2	ssize_t AudioHardware::AudioStreamOutALSA::write(const void* buffer, size_t bytes) ret = pcm_write(mHardware->getPcm(),(void*) p, bytes);

hardware/rockchip/audio/legacy_hal/alsa_pcm.c

1 2	int pcm_write(struct pcm pcm, void data, unsigned count) if (ioctl(pcm->fd, SNDRV_PCM_IOCTL_WRITEI_FRAMES, &x)) {

#define SNDRV_PCM_IOCTL_WRITEI_FRAMES _IOW('A', 0x50, struct snd_xferi)

这里可以看到，Audio通过了ioctl控制进入驱动层写数据

那么再找找pcm->fd是哪个节点

hardware/rockchip/audio/legacy_hal/alsa_pcm.c

struct pcm *pcm_open(unsigned flags) {
    const char *dfmt = "/dev/snd/pcmC%uD%u%c";
    sprintf(dname, dfmt, card, device, flags & PCM_IN ? 'c' : 'p');
    pcm->fd = open(dname, O_RDWR|O_CLOEXEC);
    // ...
}

上面我打印，得到的路经是 /dev/snd/pcmC0D0p，路经位置可以进入adb shell来验证
ioctl用法参考：http://blog.sina.com.cn/s/blog_ba08e8e00101bw4e.html
http://www.cnblogs.com/geneil/archive/2011/12/04/2275372.html

接下来就是看驱动层是怎么通过ioctl来实现播放声音的

1 2	# cd kernel/sound # grep pcmC -nr .

kernel/sound/core/pcm.c，但是发现不是在里，后面我是通过宏来定位地方的，整体目录还是在这core里的

include/uapi/sound/asound.h
#define SNDRV_PCM_IOCTL_WRITEI_FRAMES _IOW('A', 0x50, struct snd_xferi)

kernel/sound/core/pcm_native.c

static int snd_pcm_playback_ioctl1(struct file *file,
                   struct snd_pcm_substream *substream,
                   unsigned int cmd, void __user *arg)
{
    switch (cmd) {
    case SNDRV_PCM_IOCTL_WRITEI_FRAMES:
        // 具体不研究了，估计就这样映入codec内存寄存器了
        result = snd_pcm_lib_write(substream, xferi.buf, xferi.frames);
        __put_user(result, &_xferi->result);

总结，通过hw层写byte数据，然后通过ioctl定位至驱动层，写入相应寄存器发声，大致一对一的关系