ALSA PCM中间层,非常强大，驱动只需要实现底层的函数以访问硬件

浅追一下录放音流程

kernel/sound/core/pcm_native.c 

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_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_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,

        }

};

其中snd_pcm_f_ops[0]是播放，snd_pcm_f_ops[1]是录音

播放： .open-->snd_pcm_playback_open-->snd_pcm_open(file, pcm, SNDRV_PCM_STREAM_PLAYBACK)

录音： .open-->snd_pcm_capture_open-->snd_pcm_open(file, pcm, SNDRV_PCM_STREAM_CAPTURE)

snd_pcm_open的原型static int snd_pcm_open(struct file *file, struct snd_pcm *pcm, int stream)可以看到就是通过传入的int stream来区分录放音的，也就是说录放音的具体处理都是从此处开始

snd_card_file_add(pcm->card, file) //这个函数将打开的文件保存，放入snd_card的files_list链表。作用是用于跟踪连接状态，避免热插拔释放繁忙资源。

snd_pcm_open_file(file, pcm, stream) //将操作该声卡card的应用程序添加到card->files_list

snd_pcm_open_file-->snd_pcm_open_substream-->snd_pcm_attach_substream //获取snd_pcm_substream数据

snd_pcm_open_file-->snd_pcm_attach_substream-->pstr = &pcm->streams[stream]    //得到播放/录音的snd_pcm_str

  snd_pcm_open_file-->snd_pcm_open_substream-->snd_pcm_hw_constraints_init(substream); //初始化substream结构体

  snd_pcm_open_file-->snd_pcm_open_substream-->snd_pcm_hw_constraints_complete(substream);

  //重新设置SNDRV_PCM_HW_PARAM_ACCESS，SNDRV_PCM_HW_PARAM_FORMAT，等参数、最小/大值区间，设置到runtime.hw_constraints.rules里面去