alsa驱动问题

在alsa驱动源码pcm部分,有一个对pcm操作的函数指针列表,其中的内容如下:

struct file_operations snd_pcm_f_ops[2] = {
 {
  .owner =  THIS_MODULE,
  .write =  snd_pcm_write,
  .writev =  snd_pcm_writev,
  .open =   snd_pcm_playback_open,
  .release =  snd_pcm_release,
  .poll =   snd_pcm_playback_poll,
  .unlocked_ioctl = snd_pcm_playback_ioctl,
  .compat_ioctl =  snd_pcm_ioctl_compat,
  .mmap =   snd_pcm_mmap,
  .fasync =  snd_pcm_fasync,
 },
 {
  .owner =  THIS_MODULE,
  .read =   snd_pcm_read,
  .readv =  snd_pcm_readv,
  .open =   snd_pcm_capture_open,
  .release =  snd_pcm_release,
  .poll =   snd_pcm_capture_poll,
  .unlocked_ioctl = snd_pcm_capture_ioctl,
  .compat_ioctl =  snd_pcm_ioctl_compat,
  .mmap =   snd_pcm_mmap,
  .fasync =  snd_pcm_fasync,
 }
};


其中的放音部分,是否是先用snd_pcm_playback_open打开音频设备,再用snd_pcm_write向声卡中写数据就可以放音了呢?(在标准的alsa中对于应用层的编程是通过alsa-lib封装上述部分以实现的)

现在我不用alsa-lib就直接采用上述方法能行吗?

如果回答是肯定的,在我测试的过程中snd_pcm_playback_open是成功了的返回为0,substream->runtime->status->state的状态是打开的(SNDRV_PCM_STATE_OPEN).但是在snd_pcm_write中却出现了问题,如下:

 if (runtime->status->state == SNDRV_PCM_STATE_OPEN) {
  result = -EBADFD;
  goto end;
 }

运行这段代码后就退出了程序,不能向设备里面写入音频流.请问各位牛人这是怎么回事?能指教下小弟吗?