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ALSA内部函数调用流程

一直都有这样的一个疑问

就是在linux系统中我们调用snd_pcm_open后，就不知道alsa内部是怎么运行的了

用户的pcm_open()相当于先对ASoC各个驱动模块startup()，再做hw_params()。

pcm_open()pcm->fd = open("/dev/snd/pcmC0D0c")snd_pcm_capture_open()snd_pcm_open(SNDRV_PCM_STREAM_CAPTURE)snd_pcm_open_file()snd_pcm_open_substream()substream->ops->open()soc_pcm_open()cpu_dai->driver->ops->startup()platform->driver->ops->open()codec_dai->driver->ops->startup()rtd->dai_link->ops->startup()ioctl(pcm->fd, SNDRV_PCM_IOCTL_HW_PARAMS, &params)snd_pcm_hw_params_user()snd_pcm_hw_params()substream->ops->hw_params()soc_pcm_hw_params()rtd->dai_link->ops->hw_params()dai->driver->ops->hw_params()platform->driver->ops->hw_params()ioctl(pcm->fd, SNDRV_PCM_IOCTL_SW_PARAMS, &sparams)

用户的pcm_read()相当于做从内核缓冲区到用户缓冲区的copy_to_user()。即把硬件写到内核缓冲区的数据拷贝到用户缓冲区。（mmap模式例外，其没有数据拷贝的动作，性能更好。)

pcm_read()if (!pcm->running)pcm_start()pcm->running = 1ioctl(pcm->fd, SNDRV_PCM_IOCTL_READI_FRAMES, &x)snd_pcm_lib_read()snd_pcm_lib_read1(transfer)transfer(substream, appl_ofs, data, offset, frames)snd_pcm_lib_read_transfer()substream->ops->copy() *or* copy_to_user()

用户的pcm_start()相当于对ASoC的各个驱动模块做prepare()和trigger(START)动作。

pcm_start()ioctl(pcm->fd, SNDRV_PCM_IOCTL_PREPARE)snd_pcm_prepare()snd_pcm_do_prepare()substream->ops->prepare()soc_pcm_prepare()rtd->dai_link->ops->prepare()platform->driver->ops->prepare()codec_dai->driver->ops->prepare()cpu_dai->driver->ops->prepare()ioctl(pcm->fd, SNDRV_PCM_IOCTL_START)snd_pcm_action_lock_irq()snd_pcm_do_start()substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_START)soc_pcm_trigger()codec_dai->driver->ops->trigger()platform->driver->ops->trigger()cpu_dai->driver->ops->trigger()rtd->dai_link->ops->trigger()

用户的pcm_close()相当于对ASoC的各个驱动模块做trigger(STOP), hw_free()和shutdown()动作。

pcm_close()close(pcm->fd)snd_pcm_release()snd_pcm_release_substream()snd_pcm_drop(substream)snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP)snd_pcm_do_stop()substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_STOP)soc_pcm_trigger()codec_dai->driver->ops->trigger()platform->driver->ops->trigger()cpu_dai->driver->ops->trigger()rtd->dai_link->ops->trigger()if (substream->hw_opened)if (substream->ops->hw_free != NULL)substream->ops->hw_free(substream)substream->ops->close(substream)soc_pcm_close()cpu_dai->driver->ops->shutdown()codec_dai->driver->ops->shutdown()rtd->dai_link->ops->shutdown()platform->driver->ops->close()substream->hw_opened = 0

以snd开头的函数都是在alsa-core中提供的

以soc开头的函数都是在asoc-core中提供的

总结：

1.一般情况下，系统中移植了alsa,音频我们只需要修改底层asoc core 中的dai link ，让asoc core 能匹配到我们的codec，配置好iis 通信格式和时序就可以了

而alsa core 一般是不用动的，上层alsa-lib alsa-utils 也是不用动的

2.当使用了不同版本的alsa lib,会导致amixer有一些区别