浅析alsa声卡驱动snd_pcm_plugin_build函数

int snd_pcm_plugin_build(struct snd_pcm_substream *plug,
             const char *name,
             struct snd_pcm_plugin_format *src_format,
             struct snd_pcm_plugin_format *dst_format,
             size_t extra,
             struct snd_pcm_plugin **ret)
{
    struct snd_pcm_plugin *plugin;
    unsigned int channels;
    
    if (snd_BUG_ON(!plug))
        return -ENXIO;
    if (snd_BUG_ON(!src_format || !dst_format))
        return -ENXIO;
    plugin = kzalloc(sizeof(*plugin) + extra, GFP_KERNEL);
    if (plugin == NULL)
        return -ENOMEM;
    plugin->name = name;
    plugin->plug = plug;
    plugin->stream = snd_pcm_plug_stream(plug);
    plugin->access = SNDRV_PCM_ACCESS_RW_INTERLEAVED;
    plugin->src_format = *src_format; // src放音音频量化位数,比如AFMT_U8
    plugin->src_width = snd_pcm_format_physical_width(src_format->format); // src放音音频实际量化位数占用位数,字节数*8
    snd_BUG_ON(plugin->src_width <= 0);
    plugin->dst_format = *dst_format; // dst放音音频量化位数,比如AFMT_U8
    plugin->dst_width = snd_pcm_format_physical_width(dst_format->format); // dst放音音频实际量化位数占用位数,字节数*8
    snd_BUG_ON(plugin->dst_width <= 0);
    if (plugin->stream == SNDRV_PCM_STREAM_PLAYBACK)
        channels = src_format->channels; // 如果是放音,那么声道数以src放音声道个数为基准,这样如果src为1个,dst可以复制出3路
    else
        channels = dst_format->channels; // 如果是录音,那么声道数以dst录音声道个数为基准
    plugin->buf_channels = kcalloc(channels, sizeof(*plugin->buf_channels), GFP_KERNEL); // 申请管理channels个声道的控制结构体
    if (plugin->buf_channels == NULL) {
        snd_pcm_plugin_free(plugin);
        return -ENOMEM;
    }
    plugin->client_channels = snd_pcm_plugin_client_channels; // 返回plugin->buf_channels指针
    *ret = plugin;
    return 0;
}