Correct stopping capture and playback substreams?

[Pavel Hofman]

Hi Takashi,

I am working on stopping alsa streams of audio USB gadget when USB host 
stops capture/playback/USB cable unplugged.

For capture I used code from AK4114 SPDIF receiver 
https://elixir.bootlin.com/linux/latest/source/sound/i2c/other/ak4114.c#L590:

static void stop_substream(struct uac_rtd_params *prm)
{
	unsigned long _flags;
	struct snd_pcm_substream *substream;

	substream = prm->ss;
	if (substream) {
		snd_pcm_stream_lock_irqsave(substream, _flags);
		if (snd_pcm_running(substream))
			// TODO - correct handling for playback substream?
			snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING);
		snd_pcm_stream_unlock_irqrestore(substream, _flags);
	}
}

For setup I found calling snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP) 
(https://elixir.bootlin.com/linux/latest/source/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c#L63) 
  Or for both capture and playback using SNDRV_PCM_STATE_DISCONNECTED 
(https://elixir.bootlin.com/linux/latest/source/sound/core/pcm.c#L1103).

Or perhaps using snd_pcm_dev_disconnect(dev) or snd_pcm_drop(substream)?

Please what is the recommended way?

Thanks a lot,

Pavel.