Re: [alsa-devel] [RFC] ASoC: soc-pcm: crash in snd_soc_dapm_new_dai

[Stephan Gerhold <stephan@gerhold.net>]

On Mon, Feb 17, 2020 at 03:43:01PM +0000, Mark Brown wrote:
> On Mon, Feb 17, 2020 at 03:41:20PM +0100, Stephan Gerhold wrote:
> 
> > I'm a bit confused about this patch, isn't SNDRV_PCM_STREAM_PLAYBACK
> > used for both cpu_dai and codec_dai in the playback case?
> 
> It is in the normal case, but with a CODEC<->CODEC link (which was what
> this was targeting) we need to bodge things by swapping playback and
> capture on one end of the link.

I see. Looking at the code again I'm guessing the cause of the crash
"fixed" by this patch is commit a342031cdd08 ("ASoC: create pcm for
codec2codec links as well") where the codec2codec case was sort of
patched in. This is what we had before this patch:

		/* Adapt stream for codec2codec links */
		struct snd_soc_pcm_stream *cpu_capture = rtd->dai_link->params ?
			&cpu_dai->driver->playback : &cpu_dai->driver->capture;
		struct snd_soc_pcm_stream *cpu_playback = rtd->dai_link->params ?
			&cpu_dai->driver->capture : &cpu_dai->driver->playback;

This does the swapping you mentioned, so I guess rtd->dai_link->params
is only set for the codec2codec case?

		for_each_rtd_codec_dai(rtd, i, codec_dai) {
			if (snd_soc_dai_stream_valid(codec_dai, SNDRV_PCM_STREAM_PLAYBACK) &&
			    snd_soc_dai_stream_valid(cpu_dai,   SNDRV_PCM_STREAM_PLAYBACK))
				playback = 1;
			if (snd_soc_dai_stream_valid(codec_dai, SNDRV_PCM_STREAM_CAPTURE) &&
			    snd_soc_dai_stream_valid(cpu_dai,   SNDRV_PCM_STREAM_CAPTURE))
				capture = 1;
		}

		capture = capture && cpu_capture->channels_min;
		playback = playback && cpu_playback->channels_min;

And this does a part of the check in snd_soc_dai_stream_valid(),
but without the NULL check of cpu_capture/cpu_playback.
(Maybe that is the cause of the crash.)

From my limited understanding, I would say that a much simpler way to
implement this would be:

	/* Adapt stream for codec2codec links */
	int cpu_capture = rtd->dai_link->params ?
		SNDRV_PCM_STREAM_PLAYBACK : SNDRV_PCM_STREAM_CAPTURE;
	int cpu_playback = rtd->dai_link->params ?
		SNDRV_PCM_STREAM_CAPTURE : SNDRV_PCM_STREAM_PLAYBACK;

	for_each_rtd_codec_dai(rtd, i, codec_dai) {
		if (snd_soc_dai_stream_valid(codec_dai, SNDRV_PCM_STREAM_PLAYBACK) &&
		    snd_soc_dai_stream_valid(cpu_dai,   cpu_playback))
			playback = 1;
		if (snd_soc_dai_stream_valid(codec_dai, SNDRV_PCM_STREAM_CAPTURE) &&
		    snd_soc_dai_stream_valid(cpu_dai,   cpu_capture))
			capture = 1;
	}

since snd_soc_dai_stream_valid() does both the NULL-check and the 
"channels_min" check.

But I'm really not familar with the codec2codec case and am unable to
test it :) What do you think?

Thanks,
Stephan