linux-kernel - Re: [PATCH] ASoC: pcm: allow changing the playback/capture rates for symmetric links

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Message-ID: <20160427161504.GZ3217@sirena.org.uk>
Date:	Wed, 27 Apr 2016 17:15:04 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Peter Rosin <peda@...ntia.se>
Cc:	alsa-devel@...a-project.org, Liam Girdwood <lgirdwood@...il.com>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ASoC: pcm: allow changing the playback/capture rates for
 symmetric links

On Wed, Apr 27, 2016 at 10:49:19AM +0200, Peter Rosin wrote:

> The below program fails on a dai link with symmetric rates without this
> patch. The patch makes it work.

You've not articulated the problem you're trying to fix here, what in
concrete terms is the program trying to accomplish and why should it
succeed?

>     if ((fd = open("/dev/dsp", O_WRONLY, 0)) == -1) {
>             perror("open");
>             return 1;
>     }

This is using the OSS interfaces which really haven't ever been
especially supported for ASoC.

> +	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
> +		if (!cpu_dai->capture_active)
> +			return 0;
> +	} else if (substream->stream == SNDRV_PCM_STREAM_CAPTURE) {
> +		if (!cpu_dai->playback_active)
> +			return 0;
> +	}
> +
>  	rate = params_rate(params);
>  	channels = params_channels(params);
>  	sample_bits = snd_pcm_format_physical_width(params_format(params));

This means we've opened up a race where the stream is configured but not
started where the opposite direction can configure a different setup.
Since starting both directions very close together is a common operation
it seems likely to cause issues.

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