[PATCH] ASoC: qcom: q6asm-dai: Add SNDRV_PCM_INFO_BATCH flag

[Lars-Peter Clausen]

On 3/30/20 7:52 PM, Stephan Gerhold wrote:
> At the moment, playing audio with PulseAudio with the qdsp6 driver
> results in distorted sound. It seems like its timer-based scheduling
> does not work properly with qdsp6 since setting tsched=0 in
> the PulseAudio configuration avoids the issue.
>
> Apparently this happens when the pointer() callback is not accurate
> enough. There is a SNDRV_PCM_INFO_BATCH flag that can be used to stop
> PulseAudio from using timer-based scheduling by default.
>
> According to https://www.alsa-project.org/pipermail/alsa-devel/2014-March/073816.html:
>
>      The flag is being used in the sense explained in the previous audio
>      meeting -- the data transfer granularity isn't fine enough but aligned
>      to the period size (or less).
>
> q6asm-dai reports the position as multiple of
>
>      prtd->pcm_count = snd_pcm_lib_period_bytes(substream)
>
> so it indeed just a multiple of the period size.
>
> Therefore adding the flag here seems appropriate and makes audio
> work out of the box.
>
> Fixes: 2a9e92d371db ("ASoC: qdsp6: q6asm: Add q6asm dai driver")
> Cc: Srinivas Kandagatla <srinivas.kandagatla at linaro.org>
> Signed-off-by: Stephan Gerhold <stephan at gerhold.net>
> ---
> I'm still quite confused about the true meaning of SNDRV_PCM_INFO_BATCH,
> so please correct me if I'm wrong :)

The meaning might have changed over the years, but the way it is used 
right now is that it means that the position pointer has limited 
granularity. With 'limited' being a bit fuzzy, but typically means that 
the granularity is worse than a few samples.

This driver definitely falls into the limited category as the 
granularity seems to be period size.

- Lars