Re: [RFC][PATCH] ALSA: pcm_dmaengine: Use more shorter periods for SEGMENT granularity PCM DMA

[Takashi Iwai <tiwai@suse.de>]

On Fri, 27 Mar 2026 15:29:43 +0100,
Marek Vasut wrote:
> 
> The segment granularity PCM DMA updates PCM pointer at the end of every
> segment. In case the segments are too long, which happens with pipewire,
> it may happen that pcm_lib wait_for_avail() will time out while waiting
> for PCM pointer update and playback will fail with IO error.
> 
> In case of SEGMENT granularity, use 8 shorter segments over the default
> 2 long segments set in dmaengine_pcm_set_runtime_hwparams(). That gives
> the DMA engine some chance to update the PCM pointer every once in a
> while and prevent the timeout.
> 
> Signed-off-by: Marek Vasut <marex@nabladev.com>
> ---
> Cc: Jaroslav Kysela <perex@perex.cz>
> Cc: Lars-Peter Clausen <lars@metafoo.de>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Michal Simek <monstr@monstr.eu>
> Cc: Takashi Iwai <tiwai@suse.com>
> Cc: linux-sound@vger.kernel.org
> ---
>  sound/core/pcm_dmaengine.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/sound/core/pcm_dmaengine.c b/sound/core/pcm_dmaengine.c
> index 1306b04be171a..092ce958e1aee 100644
> --- a/sound/core/pcm_dmaengine.c
> +++ b/sound/core/pcm_dmaengine.c
> @@ -424,6 +424,10 @@ int snd_dmaengine_pcm_refine_runtime_hwparams(
>  			hw->info |= SNDRV_PCM_INFO_PAUSE | SNDRV_PCM_INFO_RESUME;
>  		if (dma_caps.residue_granularity <= DMA_RESIDUE_GRANULARITY_SEGMENT)
>  			hw->info |= SNDRV_PCM_INFO_BATCH;
> +		if (dma_caps.residue_granularity == DMA_RESIDUE_GRANULARITY_SEGMENT) {
> +			hw->periods_min = 8;
> +			hw->period_bytes_max /= 8;
> +		}

Overriding with the hard-coded values at hw_params refine doesn't look
like a right way.  Those values are set and dealt by the drivers in
general, as they are pretty hardware-specific.  You can't guarantee
this magic number 8 working on all available hardware.


thanks,

Takashi