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

[Stephan Gerhold]

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 tsched=0 workaround can be found in Linaro distributions
for QCOM devices for example:
  - https://git.linaro.org/ci/fai.git/commit/?id=63494268b654d80df033f4cdeccf8f115801b756
  - https://github.com/ndechesne/meta-qcom/commit/7035dfeadd1c434fc7613730ac38004670553ec0

This patch allows removing that workaround since audio then works
without any configuration changes.
---
 sound/soc/qcom/qdsp6/q6asm-dai.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sound/soc/qcom/qdsp6/q6asm-dai.c b/sound/soc/qcom/qdsp6/q6asm-dai.c
index f6c7cddf08e8..125af00bba53 100644
--- a/sound/soc/qcom/qdsp6/q6asm-dai.c
+++ b/sound/soc/qcom/qdsp6/q6asm-dai.c
@@ -78,7 +78,7 @@ struct q6asm_dai_data {
 };
 
 static const struct snd_pcm_hardware q6asm_dai_hardware_capture = {
-	.info =                 (SNDRV_PCM_INFO_MMAP |
+	.info =                 (SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_BATCH |
 				SNDRV_PCM_INFO_BLOCK_TRANSFER |
 				SNDRV_PCM_INFO_MMAP_VALID |
 				SNDRV_PCM_INFO_INTERLEAVED |
@@ -100,7 +100,7 @@ static const struct snd_pcm_hardware q6asm_dai_hardware_capture = {
 };
 
 static struct snd_pcm_hardware q6asm_dai_hardware_playback = {
-	.info =                 (SNDRV_PCM_INFO_MMAP |
+	.info =                 (SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_BATCH |
 				SNDRV_PCM_INFO_BLOCK_TRANSFER |
 				SNDRV_PCM_INFO_MMAP_VALID |
 				SNDRV_PCM_INFO_INTERLEAVED |
-- 
2.26.0