[PATCH] ASoC: qcom: q6asm: Reset PCM positions when preparing a stream

[Michael McAllister <me@michaelmcallister.org>]

Preparing an existing PCM stream closes the DSP stream and remaps its
buffers, but leaves queue_ptr, dsp_buf and hw_ptr at their previous
positions. ALSA resets appl_ptr during prepare, so q6asm_dai_ack() can
subtract a stale queue_ptr from the new appl_ptr and queue an incorrect
number of periods.

On the Motorola Moto G10, playback works after opening the PCM device,
but the first write after snd_pcm_drop() and snd_pcm_prepare() fails
with -EBUSY and "Error queuing playback buffer -16".

Reset queue_ptr in prepare, and reset the DSP buffer index and hardware
pointer when allocating the new buffer ring. This keeps the queued,
submitted and completed positions aligned with the new PCM stream.

Fixes: e386c2cf3df2 ("ASoC: qcom: q6asm-dai: schedule all available frames to avoid dsp under-runs")
Fixes: 44432315444c ("ASoC: qcom: q6asm-dai: use q6asm_get_hw_pointer")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Michael McAllister <me@michaelmcallister.org>
---
Tested on a Motorola Moto G10 using a Linux 7.2.9-based port, with
48 kHz S16_LE stereo playback, a 3840-frame buffer and 960-frame periods.
After two seconds of playback, snd_pcm_drop() followed by
snd_pcm_prepare() and snd_pcm_writei() on the same handle returned -EBUSY
without this change. With the change, both playback cycles completed.

Both affected objects also build with ARCH=arm64 W=1 against sound's
for-next branch at bef34efba74d. This maintainer-tree build has not been
booted on the device.

 sound/soc/qcom/qdsp6/q6asm-dai.c | 3 +++
 sound/soc/qcom/qdsp6/q6asm.c     | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/sound/soc/qcom/qdsp6/q6asm-dai.c b/sound/soc/qcom/qdsp6/q6asm-dai.c
index 4261c3048c..94416ddf7a 100644
--- a/sound/soc/qcom/qdsp6/q6asm-dai.c
+++ b/sound/soc/qcom/qdsp6/q6asm-dai.c
@@ -243,6 +243,9 @@ static int q6asm_dai_prepare(struct snd_soc_component *component,
 		prtd->state = Q6ASM_STREAM_STOPPED;
 	}
 
+	/* ALSA resets appl_ptr for every prepare, including xrun recovery. */
+	prtd->queue_ptr = 0;
+
 	ret = q6asm_map_memory_regions(substream->stream, prtd->audio_client,
 				       prtd->phys,
 				       (prtd->pcm_size / prtd->periods),
diff --git a/sound/soc/qcom/qdsp6/q6asm.c b/sound/soc/qcom/qdsp6/q6asm.c
index de0bd8fd08..8dde9a3b9c 100644
--- a/sound/soc/qcom/qdsp6/q6asm.c
+++ b/sound/soc/qcom/qdsp6/q6asm.c
@@ -514,6 +514,8 @@ int q6asm_map_memory_regions(unsigned int dir, struct audio_client *ac,
 
 
 	ac->port[dir].buf = buf;
+	ac->port[dir].dsp_buf = 0;
+	atomic_set(&ac->port[dir].hw_ptr, 0);
 
 	buf[0].phys = phys;
 	buf[0].size = period_sz;

base-commit: bef34efba74d87d5c20e3bb93539fa45b3cb8a83
-- 
2.50.1 (Apple Git-155)