[PATCH 6.18.y] ASoC: qcom: q6apm-dai: wrap the position of a full ring to 0

[Sasha Levin <sashal@kernel.org>]

From: Vladyslav Tymoshchuk <tvlad744@gmail.com>

[ Upstream commit 0ea3e0b196f9dcc17b40ac881a6efef06459975b ]

q6apm_dai_pointer() turns the number of period buffers the DSP has
returned in the current lap, 1..periods, into a position and reports
one frame less than that. Commit a93dad6f4e6a ("ASoC: q6apm-dai: make
use of q6apm_get_hw_pointer") added the "- 1" so that the end of the
ring is not reported as a position equal to the buffer size.

One frame short has a cost once the application stops feeding the
stream. The DSP returns the last queued buffer and the position stops
at appl_ptr - 1, so avail stays at buffer_size - 1, below the default
stop_threshold. The DSP sends nothing more as its queue is empty, so
no period interrupt comes either: ALSA never detects the underrun and
the stream stays RUNNING. A poll-driven application that missed its
last wakeup waits forever, and drain only ends on its timeout, with
-EIO. On capture, the last frame of each period is held back until
the next one.

Before that commit the position wrapped to 0 at the end of the ring.
Do that again. A full lap is the start of the ring, and
snd_pcm_update_hw_ptr0() accounts for the wrap: the position cannot
move by more than one lap between two updates, as each returned buffer
calls snd_pcm_period_elapsed() and the DSP holds at most periods
buffers.

Seen on a OnePlus Pad 3 (SM8750) with PipeWire in interrupt-driven
scheduling: after a stall the PCM sat at hw_ptr 78239, appl_ptr 78240
(avail 3839 of 3840) for two hours.

Fixes: a93dad6f4e6a ("ASoC: q6apm-dai: make use of q6apm_get_hw_pointer")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Vladyslav Tymoshchuk <tvlad744@gmail.com>
Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@oss.qualcomm.com>
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@oss.qualcomm.com>
Link: https://patch.msgid.link/20261014162703.10172-2-tvlad744@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>
[ sashal: Adjusted context in q6apm_dai_pointer() because the push/pull-mode branch is absent. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 sound/soc/qcom/qdsp6/q6apm-dai.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/sound/soc/qcom/qdsp6/q6apm-dai.c b/sound/soc/qcom/qdsp6/q6apm-dai.c
index 54c1a33f9f937..4ea2c9869d5a1 100644
--- a/sound/soc/qcom/qdsp6/q6apm-dai.c
+++ b/sound/soc/qcom/qdsp6/q6apm-dai.c
@@ -442,11 +442,15 @@ static snd_pcm_uframes_t q6apm_dai_pointer(struct snd_soc_component *component,
 	struct q6apm_dai_rtd *prtd = runtime->private_data;
 	snd_pcm_uframes_t ptr;
 
-	ptr = q6apm_get_hw_pointer(prtd->graph, substream->stream) * runtime->period_size;
-	if (ptr)
-		return ptr - 1;
+	/*
+	 * The hw pointer counts the period buffers the DSP has returned in
+	 * this lap, 1..periods. A full lap is the start of the ring again:
+	 * report 0 rather than one frame less, so that the position of a
+	 * drained buffer reaches appl_ptr and ALSA sees the underrun.
+	 */
+	ptr = q6apm_get_hw_pointer(prtd->graph, substream->stream) % runtime->periods;
 
-	return 0;
+	return ptr * runtime->period_size;
 }
 
 static int q6apm_dai_hw_params(struct snd_soc_component *component,
-- 
2.53.0