[PATCH 1/3] ASoC: qcom: q6apm-dai: wrap the position of a full ring to 0

[Vladyslav Tymoshchuk <tvlad744@gmail.com>]

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>
---
Tested on a OnePlus Pad 3 (SM8750, AudioReach, queue mode) together
with patch 2, with a small ALSA test program and no sound server,
buffer 3840 frames, period 480:
 - write 4 periods and stop feeding: before, the PCM stays RUNNING at
   hw_ptr 1919, appl_ptr 1920; after, it goes to XRUN and the next
   write gets -EPIPE;
 - drain after 8 periods: before, -EIO after 182 ms; after, done in
   78 ms (twice), and 18 ms for 2 periods;
 - 60 s of tone through PipeWire without an xrun.

 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 b0b10ba51c99..58beaf906b9f 100644
--- a/sound/soc/qcom/qdsp6/q6apm-dai.c
+++ b/sound/soc/qcom/qdsp6/q6apm-dai.c
@@ -613,11 +613,15 @@ static snd_pcm_uframes_t q6apm_dai_pointer(struct snd_soc_component *component,
 		ptr = bytes_to_frames(runtime, index);
 		return 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,
-- 
2.43.0