Re: [PATCH] dmaengine: apple-admac: Fix position one period ahead at period boundary

[Frank Li]

On Thu, Oct 29, 2026 at 11:59:08AM +0100, Jacob Procee wrote:
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>
> Speaker playback on a MacBook Pro 14" (M1 Pro, J314s) underruns at
> irregular intervals, audible as a tick: 8 times in 3.7 hours of
> playback. Where an underrun was caught (4 of 8), the ALSA hw_ptr had
> jumped by one buffer size while about 70 ms of audio was still queued,
> and a kernel trace showed the DMA position stepping back by exactly one
> period at each of the 6 underruns it covered.
>
> admac_cyclic_read_residue() derives the position from the report ring
> (periods finished but not yet reclaimed) and REG_RESIDUE (bytes left in
> the current descriptor). When a descriptor finishes, the residue first
> drops to zero, then the report is posted and the next descriptor is
> loaded. Usually the last two happen together, but occasionally the
> report comes first. A read in that window counts the finished period
> twice:
>
>  - residue2 == 0 with the report already in ring2: the period is
>    counted in nreports and again as the fully consumed current
>    descriptor;
>  - residue1 == 0 with the report already in ring1, and the next
>    descriptor loaded by the time residue2 is read: the
>    residue2 > residue1 branch adds one to a ring1 that already holds
>    the report.
>
> Either way the position is one period ahead. The next read is behind
> it, snd_pcm_update_hw_ptr0() takes the backwards step for a wrap of the
> whole buffer, and the stream underruns.
>
> While the channel runs, a zero residue only occurs between descriptors,
> so read the registers again while either residue is zero. The retries
> are bounded, as the residue also stays zero on a stopped channel or one
> that ran out of descriptors; after the last retry the position is
> computed as before.
>
> With a debug build counting retries, one hour of playback needed a
> retry for 2210 of 3.39 million reads, at most three retries (3.5 us),
> also across pause and stream restart. Both states above showed up in
> the logged register values. There were no underruns in three hours of
> playback with the fix.
>
> Fixes: b127315d9a78 ("dmaengine: apple-admac: Add Apple ADMAC driver")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Jacob Procee <sn125164@xxxxxxxxx>
> ---
Reviewed-by: Frank Li <Frank.Li@xxxxxxx>