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

[Jacob Procee <sn125164@gmail.com>]

On 10/30/26 08:35, Jacob Procee wrote:
> Thanks for the ack and the extra background info. I would love to take a
> look at a race-free version of admac_cyclic_read_residue() as a separate
> follow-up patch; v2 of this fix follows shortly.
Hello Martin,

I looked into this using REG_DESC_RING. It only contains descriptors
that the hardware hasn't fetched yet, and its read slot is effectively
the hardware's fetch pointer.

Unfortunately, it isn't updated atomically with the residue reload
either. Across 1.49 M reads, I hit 126 period boundaries. In 4 cases,
DESC_RING still showed the old state even though the residue had already
reloaded. In another case, DESC_RING had advanced while the residue was
still 0 and no report had been posted. So I don't see a way to get a
consistent, retry-free read from these registers.

One could make the retry check more comprehensive by using three
counters: outstanding = queued + playing + reported, with playing being
1 unless the residue is 0. This would also catch the reverse ordering,
where the report is posted after the descriptor is loaded, which the v2
check doesn't cover.

However, I left the debug module running overnight (10.65 h, with music
playing through the speakers), and never observed that reverse ordering.
There were no XRuns or backward steps. The speaker channel had 96.6 M
reads and 15,483 boundary hits; the second channel had 29.3 M reads and
86 hits.

All 1,498 snapshots that failed the three-counter check were explained
by read skew: 822 where DESC_RING was read before the fetch while the
residue was already full, and 676 where it was read after the fetch
while the residue was still 0. The positions returned to ALSA never
stepped backwards.

Given that the more complicated check would only handle a case I haven't
observed, I'd prefer to stick with the simpler v2 retry.

Unless you know something about the hardware ordering that these traces
wouldn't show, I'd prefer to stick with the simpler v2 retry.

Kind regards,
Jacob