linux-kernel - Re: [PATCH 4.9 026/191] ALSA: usb-audio: Improve frames size computation

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Date:   Tue, 30 Jun 2020 12:54:07 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     Alexander Tsoy <alexander@...y.me>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH 4.9 026/191] ALSA: usb-audio: Improve frames size
 computation

On Tue, Jun 30, 2020 at 01:49:50PM +0300, Alexander Tsoy wrote:
>В Пн, 29/06/2020 в 11:37 -0400, Sasha Levin пишет:
>> From: Alexander Tsoy <alexander@...y.me>
>>
>> [ Upstream commit f0bd62b64016508938df9babe47f65c2c727d25c ]
>>
>> For computation of the the next frame size current value of fs/fps
>> and
>> accumulated fractional parts of fs/fps are used, where values are
>> stored
>> in Q16.16 format. This is quite natural for computing frame size for
>> asynchronous endpoints driven by explicit feedback, since in this
>> case
>> fs/fps is a value provided by the feedback endpoint and it's already
>> in
>> the Q format. If an error is accumulated over time, the device can
>> adjust fs/fps value to prevent buffer overruns/underruns.
>>
>> But for synchronous endpoints the accuracy provided by these
>> computations
>> is not enough. Due to accumulated error the driver periodically
>> produces
>> frames with incorrect size (+/- 1 audio sample).
>>
>> This patch fixes this issue by implementing a different algorithm for
>> frame size computation. It is based on accumulating of the remainders
>> from division fs/fps and it doesn't accumulate errors over time. This
>> new method is enabled for synchronous and adaptive playback
>> endpoints.
>>
>> Signed-off-by: Alexander Tsoy <alexander@...y.me>
>> Link:
>> https://lore.kernel.org/r/20200424022449.14972-1-alexander@tsoy.me
>> Signed-off-by: Takashi Iwai <tiwai@...e.de>
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>> ---
>>  sound/usb/card.h     |  4 ++++
>>  sound/usb/endpoint.c | 43 ++++++++++++++++++++++++++++++++++++++--
>> ---
>>  sound/usb/endpoint.h |  1 +
>>  sound/usb/pcm.c      |  2 ++
>>  4 files changed, 45 insertions(+), 5 deletions(-)
>
>Please drop this patch from the queue for now (and for 4.4 as well). It
>introduced a regression for some devices. The fix is available, but not
>accepted yet.

I've dropped it from the older branches, but note that it's already in
newer released stable kernels. Should it be reverted or should we wait
for the fix?

-- 
Thanks,
Sasha