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

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Date:   Tue, 30 Jun 2020 13:49:50 +0300
From:   Alexander Tsoy <alexander@...y.me>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH 4.9 026/191] ALSA: usb-audio: Improve frames size
 computation

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.