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

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Date:   Tue, 30 Jun 2020 20:33:28 +0200
From:   Greg KH <greg@...ah.com>
To:     Sasha Levin <sashal@...nel.org>
Cc:     Alexander Tsoy <alexander@...y.me>, 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 12:54:07PM -0400, Sasha Levin wrote:
> 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?

I was going to wait for the fix.