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

[Sasha Levin]

On Tue, Jun 30, 2020 at 01:49:50PM +0300, Alexander Tsoy wrote:
> Ð ÐÐ, 29/06/2020 Ð 11:37 -0400, Sasha Levin ÐÐÑÐÑ:
> > From: Alexander Tsoy <alexander@xxxxxxx>
> >
> > [ 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@xxxxxxx>
> > Link:
> > https://lore.kernel.org/r/20200424022449.14972-1-alexander@xxxxxxx
> > Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > ---
> >  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