[PATCH] ALSA: usb-audio: Apply async workaround for Scarlett 2i4 2nd gen

[Alexander Tsoy]

В Чт, 23/04/2020 в 21:24 +0300, Alexander Tsoy пишет:
> В Чт, 23/04/2020 в 19:35 +0200, Takashi Iwai пишет:
> > On Thu, 23 Apr 2020 19:29:08 +0200,
> > Takashi Iwai wrote:
> > > On Thu, 23 Apr 2020 18:57:34 +0200,
> > > Alexander Tsoy wrote:
> > > > And some further notes:
> > > > 
> > > > - I removed locking from snd_usb_endpoint_next_packet_size()
> > > > and
> > > > this
> > > > seems completely fixed an issue with large URBs I reported
> > > > here:
> > > > 
> > > > https://bugzilla.kernel.org/show_bug.cgi?id=199327#c28
> > > > 
> > > > So playing at 96 kHz, driver packs 48 frames per URB and no
> > > > more
> > > > audio
> > > > discontinuities.
> > > 
> > > Hmm, that's weird.
> > > 
> > > If removing the lock from snd_usb_endpoint_next_packet_size()
> > > really
> > > fixes the problem, it implies the lock contention.  But as far as
> > > I
> > > see the code performed in this lock isn't conflicting so
> > > much.  The
> > > URB processing shouldn't happen in parallel for the same EP.
> > 
> > BTW, one potential racy code I found while looking at the code is
> > the
> > list management in queue_pending_output_urbs().  The fix patch is
> > below.
> 
> OK, it seems like it was just a luck. I'm still getting clicking
> artifacts with and without your patch, with and without locking. Will
> investigate further.

After more testing, it seems that with large URBs the transfer size is
too large for timer-based scheduling to work correctly in pulseaudio.
And looks like pulseaudio sometimes fail to adjust tsched watermark or
something like that. And it is not 100% reproducible.