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

[Takashi Iwai]

On Sun, 26 Apr 2020 19:12:41 +0200,
Alexander Tsoy wrote:
> 
> 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.

Aha, that's interesting.  Basically that's natural that the timer
scheduling doesn't work reliably for the device buffer management like
USB-audio, but it was enabled on PA because it works in most of
cases.


Takashi