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

[Alexander Tsoy]

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.