[arch-general] nvidia causing audio glitches

[Paul Dann]

On Wed, 17 Mar 2021 at 11:37, Olivier Langlois via arch-general
<arch-general at lists.archlinux.org> wrote:
> The glitch that you hear is the symptom of the sound card interrupt not
> delivered in time.

Thanks for the reply. So do you think in the case of bluetooth, it's
the BT controller that doesn't receive the interrupt in time?

> You can validate this symptom by using the ALSA command line tools such
> as aplay.

The interesting thing is that aplay appears unaffected. See some
discussion here:

* https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/issues/1122#note_790489
* https://bbs.archlinux.org/viewtopic.php?id=262706

> You would see the error about an underflow each time that you hear a
> glitch.

I see errors in pulseaudio and pipewire logs, but nothing in kernel logs.

> Usually, it is caused by a badly written driver that is masking the
> interrupts for too long.

Yes, I reached the same conclusion, but given that it seems to go away
when the discrete graphics card is operating at full power, I figured
it was something to do with the card reacting slowly to messages on
the PCI bus.

> Even if the driver is masking locally the interrupts on a single core,
> it will end up putting all the system interrupts to a stall because of
> the round-robin interrupt balance algo, they will all end up on the
> masked core pretty fast.

Thanks; that's helpful information. How could I go about diagnosing
this? I'm not sure what to look for in /proc/interrupts, for instance.

> If possible, try to switch drivers from Nvidia to the open source one.
> As a workaround, you may explore the option to pin your sound card to a
> specific core to see if it helps.

I can't really switch to nouveau, as I do use the discrete graphics
(with PRIME). I do at least have a workaround, which is something.

Thanks again for the reply,
Paul