[LAU] Scarlett 3rd gen glitches (again) + Raspi 4

Iain Mott mott at escuta.org
Sun Jul 19 13:36:08 CEST 2020

Thanks Florian. I tried using the schedtool line that you suggest and 
changing the priority value in Jack but the problem continues. I believe 
the 3rd generation Scarlett interfaces are a special case with many 
Linux users giving up on them entirely. Yesterday evening I found a 
thread which reports a solution and a patch for the kernel file 
sound/usb/pcm.c. I've not yet managed to apply the patch, for some 
reason, but I'll keep trying and report back if i get anywhere. The 
patch is attached in the 3rd message on this page (you need to be logged 
in to see it): https://linuxmusicians.com/viewtopic.php?f=6&t=21163&start=60



Em 19/07/2020 03:41, Florian Paul Schmidt escreveu:
> On 7/18/20 8:28 PM, Iain Mott wrote:
>> Hi list,
>> I have a Focusrite Scarlett 3rd Gen interface that is producing audio
>> glitches with a Raspberry Pi 4 running a realtime kernel (or not). I had
>> similar problems on a laptop and on a desktop. On the laptop however it
>> now runs perfectly with the very latest Linux Mint and the standard
>> kernel. On my desktop I get acceptable results with AV Linux although
>> haven't tried it with the latest Mint. There are no error messages
>> reported or overruns, just glitches which are independent of buffer
>> sizes and sample rates.
> Hi,
> on a rpi 4 and with a "RT-PREEMPT-FULL y" kernel you also want to change
> the IRQ thread priority of the xhci (usb host controller). I think it's
> always on the same irq line (at least it's for me on two different 
> rpi4s).
> This should do the trick:
> schedtool -F -p 90 $(pidof irq/38-xhci_hcd)
> This seems to make the audio processing on my rpi4 *much* more robust to
> SD-card accesses. Before my daily borgbackup job at 6am caused a storm
> of xruns. Now none at all anymore.
> You mentioned no xruns reported by the software so in principle this
> _should_ have no effect. Possibly the software xrun reporting is broken
> though, so give it a shot.
> Also make sure that you run your audio processing threads with a realtie
> priority higher than 50 (the default priority for irq handler threads in
> the kernel). With jack I'd recommend running it with
> jackd -R -P 80 ...
> Regards,
> -- 
> https://fps.io
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