USB patch needed for pi4?

[micahvdm]

Is the usb dwc patch needed for pi4 rt kernel or does it not need it?

[kdoren]

The patch was required for 5.10 RT kernel to run reliably on Pi3 (without lockup), and I think also on 5.15. I haven't built 6.1 yet.

I'm not completely certain if it worked reliably without the patch on Pi4. The failure was fairly easy to reproduce on Pi3, and I wanted it to work on both Pi3 and Pi4. Failure mode was a complete lockup under USB load. I don't recall seeing failures on 5.15 kernel on Pi4, but I think the possibility may still be there.

The reason for the patch IIRC is that Raspberry Pi uses chipset FIQ (Fast IRQ) to improve USB performance, but FIQ is not compatible with PREEMPT_RT. The patch I think causes certain places to spin until it's safe to proceed.

So my belief is that if you are using USB, it's not safe to omit the patch, even on Pi4.

[micahvdm]

Initial test show that usb is not an issue with fiq anymore on pi4, but I’ll do more investigating and tests first

[kdoren]

@micahvdm ,

I did builds of PREEMPT_RT kernel 6.1.54-rt15, both with and without the Raspberry Pi specific FIQ patches. I ran the unpatched kernel for a week running USB audio, with no lockups. That's not exactly a definitive test, but it is an indication that maybe the patches aren't required for Pi4. Including the patches doesn't seem to hurt though, and if that makes it work on Pi3, I'd be inclined to leave the patches in. I haven't tested if I can still reproduce the lockups on Pi3.

-Kevin