Raspberry Pi 3 and PREEMPT_RT

[kdoren]

The rpi-4.19.y-rt branch, which works on RPi3, was built built by applying some additional patches on top of the "official" PREEMPT_RT patch set: https://github.com/raspberrypi/linux/commits/rpi-4.19.y-rt

I tried these additional patches on 5.10 (the patches appear to apply correctly and cover all the intended cases). It boots on RPi3 but is not stable. So something more is needed but it's not clear what.

On RPi4 I have been using 5.10 with only the PREEMPT_RT patch set. (https://github.com/kdoren/linux/releases) This has worked well in most cases I care about (audio apps with jack & USB audio, lots of people successfully using it).

However there is at least one issue with this approach on Pi4: Jamulus server with multithreading enabled [no jack audio] can cause one of the cores running lxpanel to go to 100% CPU, This does not happen on low-latency PREEMPT kernel build. I don't know if this is a general problem with PREEMPT_RT, or is specific to my build for RPi4.

[kdoren]

The patches from 4.19,y-rt branch make PREEMPT_RT kernel 5.10 run on RPi3.
Release 5.10.52-rt47 (see releases) is now stable with Jamulus server.