Raspberry Pi 4

Turned off GPU accel and video4linux and with a 100,000nS servo thread latency is <25,000nS after several hours, but without playing a video.
Kernel 4.19.71, the one that comes with the Rpi4 2.8 img on the LCNC website.
www.linuxcnc.org/iso/linuxcnc-2.8.0-pi4.zip
Video is limited to 1920x1080. Overclocked to 2.1Ghz, isolcpus=1,2,3, idle=poll

With video playback latency jumps to 102,000nS after a few seconds. Video is choppy.

A USB GPU might work well enough with the Rpi4. Will try that soon.

We had Xenomai on the allwinners 6 years ago (cubieboards) at 40,000 worst case while playing videos.

Bari wrote: You don't have to install using all the steps at JT's site anymore. There is a full working SDcard image on the wiki now.

LinuxCNC 2.8.0 Raspberry Pi OS based on Debian 10 Buster
www.linuxcnc.org/iso/linuxcnc-2.8.0-pi4.zip

Raspberry Pi 4 Uspace compatible with Mesa Ethernet and SPI interface boards.

A StepConfig app could be written for Rpi4.

I'm curious, is this rpi distro 64 bit?

It's 32bit.

Run "uname -m" to confirm. If it says aarch64 then it is 64 bit. If it says armv7l then it is 32 bit.

If you want 64b I'd start with github.com/sakaki-/gentoo-on-rpi-64bit
Bootable 64-bit Gentoo image for the Raspberry Pi 4 Model B, and Pi 3 B and B+, with Linux 5.4, OpenRC, Xfce4, VC4/V3D, camera & h/w codec support, profile 17.0, weekly-autobuild binhost

To turn off hardware acceleration on the Rpi4:

1. Download and copy this kernel module blacklist file pastebin.pl/view/2f57585f to your /rootfs/etc/modprobe.d/
2. Reboot and you should be software rendering now.

To turn off software rendering and go back to hardware acceleration:

1. Remove the kernel module blacklist file from your /rootfs/etc/modprobe.d/ directory.
2. Reboot and you should be back to hardware rendering.

A copy of the kernel module blacklist file is also attached.

thank you for all your effort!

We were using isolcpus=1,2,3, idle=poll with software rendering.

I haven't tried testing software rendering with just isolcpus=3, idle=poll and the core clock speed lower. Theoretically isolcpus=3 should keep RT on the highest core which is 3 (counting up from 0). And this should make cores 0, 1 and 2 available for software rendering and non-RT.

Somebody also mentioned that force_turbo=1 does not actually keep the cores at max frequency when the core gets warmer. So it might still speed step/throttle cpu core speed even though we are asking for max speed at all times. Keeping the max core speed lower might keep it from throttling even though we told the cpu not to.