Version 2.8.2 Debian Buster PREEMPT-RT ISO Freezes

Certainly not knowingly. And there's no point whatsoever in having any wireless drivers at all as there's no wireless hardware.

My reading of "but 2016.06.10-1 to be installed" is that installing ethtool would result in the installation of a 2016 version of wireless-regdb, and that the RT kernel I've got now requires a newer version (something later than 2019.06.03?). Is this correct?

I've done a bit of poking around. wireless-regdb is not a driver but the wireless regulatory database. Kernels depend on it, and so does ethtool. And as the 5.10 image and ethtool appear to require different versions of wireless-regdb, one evidently can't have both installed at the same time.

Looking at my list of installed packages, I see that the 4.19 kernel is still there. Does this means I can get back to where I was by just removing linux-image-5.10.0-13-rt-amd64?

Actually one can remove the working kernel. There's a warning message but it is possible to proceed anyway. After removing 5.10, I reinstalled 4.19, and then rebooted. Worked fine.

Now that I'm back to 4.19, everything seems to be as it was. Those strange LinuxCNC read errors have vanished, and LinuxCNC is now back to working normally apart from the fact that after some time it and the machine all freeze up.

While still using 5.10 I did some more long term pinging and in one case the worst case round trip time was 1.5ms. Truly horrendous and greater than the servo thread interval. I also noticed something of a pattern. Things would go on for a while with all round trip times close to .11ms, then there'd be a period when times were significantly higher (by a factor of 3 or 4), and then things would go back to normal. I wondered whether whether this might be due to the Intel NM10 chip set applying some form of temperature control, and applied some new thermal grease between .it and the heat sink. I haven't seen the pattern since, but then I haven't done much testing.

Basically I'm right back to where I was when I started, except in that I'm now a little bit less helpless when it comes to finding my way around Linux. Progress of a sort.....

P.S. ethtool is now installed. The 5.10 dependency Catch-22 doesn't apply to 4.19 as both 4.19 and ethtool require the same version of ethtool.

I had hoped that

sudo ethtool -C eth0 rx-usecs 0

might be a silver bullet, but when I checked the current parameters for the interface (actually named enp1s0, in case this means anything to anybody) I found that rx-usecs is already zero.

I left ping (to the Mesa card) running last night and when I stopped it this morning the reported average and maximum round trip times were 0.114ms and 2.140ms.  I don't know whether the high maximum is in any way related to the system freezes I'm experiencing, but I take it as a sign that something very strange is happening now and then.

I would be very interesting in knowing whether running the same test on other machines does or does not produce similar results.