Latency, error finishing read, and IRQ affinity

Is the processor 4 core or 2 core 4 thread one? Set that to 2 and do some tests just in case. Also try 1, but that might make the PC slouchy.
In general, disable hyperthreading, power saving, virtualisation, TPM, turbo mode, pci-e agressive link management, C states.
In some cases disabling sound helps, also disable serial ports if any and modem.
Open a terminal, type
Sensors
Then start LinuxCNC, enable it and try to jog, then go to terminal and again type sensors, notice the processor temperature. If it goes above 60-70C, open the PC, remove CPU cooler, clean, repaste, mount the cooler. Test again.

It is four core, at least according to everything I have read.  I increased the servo-thread to 2000000 and halved the P values and that actually had an effect, but it still errored out after a while, it just took longer.  Temps are nil, 30° while running a program in LCNC.  I am going to test everything again under 2.8 and maybe try a SEEED Odyssey I have here.  

 
Can you post the results of running this script:



to run

mv checkmask.txt checkmask
chmod +x checkmask

then

./checkmask enp2s0

Here the results from the Debian 12 system that is not working:

And here the results from the Debian 10 system that is working (same computer, just swapping HDD back and forth):

Can you try pinning the Ethernet IRQ to the last processor:

 

Use by

mv pinirq.txt pinirq
chmod +x pinirq

sudo ./pinirq enp2s0

and then try LinuxCNC again

NOTE: Look at the script before you run it as with any script
from unknown sources, especially scripts that need sudo...
 

Question, why and where did you get the info about using i915 in the kernel line?
Remove that, update grub and try again.
-
Pretty sure i read hundreds of times about i915 having to be blacklisted a few years back, so using it now seems really strange.
It was an intel chipset that lasted till i965 or i945, so that is superseded, probably 15 or more years ago.

Strange, for some reason I thought the rt_setup script had already done that.  Obviously I was mistaken as that does seem to have done the trick, have the original servo_thread value and have run multiple times through the test program. 

Really can not thank you enough and sorry that I missed that in all my other troubleshooting.

I do not know where the i915 came from, this is a vanilla install from the distro and the 2.8 was the same. Ah, I see, it is in the rt_setup script pinned to the start of this thread. I did run that in both setups trying to bring latency down.

The i915 driver supports basically all Intel graphic chips except the GMA500 series

That kernel command line is fine

Mozmcks script requires editing to match your IP address setup
or it will not setup the irq mask.

Note that my simple script just sets the mask temporarily and would
need to be re-run after re-boot. In addition, if irqbalance is not disabled
the mask setting  will be eventually undone by irq balance.

The main point of my script is for testing