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bcm2712_defconfig.txt is the 16k-page one, it's broken by design, don't use it that's also pretty much the only difference between it and 2711, look at the diff between the two.
bcm2711_rt_defconfig.txt works on both Pi versions, I've tested it on my setup. it's the one that linux-image-6.12.x+rpt-rpi-v8-rt is built from. I only use SPI though (with my own drivers that mmap the registers), so I'm not sure how Mesa cards or GPIO would behave.

After some testing and having my machine up for 39 days a network manager doesn't really affect latency, well at least on the RPi5 (I replaced the Odroid with the RPi5). And it does make it easier to turn wifi off when needed.

A couple of things related to choosing a Kernel for Linuxcnc.

For the RPi4 you require options for the bcm2711 in the config.
For the RPi5 you require options for the bcm2712 in the config.

The change of the SoC broke a lot of stuff that broke GPIO stuff. What previously worked on the RPi4 failed on the RPi5. In the context of Linuxcnc this required a rewrite of the SPI driver for the RPi5.

For both SoCsmust be set. Otherwise we dont get the real time model required.

In the link you gave for the configs only the bcm_2711 rt config has that option set. The bcm_2712 config does not.

With regards to the kernel version the one on the image is uname -a
Linux raspberrypi 6.12.11 #1 SMP PREEMPT_RT Mon Jan 27 00:10:15 AEDT 2025 aarch64 GNU/Linux

This was built from the 6.12.y branch in January. Whilst not to up to date 6 months later it was in January and has proved to be stable. With regards to the kernel itself there have been no reports of crashing, except one user whilst using GMOCCAPY but this was traced to zram swap issue.

This is the one running my RPi5 on the mill, it's been up for 39 odd days with no issues. Having the power saving stuff turned off on the wifi has been constantly connected for those 39 days. Usually it powers down after some time.

I bought a spindle servo and i am unsure about the wiring on the 7i96s. Do I need 4 resistors for the 4 step/dir inputs or can I do it without them?

I connect 5V and 0V as well as step+, step-, dir+, and dir-. Which encoder outputs do I need for rigid tapping on the milling machine?
 
 

The issue has been with the live-build prior to the Trixie release.

The function that installs the firmware prior to Trixie doesn't blacklist unneeded firmware packages depending on the architecture. So when you run the installer from the ISO it installs all firmware packages regardless of architecture. This could have been mitigated by removing the raspi-firmware at the final stage of the installer by adding a line to preseed that instructs the installer to remove the raspi-firmware package. Basically what a user is needs to do when the error pops it's head up during an update.
On my bookworm machine instead of using the package that is in the bookworm repo I built a live-build package on bookworm using the Trixie sources. An extra line is added to the config, indicating that it's amd64 we're building an ISO for and the "blacklist function" kicks in and the raspi-firmware (and some others unrelated to amd64) are never installed when the installer is run from the ISO.

So to conclude:
It comes down to the version of live-build being used.
Bookworm version of live-build is (1:20230502), this causes the issue.
Trixie version of live-build is (1:20250505+deb13u1), this has no issues as long as --architectures amd64 is in the config file used by live build.
To install live-build (1:20250505+deb13u1) on Bookworm you need to build the package from source and install the package you just built. Also the additional line in the config file.

Building an ISO of a machine running Trixie that was successful gave me an idea as what the issue maybe. Installing the package for Trixie on Bookworm was no help, it was required to build from source.

I pretty much describe the issue in this post:
forum.linuxcnc.org/9-installing-linuxcnc...-iso?start=20#336448

unknown wrote:

I've tried looking for a RT_PREEMPT kernel in the repos, but could only find kernels that were configured for Real time not Preemptive real time (which is what Linuxcnc requires).

the packages are on archive.raspberrypi.org . you want linux-image-6.12.34+rpt-rpi-v8-rt, it works for both RPi 4 and 5. there's a meta package linux-image-rpi-v8-rt that depends on the latest version of the RT kernel, but it gives you auto-updates with apt, which is not necessarily a good idea (e.g. the currently-latest 6.12.47 is hanging my Pi5 sometimes).

it is a proper PREEMPT_RT kernel:

in order for bootloader to use it, you need to add kernel=kernel8_rt.img to /boot/firmware/config.txt.

the kernel configs themselves are here .