RPI support / Install hook

[kfox1111]

I've been using bootc with RPI5's for a bit over a year now. I love bootc for managing my PI images. I dont want to manage my PI's any other way anymore.

But there are still some painful rough edges.

The built in bootloader in the PI doesn't support UEFI. There seem to be 2 chain loaders that can get you to an UEFI environment enough to get grub going. But one went unsupported, and one takes a lot of patching and only supports some of the RPI5 variants so far. So it can be hit or miss.

In addition, the PI likes to (and kind of needs to) assemble its own device tree from overlays. So even when the UEFI chain loading of the kernel is worked out, the dtb's are a pain to maintain currently. They are using it for some of their board variants now. so even getting some PI's to boot requires overlays.

So, I'd like the ability to work with the RPI5's bootloader rather then work against it.

How?

I think maybe I need a post install hook to run after the image is updated that can look at the grub files, generate an updated config.txt, and make a copy of the dtb/overlay dir to the right places on the rpi boot partition.

This would also enable some special features of the pi bootloader that would be really useful to unlock.

The first is Tryboot. The bootloader can be setup to try to boot a particular os. If it fails, the next boot after that will revert to the known good one. This would allow bootc switch to automatically rollback if it fails to boot properly. This would be extremely useful for headless PI nodes. It can even be used in conjunction with the built in watchdog timer to ensure if the os boot is unresponsive enough for long enough, it automatically reverts.

Another is hardware based bootc image selection. Its possible to wire a button to one of the gpio pins of a PI and affect the config.txt's settings based on if the button is pressed or not. This would allow booting the previous image by just holding down a button during powerup, without needing a screen/keyboard or a serial to usb adapter and laptop to select a grub menu entry. Again, making for a cheep headless PI node setup.

What can we do to unlock this functionality?

[kfox1111]

Some more thoughts on how this could be implemented

we should copy the device tree/overlay stuff from the bootc image to /boot/loader.0/ostree-1 or ostree-2. This allows the entire device tree and overlay structure to come from the image and be selected as part of the OS. This prevents the wrong device trees from being loaded when switching images back and forth. This also allows the rpi firmware to assemble the final dtb with overlay logic selection that it has built in, to automatically load the correct overlays for model variants that is needed for some boards.

In /boot/config.txt, use the os_prefix option to specify either loader.0/ostree-1 or 2.

Setup a gpio pin in the config for image selection:

os_prefix=loader.0/ostree-1
[gpio21=0]
os_prefix=loader.0/ostree-2

Then, holding the button during boot will cause it to go to the previous image.

If the user wants to setup a tryboot session, we can:

1. remove the gpio21=0 section from config.txt. copy config.txt to tryboot.txt. set ostree-1 in tryboot and ostree-2 in config.txt
2. if the user wants to automatically fail back to the current image, ensure the current image and new image both has watchdog support in it and enabled, and config.txt/tryboot.txt has dtparam=watchdog=on
3. the user can then reboot the system with the tryboot flag reboot tryboot to attempt to use the new image
4. rebooting again will go back to the previous image. If the new images watchdog support doesnt come up in time, and watchdog enabled, it will automatically happen.