Fallback kernel.img possible in config.txt

Question

I have a raspberry with a customized OS. The image it should load is described in the config.txt on the boot partition. The image is called zImage and the line in the config.txt is:

kernel=zImage

I expect to update it from time to time by updating the whole zImage. Now I am wondering that if I accidentally upload a faulty new zImage.img which causes a faulty system start (no start or partial start). Is there a way that I can use a fallback zImage?

So my ideal boot procedure would be:

1. Raspberry checks if a certain file is present on the boot partion (for example fallback.txt).
2. If fallback.txt is present use the kernel image fallback.img
3. If not, use the zImage.img and as a first step write the file fallback.txt
4. Boot and perform some checks. If everything is ok, delete fallback.txt

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In this configuration I would always have a bootable system, even if I mess up the update of the system. I have been searching if this is possible but I only find procedures to boot through USB.

Is this possible?

Answer 1

If there's no

kernel=zImage

line in /boot/config.txt then your system will load kernel.img, kernel7.img or kernel7l.img (depending on the model). So the simple fix is to make

kernel=zImage

into a comment. You can do that on Windows or with the NOOBS recovery console.

#kernel=zImage

and it will boot normally (as long as you've got the original kernel and original matching /lib/modules/$(uname -r) folder that goes with it.

The other quick fix is a second bootable SDCard and a USB reader.

Answer 2

Modifying boot procedure is very difficult on Raspberry Pi because it is a custom solution for Raspberry Pi and deep integrated into the firmware. As far as I know there are parts closed source. You will find a description from the official site at Raspberry Pi boot modes. Have a look at Boot flow there.