in case one wants to test boot a new kernel on a chromebook

using the native depthcharge/kpart mechanism without risking

to have a non working system in case the new kernel does not

boot properly, the following command can be used to instruct

the bootloader to boot once from the second kernel partition

and go back to the regular first kernel partition on the

next boot afterwards:

cgpt add -i 2 -S 0 -T 1 -P 15 /dev/mmcblk0

the above command sets the number of tries (-T) to boot from

the second partition (-i) to 1 and increases its boot

priority to 15 (-P), which is higher than the priority of

the default kernel partition 1 (10) - see "cgpt add --help"

for more info on those options. after the defined single

boot from kernel partition 2 depthcharge will reset the boot

parameters, so that the next boot will happen as usual from

kernel partition 1. of course to be able to boot from the

second kernel partition one has to install a proper kpart

format kernel to the second kernel partition via the dd

command.

for this to work well it is important to have a fixed root

filesystem defined in the kernel cmdline like

root=LABEL=xyz

or

root=/dev/xyz

this is the case for all the chromebook kernels used in

imagebuilder images using the dephcharge/kaprt meachnism

for booting (and in general for the other imagebuilder

images as well).

the relative root filesystem syntax used on chromebooks at

other places sometimes like

root=PARTUUID=%U/PARTNROFF=1

will not work, as it references the root partition relative

to the kpart partition, which does no longer work if the

kernel is booted from the kernel partition 2 instead of 1

(in the example above the root filesystem is expected at the

next partition after the kernel partition, i.e. offset=1)

for testing and debugging kernel topics it might be useful to

be able to see the kernel console output during boot. for

this a special suzyqable is required and the following should

be added to the kernel commandline when building the kpart

kernel image (after the already existing console= entry, as

only the last console= entry has the highest priority for

displaying the kernel boot log)

console=ttyS0,115200n8 earlycon