On Sat, Mar 10, 2018 at 6:34 AM, Andy Rea andy@azureussolutions.com wrote:

I have an ARM CPU on a board that has 3 optional sub boards plugged into it, all of which have 3 hardware rev bits connected to the CPU

Rather than getting the CPU to do mmap type stuff or sys/class/gpio stuff to read those versions in the kernel, is there a way for u-boot to push that information ( even it's a U32 or a string ) into the kernel somewhere that can be parsed? I tried some code in the board startup to create env vars to append into the bootargs but they aren't evaluated each boot properly ( boards are optional and revs can change ) so cat /proc/cmdlne stayed the same value all the time

Is there a better way to do this?

I've done something similar in the past by having u-boot manipulate the device-tree passed to the kernel with a few calls to fdt_setprop(). It's practical when you have a few tweaks to make. I've used it for telling the kernel to disable bits of hardware on prototype boards.

For more substantial configurations you could use a FIT image[1]. I've also used this for having u-boot tell the kernel about plug-in boards. Ultimately we ended up switching to device-tree overlays once the kernel support caught up so that we could support hot-plugging boards.

[1] - http://git.denx.de/?p=u-boot.git;a=blob_plain;f=doc/uImage.FIT/howto.txt;hb=...