fdt_high default value causes unbootable kernel due to misalignment

[Tim van der Staaij | Zign]

Hello,

When I was using U-Boot with a dart_6ul board I occasionally had fitimages that U-Boot would attempt to boot just fine, but the kernel wouldn't actually boot. Debugging found that this was correlated with the header length of the fitImage. Then I noticed this remark in doc/usage/environment.rst about fdt_high:

>If this is set to the special value 0xffffffff (32-bit machines) or
>0xffffffffffffffff (64-bit machines) then the fdt will not be copied at
>all on boot. For this to work it must reside in writable memory, have
>sufficient padding on the end of it for u-boot to add the information
>it needs into it, and the memory must be accessible by the kernel.
>This usage is strongly discouraged however as it also stops U-Boot
>from ensuring the device tree starting address is properly aligned
>and a misaligned tree will cause OS failures.

And indeed, fdt_high is set to 0xffffffff in the include/configs/dart_6ul.h that I was using. Since mkimage only guarantees 32-bit alignment of the device tree blob within the fitimage, and not the 64-bit alignment that Linux requires, this is basically a 50/50 coin flip. Something simple like adding a few characters to the description string in the fitimage header can make or break the alignment.

Personally I fixed this by removing the fdt_high altogether, which makes U-Boot copy the device tree to an appropriate location. I assume this only works in my situation because I only have half a GiB of RAM and therefore do not have any high memory. I can imagine my fix would be problematic for board variants with more RAM, the setting doesn't exist without a reason of course.

It looks like this issue is not limited to my board; the same usage of fdt_high can be found in many other board configs. I'm not going to suggest what the best fix is because I lack detailed knowledge, but I will pitch that perhaps omitting fdt_high by default is at least preferable to a value that is "strongly discouraged" and causes Linux boot failures somewhat arbitrarily.

Kind regards,

Tim