Hi,
I've tried to play around with Device Tree overlays (.dtbo files):
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Docu...
First of all, that document refers to a non-existing Documentation/devicetree/dt-object-internal.txt. Could someone please fix that one way or another?
In my particular example I've tried to extend the &i2c1 and &gpio nodes of 4.11-rc5 arm64 broadcom/bcm2837-rpi-3-b.dts. The above documentation prominently claims that this can be done via target = <&foo> syntax, but U-Boot's fdt apply command fails for such a file. If instead I use the alternative target-path = "/soc/..." then it works just fine.
As mentioned in the very bottom of the documentation, resolution of phandle target references requires a __symbols__ node in the base .dtb. IIUC this is only generated when passing the -@ dtc command line flag.
At first I thought this were an issue with how we build the .dtb files in openSUSE [1], but by my reading of the kernel Makefiles not passing -@ in DTC_FLAGS or cmd_dtc, you should run into the exact same issue.
I could think of a few ARMv7-M systems where such DT bloat might be undesired (small flash sector sizes), but then it would seem easier to suppress -@ where needed than to have a feature that by all practical means is half unusable by default.
U-Boot itself appears to face a similar issue in that its internal Device Trees are built without -@, and via Alex' distro boot extensions this internal DT is passed on via UEFI as fallback when no external .dtb file is found. So in the non-SPL case the DT should probably be built with -@, too.
Or am I misunderstanding something here?
Thanks, Andreas
[1] https://build.opensuse.org/package/view_file/Kernel:HEAD/dtb-aarch64/dtb-aar...