On Mon, Oct 5, 2020 at 4:22 PM Ard Biesheuvel [off-list ref] wrote:

I think the problem here may be that early_init_fdt_reserve_self()
uses the wrong translation for obtaining the PA of the device tree
blob. This is also the reason we no longer use it on arm64 IIRC.

When I add the following on top, everything works as expected with
MT_ROM, including the ability to dump the firmware provided DT from
/sys/firmware/fdt

(...)

-       if (atags_vaddr)
+       if (atags_vaddr) {
                mdesc = setup_machine_fdt(atags_vaddr);
+               if (mdesc)
+                       memblock_reserve(__atags_pointer,
fdt_totalsize(atags_vaddr));

I tested with this too and it works fine, and makes perfect sense,
it should protect the physical memory used by the DT even if
that happens to be in lowmem.

Please fold this into the patch! (Reviewed/Tested-by).

If we go for copying and unflattening the
devicetree instead of just unflattning it the memblock should
be removed, which is nice and clean to my OCD because
then we don't have this random "hole" in the memory, but I
suspect then we should also drop the virtual mapping and
that seems like it could be hard.

Yours,
Linus Walleij

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