Requirements for FDT load address on ARM

[Nicolas Pitre]

On Wed, 24 Jul 2013, Ian Campbell wrote:

> [apologies if this is a repeat, I don't see it in the archives and at
> least one CC'd recipient (Julien) didn't receive it either. Beasties in
> our email server I expect...]
> 
> Are there any requirements about where the FDT may be loaded by the
> bootloader?

It must be located where it is unlikely to be overwritten by the zImage 
decompressor relocating itself.  Same goes for the initrd.

> I'm finding that if I load it "too high" I get a fault (the specifics
> aren't that relevant but details are below).

Do you have commit 6f16f4998f98 in your kernel tree?

> FWIW with RAM from 0x8000000-0xffffffff and a kernel loaded at 
> 0x80008000 "too high" means above somewhere around 0xaf800000.

Sorry, I don't understand the above sentence.

Is RAM actually starting at 0x08000000 ?  Note that the kernel must be 
loaded in the first 128MB of RAM.

> I expect this is because the kernel doesn't use an explicit mapping of
> the fdt but expects it to be within the RAM directly mapped by the
> lowmem area (i.e. it uses phys_to_virt, since day one aka 93c02ab40ae6
> "arm/dt: probe for platforms via the device tree").

The fdt is expected to be located in lowmem.  So yes, that's actually a 
restriction.


> This effectively
> limits the FDT to being within the first ~700MB (give or take based
> on .config etc, I suppose <500MB is a pretty safe rule of thumb). Is
> this expected or is it actually not intentional and the kernel should
> really be using an explicit mapping here? Should I send a patch against
> Documentation/arm/Booting?

Yes please.


Nicolas