[U-Boot] [PATCH v2 5/6] sunxi: add code for recalculating the DRAM size in U-Boot

[Maxime Ripard]

On Tue, Apr 03, 2018 at 11:13:17AM +0100, Andre Przywara wrote:
> >>>> For hacking it, see my implementation in v1, which assumes the
> >>>> only size supported bigger than 2GiB is 3GiB (which is
> >>>> acceptable on sunxi, but might not work on other platforms).
> >>>>
> >>>> As Andre said, that function has another big problem -- it detects
> >>>> memory with writing to it. This is risky.
> >>>
> >>> How is it risky when it's done by the SPL?
> >>
> >> Originally that was my confusion as well: It's not the SPL calling that
> >> function. The DRAM controller init function in there knows very
> >> precisely how much DRAM we have, but we don't communicate this to U-Boot
> >> proper. So U-Boot *proper* goes ahead and probes the DRAM. This means it
> >> could step into secure memory, for instance. On sunxi64 we have the ATF
> >> running between SPL and U-Boot, also all kind of secure payloads could
> >> already have been registered.
> >> So I wonder if it would be easier to somehow pass on this *one* word of
> >> information between SPL and U-Boot proper to avoid calling this function
> >> altogether?
> > 
> > That would definitely make sense yes.
> 
> So since the SPL loads the DT anyway (from the FIT image) and puts it at
> the end of the U-Boot (proper) binary, wouldn't it be the easiest to
> just patch the actual DRAM size in there?
> IIRC we don't have any FDT write code in the SPL at the moment, and
> pulling it in would probably push it over the edge again, but:

That assumes that you are loading a FIT image, which might or might
not be the case, and on most arm32 chips, most likely won't.

I guess we'd need to find another way (put some information in an
SRAM somewhere?) to try to do that for all variants.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com
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