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

[Andre Przywara]

Hi,

On 03/04/18 12:34, Maxime Ripard wrote:
> 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.

That's true, but my understanding is that this >4GB is only relevant for
64-bit SoCs, where we mandate FIT images, don't we?

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

Oh, so your aim at getting rid of the call to the memory probing
function at all? I was just assuming that we change it for the purpose
of this 3GB support, which would be 64-bit only?

Cheers,
Andre