Change in coreboot[master]: payloads/external/tianocore: Add build argument for 4G Decode

[Nico Huber (Code Review)]

Attention is currently required from: Sean Rhodes, Martin Roth, Matt DeVillier, Andy Pont, Paul Menzel, Stefan Reinauer, Angel Pons.
Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/56223 )

Change subject: payloads/external/tianocore: Add build argument for 4G Decode
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Patch Set 19:

(1 comment)

File payloads/external/tianocore/Kconfig:

https://review.coreboot.org/c/coreboot/+/56223/comment/4bcce950_49878460 
PS16, Line 86: 	bool "Avoid above 4G memory"
             : 	default n
             : 	help
             : 	  Select this option to not reserve memory above 4G. This will allow
             : 	  the payload to use all of the memory.
> > I still don't fully understand that tianocore patch btw. I wonder if
> > it only worked by luck without that HOB? Is it actually the
> > `EfiMaxMemoryType, 0` that makes it avoid additional memory? Also,
> > if the HOB only gives numbers of pages, isn't it just luck again
> > that tianocore will choose pages below 4G?
>
> No, the HOB is needed.

How does it work if Above4GDecode() is not called? What is
providing the HOB in that case?
> > I've searched around a bit and still can't find any trace of anybody calling
> > it like this. Some similar tianocore patches by Intel showed up and they don't
> > mention any "decoding". Am I just unlucky?
>
> Guess so, that search term is how we got here!

It's not that there are no hits. Just that there is a forest
of results about actual decoding (in the notion I'm used to)
and nothing related to the problem at hand :-/

That there are so many BIOSes that use "Above 4G decoding" in
a very different sense will most likely confuse people.