Re: [PATCH] Start arch/powerpc/boot code reorganization

[Segher Boessenkool <segher@kernel.crashing.org>]

> It was pointed out on IRC that the "address" property is defined in  
> the
> OF spec for specifying virtual address mappings.  This is exactly what
> we need in that it allows the zImage wrapper to use the fw defined  
> UART
> mapping, but the kernel still gets the real physical address later on.
> And other things that don't use zImage wrappers, like u-boot, can  
> simply
> ignore the "address" property defined within the UART node.

The "address" property has several problems.  An obvious one is that the
name is too generic.  A nastier one is that once you start making new  
MMU
mappings, you have to keep *all* old mappings, or blow away the mapping
for this "address" as well (you don't know the size of the mapping).   
And
a third problem is that it can only encode 32-bit virtual addresses.

Now, if it's only used for the very-early-debug UART console on machines
that cannot accesses physical addresses directly, in things like a boot-
wrapper that cannot be bothered to set up a MMU mapping themselves (and
there might be good reasons not to), and only when there is no client
interface (i.e., it uses the flat tree only); then it might be a  
reasonable
approach.  All alternatives I can think of have their own nasty  
problems.

So please comment the nastiness with a big "HACK HACK HACK" comment and
make sure it only ever gets used on systems where nothing better is
available, and all should be fine.


Segher