Re: [PATCH] [v3] powerpc/4xx: work around CHIP11 errata in a more PAGE_SIZE-friendly way

[Josh Boyer <jwboyer@linux.vnet.ibm.com>]

On Thu, 13 Nov 2008 07:44:56 +1100
Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> On Wed, 2008-11-12 at 09:11 -0600, Hollis Blanchard wrote:
> > Forget pages. The errata is about the last 256 bytes of physical
> > memory.
> > 
> > > I still find it a bit tricky to have memory nodes not aligned on
> > nice
> > > fat big boundaries tho.
> > 
> > I don't know what you're referring to. The patch I sent doesn't touch
> > memory nodes, so they are indeed still aligned on nice fat big
> > boundaries.
> 
> My last comment was about the approach of modifying the memory node.
> 
> > I don't think this is overengineering at all. We can't touch the last
> > 256 bytes, so we mark it reserved, and then we won't. Altering memory
> > nodes is far more complicated and error-prone.
> 
> But your approach is going to be painful for kexec which will have to
> duplicate that logic.
> 
> Again, why can't we just stick something in the kernel code that
> reserves the last page ? It could be in prom.c or it could be called by
> affected 4xx platforms by the platform code, whatever, but the reserve
> map isn't really meant for that and will not be passed over from kernel
> to kernel by kexec.

Again, because newer U-Boot is doing the fixup on memsize for us
already.  This is why it was done in the wrapper to begin with, since
it depends on the version of U-Boot that you happen to be using.

If you have a good idea on how to figure that out in-kernel, do the
fixup when needed, and not make people's eyes bleed, I'm all for it.

josh