realview compulsory enables CONFIG_ARM_PATCH_PHYS_VIRT on me

[Russell King - ARM Linux]

On Mon, Dec 05, 2011 at 05:08:12PM -0500, Nicolas Pitre wrote:
> On Mon, 5 Dec 2011, Rob Herring wrote:
> 
> > On 12/05/2011 03:22 PM, Linus Walleij wrote:
> > > Hi Nico,
> > > 
> > > now I'm lost, I discussed by boot problems on ARM PB1176
> > > with Marc and after some investigation I found this strange thing on
> > > the lastest rc:s since -rc1:
> > > 
> > > In arch/arm/Kconfig ARM_PATCH_PHYS_VIRT is forcibly
> > > disabled like so:
> > > 
> > > depends on !ARCH_REALVIEW || !SPARSEMEM
> > 
> > should be !ARCH_REALVIEW && !SPARSEMEM ?
> 
> Indeed.

No, it's correct.  None of you are understanding the logic.

If you don't understand it, draw up a truth table:

ARCH_REALVIEW	SPARSEMEM	result
n		n		y
n		y		y
y		n		y
y		y		n

And this clearly shows that if we have realview _and_ sparsemem enabled,
then we don't want P2V patching in place.  The reason for this is quite
simple - here's Realview's memory.h:

#ifdef CONFIG_SPARSEMEM

#define __phys_to_virt(phys)                                            \
        ((phys) >= 0x80000000 ? (phys) - 0x80000000 + PAGE_OFFSET2 :    \
         (phys) >= 0x20000000 ? (phys) - 0x20000000 + PAGE_OFFSET1 :    \
         (phys) + PAGE_OFFSET)

#define __virt_to_phys(virt)                                            \
         ((virt) >= PAGE_OFFSET2 ? (virt) - PAGE_OFFSET2 + 0x80000000 : \
          (virt) >= PAGE_OFFSET1 ? (virt) - PAGE_OFFSET1 + 0x20000000 : \
          (virt) - PAGE_OFFSET)

#endif  /* CONFIG_SPARSEMEM */

Or to put it another way: on Realview if sparsemem is enabled, then we
are incompatible with P2V (due to the non-linear translation) so we have
a dependency there to disable it.