Building for MMU-less vexpress targets

[Russell King - ARM Linux]

On Tue, Nov 06, 2012 at 09:14:49PM +0000, Arnd Bergmann wrote:
> The other point is being able to build such a kernel, and this is what Will
> seems to be interested in more. We have made VEXPRESS depend on
> MULTIPLATFORM, which broke support for building a non-MMU vexpress kernel,
> and I think we should fix that. The two options are either to make
> vexpress be single-platform when building for !MMU, or to allow multiplatform
> kernels to be built without MMU support in principle. I think the second
> option is more logical and avoids complex Kconfig constructs.

The other thing here is... why does a platform which _was_ able to be
built in isolation from every other platform suddenly become incapable
of being so when they join the multiplatform conglomerate?  This just
sounds totally perverse and wrong.

Surely it should be: platforms _not_ yet converted to multiplatform
can't be selected with multi-platform support enabled?

So, maybe the _proper_ solution here is:

- change the big choice to be: config SINGLE_xxx
  - these select config MACH_FOO / PLAT_FOO / ARCH_FOO
  eg,
	config SINGLE_FOO
		bool "Support for foo platforms in single kernel"
		select MACH_FOO
- add a final option: config MULTIPLATFORM
- then add:

config MULTI_FOO
	bool "Include support for foo platforms"
	select MACH_FOO
	depends on MULTIPLATFORM || !MMU
...

config MACH_FOO
	bool

Now, we don't _have_ to have the single and multi variants if they aren't
appropriate for the platform, but we can cover all the cases: a platform
where it's part of the multi-platform kernel when built for MMU, but is
incapable of being a multi-platform kernel when built without MMU.

And we can do it without _too_ much Kconfig pain, and certainly without
having to delve into anything beyond arch/arm/Kconfig.

I'd suggest at that point we separate out this stuff into a separate
file - arch/arm/Kconfig.mach, which contains all the platform selection
stuff.