Re: [PATCH] 2.4: export the symbol "mmu_cr4_features" for XFree86 DRM kernel drivers

[Calum Mackay <calum.mackay@sun.com>]

Christoph Hellwig wrote:
> Can you explain why they need it and why they magically need it
> because of vmap() ?

Sorry, of course.

[as Mitch has already pointed out privately] e.g the XFree86 DRM radeon 
Makefile includes this:

[xc/programs/Xserver/hw/xfree86/os-support/linux/drm/kernel/Makefile.linux]

	# Check for 4-argument vmap() in some 2.5.x and 2.4.x kernels
	VMAP := $(shell grep -A1 'vmap.*count,$$' 
$(LINUXDIR)/include/linux/vmalloc.h | grep -c prot)

	ifneq ($(VMAP),0)
	EXTRA_CFLAGS += -DVMAP_4_ARGS
	endif

Christoph's vmap() backport introduced this reference in 
linux/vmalloc.h, so the DRM module is now compiled with VMAP_4_ARGS defined.

The radeon kernel driver source includes (via drm_memory.h) 
linux/vmalloc.h, iff VMAP_4_ARGS is defined. The latter includes (on 
i386) asm/asm-i386/pgtable.h, which includes asm/asm-i386/processor.h.

processor.h defines some inline functions [set_in_cr4() & 
clear_in_cr4()] which reference mmu_cr4_features.

mmu_cr4_features is currently declared in arch/i386/kernel/setup.c [and 
also arch/x86_64/kernel/setup.c] but not currently exported.

[The problem is not restricted to the radeon kernel driver; I've just 
used it as an example]

cheers,
Calum.