sparc64: Add preprocessor symbols for PAGE_* pgprot_t values.

Kernel code assumes that the PAGE_* values are preprocessor symbols, and that therefore arch support can be checked for with #ifdef.

At the moment, sparc64 does not implement any of the symbols checked for, so these checks happen to work.

To prevent potential breakage when another #ifdef check is added or when sparc64 implements another PAGE_ value, make such #ifdef checks work by adding preprocessor symbols on top of the PAGE_* variables.

---
 arch/sparc/include/asm/pgtable_64.h |    4 ++++
 1 file changed, 4 insertions(+)

index bfeb626..a835fe9 100644
@@ -216,9 +216,13 @@ pte_t mk_pte_io(unsigned long, pgprot_t, int, unsigned long);
 unsigned long pte_sz_bits(unsigned long size);
 
 extern pgprot_t PAGE_KERNEL;
+#define PAGE_KERNEL		PAGE_KERNEL
 extern pgprot_t PAGE_KERNEL_LOCKED;
+#define PAGE_KERNEL_LOCKED	PAGE_KERNEL_LOCKED
 extern pgprot_t PAGE_COPY;
+#define PAGE_COPY		PAGE_COPY
 extern pgprot_t PAGE_SHARED;
+#define PAGE_SHARED		PAGE_SHARED
 
 /* XXX This uglyness is for the atyfb driver's sparc mmap() support. XXX */
 extern unsigned long _PAGE_IE;

From: Clemens Ladisch <[email protected]>
Date: Sun, 02 Nov 2014 21:15:12 +0100

"This change actually doesn't have any effect, and doesn't matter,
so let's make this change."

I really don't buy this.

I'd also rather the kernel use Kconfig based symbols to detect for arch availability of feature X or Y, assuming things are CPP symbols is very fragile at best.

I'm not applying this, sorry. I mean, you didn't even bother to mention what symbol this does matter for.

At the moment, sparc64 does not support symbols such as PAGE_KERNEL_RO, and has no mechanism for arch-independent code to detect this. Some code tries anyway:

$ git grep '#ifn\?def PAGE_KERNEL_'
drivers/base/firmware_class.c:#ifndef PAGE_KERNEL_RO
drivers/staging/comedi/comedi_buf.c:#ifdef PAGE_KERNEL_NOCACHE
mm/nommu.c:#ifndef PAGE_KERNEL_EXEC
mm/vmalloc.c:#ifndef PAGE_KERNEL_EXEC

I don't want to add more such code to the kernel without a guarantee that it actually works, or without replacing it with some other kind of check that does work.

It is certainly possible to use Kconfig-based symbols. However, this would have its own fragility: adding, e.g., PAGE_KERNEL_SO to an arch requires that one remembers to update Kconfig, and if one forgets, a check like this:

#ifndef CONFIG_ARCH_HAS_PAGE_KERNEL_RO
#define PAGE_KERNEL_RO PAGE_KERNEL /* fallback for this code */ #endif

will not detect the error on sparc64 (if PAGE_KERNEL_RO were a CPP symbol, the compiler would complain about the redefinition).

So even if PAGE_KERNEL_RO is a variable, making it into a CPP symbol is beneficial. And once it is a CPP symbol, introducing a separate Kconfig-based symbol is not necessary and just increases the chances of an error.

Regards,
Clemens

From: Clemens Ladisch <[email protected]>
Date: Mon, 03 Nov 2014 10:44:40 +0100

I'd rather code then use the symbol unconditionally, and we declare that it's something every architecture must provide in some shape or form.

This CPP check business is fragile as hell and I'm not going to promote it in the architectures I maintainer by applying patches like your's.

Sorry.