[ofa-general] Write combining on PPC64

[Jack Morgenstein]

Hi Shirley,

I was reviewing write-combining for the PPC on kernel 2.6.30, and noticed the following
in file arch/powerpc/include/asm/pgtable.h:

#define _PAGE_CACHE_CTL (_PAGE_COHERENT | _PAGE_GUARDED | _PAGE_NO_CACHE | \
                         _PAGE_WRITETHRU)
...
#define pgprot_noncached_wc(prot) (__pgprot((pgprot_val(prot) & ~_PAGE_CACHE_CTL) | \
                                            _PAGE_NO_CACHE))

It seems to me that this gives the same result as the fix I put in OFED 1.5 for the PPC:
(kernel_patches/fixes/mlx4_0010_add_wc.patch):

+pgprot_t pgprot_wc(pgprot_t _prot)
+{
+       return __pgprot((pgprot_val(_prot) | _PAGE_NO_CACHE) &
+                                    ~(pgprot_t)_PAGE_GUARDED);
+}

( That is, PAGE_NO_CACHE set, and _PAGE_GUARDED cleared).

Differences are that _PAGE_COHERENT and _PAGE_WRITETHRU are also cleared in pgprot_noncached_wc()
and the patch I put in is only for PPC64, not all PPCs.

Questions:
1. Can I use the pgprot_noncached_wc() macro to implement write-combining on all PPCs? (it is defined
   thus in arch/powerpc/include/asm/pgtable.h for all PPC platforms)

2. Is there any reason that there is no define:

    #define  pgprot_writecombine  pgprot_noncached_wc
   
   in file arch/powerpc/include/asm/pgtable.h  (so that the general
     pgprot_writecombine() call can be used for PPCs as well).

-Jack