powerpc/8xx: remove unused _PAGE_WRITETHRU

_PAGE_WRITETHRU is only used in:
* AMIGA_Z2RAM block driver which is never activated on powerPC
* Video/FB driver which is for PPC_PMAC

Therefore, no need to spend time in 8xx TLB miss handlers for
handling it.

And by removing it, we free up bit 20 which then avoids having
to clear it on each TLB miss.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/nohash/32/pte-8xx.h

@@ -41,8 +41,7 @@
 #define _PAGE_GUARDED	0x0010	/* Copied to L1 G entry in DTLB */
 #define _PAGE_USER	0x0020	/* Copied to L1 APG lsb */
 #define _PAGE_EXEC	0x0040	/* Copied to L1 APG */
-#define _PAGE_WRITETHRU	0x0080	/* software: caching is write through */
-#define _PAGE_ACCESSED	0x0800	/* software: page referenced */
+#define _PAGE_ACCESSED	0x0080	/* software: page referenced */
 
 #define _PAGE_RO	0x0600	/* Supervisor RO, User no access */
 

arch/powerpc/include/asm/nohash/pgtable.h

@@ -235,8 +235,10 @@ extern int ptep_set_access_flags(struct vm_area_struct *vma, unsigned long addre
 #define pgprot_cached(prot)       (__pgprot((pgprot_val(prot) & ~_PAGE_CACHE_CTL) | \
 				            _PAGE_COHERENT))
 
+#if _PAGE_WRITETHRU != 0
 #define pgprot_cached_wthru(prot) (__pgprot((pgprot_val(prot) & ~_PAGE_CACHE_CTL) | \
 				            _PAGE_COHERENT | _PAGE_WRITETHRU))
+#endif
 
 #define pgprot_cached_noncoherent(prot) \
 		(__pgprot(pgprot_val(prot) & ~_PAGE_CACHE_CTL))

arch/powerpc/kernel/head_8xx.S

@@ -486,10 +486,6 @@ _ENTRY(DTLBMiss_jmp)
 	 * above.
 	 */
 	rlwimi	r11, r10, 0, 26, 27
-	/* Insert the WriteThru flag into the TWC from the Linux PTE.
-	 * It is bit 25 in the Linux PTE and bit 30 in the TWC
-	 */
-	rlwimi	r11, r10, 32-5, 30, 30
 	mtspr	SPRN_MD_TWC, r11
 
 	/* In 4k pages mode, SPS (bit 28) in RPN must match PS[1] (bit 29)
@@ -523,7 +519,6 @@ _ENTRY(DTLBMiss_jmp)
 #else
 	rlwimi	r10, r11, 0, 24, 28	/* Set 24-27, clear 28 */
 #endif
-	rlwimi	r10, r11, 0, 20, 20	/* clear 20 */
 	mtspr	SPRN_MD_RPN, r10	/* Update TLB entry */
 
 	/* Restore registers */