sparc64: Document the shift counts used to validate linear kernel addresses.

commit bb7b4353 upstream.

This way we can see exactly what they are derived from, and in particular
how they would change if we were to use a different PAGE_OFFSET value.
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Bob Picco <bob.picco@oracle.com>

arch/sparc/include/asm/page_64.h

@@ -121,6 +121,22 @@ typedef pte_t *pgtable_t;
 #define PAGE_OFFSET_BY_BITS(X)	(-(_AC(1,UL) << (X)))
 #define PAGE_OFFSET		PAGE_OFFSET_BY_BITS(MAX_SUPPORTED_PA_BITS)
 
+/* The "virtual" portion of PAGE_OFFSET, used to clip off the non-physical
+ * bits of a linear kernel address.
+ */
+#define PAGE_OFFSET_VA_BITS	(64 - MAX_SUPPORTED_PA_BITS)
+
+/* The actual number of physical memory address bits we support, this is
+ * used to size various tables used to manage kernel TLB misses.
+ */
+#define MAX_PHYS_ADDRESS_BITS	41
+
+/* These two shift counts are used when indexing sparc64_valid_addr_bitmap
+ * and kpte_linear_bitmap.
+ */
+#define ILOG2_4MB		22
+#define ILOG2_256MB		28
+
 #ifndef __ASSEMBLY__
 
 #define __pa(x)			((unsigned long)(x) - PAGE_OFFSET)

arch/sparc/kernel/ktlb.S

@@ -153,12 +153,12 @@ kvmap_dtlb_tsb4m_miss:
 	/* Clear the PAGE_OFFSET top virtual bits, shift
 	 * down to get PFN, and make sure PFN is in range.
 	 */
-	sllx		%g4, 21, %g5
+	sllx		%g4, PAGE_OFFSET_VA_BITS, %g5
 
 	/* Check to see if we know about valid memory at the 4MB
 	 * chunk this physical address will reside within.
 	 */
-	srlx		%g5, 21 + 41, %g2
+	srlx		%g5, PAGE_OFFSET_VA_BITS + MAX_PHYS_ADDRESS_BITS, %g2
 	brnz,pn		%g2, kvmap_dtlb_longpath
 	 nop
 
@@ -176,7 +176,7 @@ valid_addr_bitmap_patch:
 	or		%g7, %lo(sparc64_valid_addr_bitmap), %g7
 	.previous
 
-	srlx		%g5, 21 + 22, %g2
+	srlx		%g5, PAGE_OFFSET_VA_BITS + ILOG2_4MB, %g2
 	srlx		%g2, 6, %g5
 	and		%g2, 63, %g2
 	sllx		%g5, 3, %g5
@@ -189,9 +189,9 @@ valid_addr_bitmap_patch:
 2:	 sethi		%hi(kpte_linear_bitmap), %g2
 
 	/* Get the 256MB physical address index. */
-	sllx		%g4, 21, %g5
+	sllx		%g4, PAGE_OFFSET_VA_BITS, %g5
 	or		%g2, %lo(kpte_linear_bitmap), %g2
-	srlx		%g5, 21 + 28, %g5
+	srlx		%g5, PAGE_OFFSET_VA_BITS + ILOG2_256MB, %g5
 	and		%g5, (32 - 1), %g7
 
 	/* Divide by 32 to get the offset into the bitmask.  */

arch/sparc/mm/init_64.h

 #ifndef _SPARC64_MM_INIT_H
 #define _SPARC64_MM_INIT_H
 
+#include <asm/page.h>
+
 /* Most of the symbols in this file are defined in init.c and
  * marked non-static so that assembler code can get at them.
  */
 
-#define MAX_PHYS_ADDRESS	(1UL << 41UL)
+#define MAX_PHYS_ADDRESS	(1UL << MAX_PHYS_ADDRESS_BITS)
 #define KPTE_BITMAP_CHUNK_SZ		(256UL * 1024UL * 1024UL)
 #define KPTE_BITMAP_BYTES	\
 	((MAX_PHYS_ADDRESS / KPTE_BITMAP_CHUNK_SZ) / 4)