sh: Reworked swap cache entry encoding for SH-X2 MMU.

In the 64-bit PTE case there's no point in restricting the encoding
to the low bits of the PTE, we can instead bump all of this up to
the high 32 bits and extend PTE_FILE_MAX_BITS to 32, adopting the
same convention used by x86 PAE.

There's a minor discrepency between the number of bits used for the
swap type encoding between 32 and 64-bit PTEs, but this is unlikely
to cause any problem given the extended offset.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

include/asm-sh/pgtable.h

@@ -508,14 +508,48 @@ struct vm_area_struct;
 extern void update_mmu_cache(struct vm_area_struct * vma,
 			     unsigned long address, pte_t pte);
 
-/* Encode and de-code a swap entry */
 /*
+ * Encode and de-code a swap entry
+ *
+ * Constraints:
+ *	_PAGE_FILE at bit 0
+ *	_PAGE_PRESENT at bit 8
+ *	_PAGE_PROTNONE at bit 9
+ *
+ * For the normal case, we encode the swap type into bits 0:7 and the
+ * swap offset into bits 10:30. For the 64-bit PTE case, we keep the
+ * preserved bits in the low 32-bits and use the upper 32 as the swap
+ * offset (along with a 5-bit type), following the same approach as x86
+ * PAE. This keeps the logic quite simple, and allows for a full 32
+ * PTE_FILE_MAX_BITS, as opposed to the 29-bits we're constrained with
+ * in the pte_low case.
+ *
+ * As is evident by the Alpha code, if we ever get a 64-bit unsigned
+ * long (swp_entry_t) to match up with the 64-bit PTEs, this all becomes
+ * much cleaner..
+ *
  * NOTE: We should set ZEROs at the position of _PAGE_PRESENT
  *       and _PAGE_PROTNONE bits
  */
+#ifdef CONFIG_X2TLB
+#define __swp_type(x)			((x).val & 0x1f)
+#define __swp_offset(x)			((x).val >> 5)
+#define __swp_entry(type, offset)	((swp_entry_t){ (type) | (offset) << 5})
+#define __pte_to_swp_entry(pte)		((swp_entry_t){ (pte).pte_high })
+#define __swp_entry_to_pte(x)		((pte_t){ 0, (x).val })
+
+/*
+ * Encode and decode a nonlinear file mapping entry
+ */
+#define pte_to_pgoff(pte)		((pte).pte_high)
+#define pgoff_to_pte(off)		((pte_t) { _PAGE_FILE, (off) })
+
+#define PTE_FILE_MAX_BITS		32
+#else
 #define __swp_type(x)			((x).val & 0xff)
 #define __swp_offset(x)			((x).val >> 10)
-#define __swp_entry(type, offset) ((swp_entry_t) { (type) | ((offset) << 10) })
+#define __swp_entry(type, offset)	((swp_entry_t){(type) | (offset) <<10)})
+
 #define __pte_to_swp_entry(pte)		((swp_entry_t) { pte_val(pte) >> 1 })
 #define __swp_entry_to_pte(x)		((pte_t) { (x).val << 1 })
 
@@ -525,6 +559,7 @@ extern void update_mmu_cache(struct vm_area_struct * vma,
 #define PTE_FILE_MAX_BITS	29
 #define pte_to_pgoff(pte)	(pte_val(pte) >> 1)
 #define pgoff_to_pte(off)	((pte_t) { ((off) << 1) | _PAGE_FILE })
+#endif
 
 typedef pte_t *pte_addr_t;