powerpc/mm: Lower the max real address to 53 bits

Max value supported by hardware is 51 bits address. Radix page table define
a slot of 57 bits for future expansion. We restrict the value supported in
linux kernel 53 bits, so that we can use the bits between 57-53 for storing
hash linux page table bits. This is done in the next patch.

This will free up the software page table bits to be used for features
that are needed for both hash and radix. The current hash linux page table
format doesn't have any free software bits. Moving hash linux page table
specific bits to top of RPN field free up the software bits for other purpose.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Reviewed-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -39,6 +39,30 @@
 #define _RPAGE_RPN0		0x01000
 #define _RPAGE_RPN1		0x02000
 
+/* Max physical address bit as per radix table */
+#define _RPAGE_PA_MAX		57
+
+/*
+ * Max physical address bit we will use for now.
+ *
+ * This is mostly a hardware limitation and for now Power9 has
+ * a 51 bit limit.
+ *
+ * This is different from the number of physical bit required to address
+ * the last byte of memory. That is defined by MAX_PHYSMEM_BITS.
+ * MAX_PHYSMEM_BITS is a linux limitation imposed by the maximum
+ * number of sections we can support (SECTIONS_SHIFT).
+ *
+ * This is different from Radix page table limitation above and
+ * should always be less than that. The limit is done such that
+ * we can overload the bits between _RPAGE_PA_MAX and _PAGE_PA_MAX
+ * for hash linux page table specific bits.
+ *
+ * In order to be compatible with future hardware generations we keep
+ * some offsets and limit this for now to 53
+ */
+#define _PAGE_PA_MAX		53
+
 #define _PAGE_SOFT_DIRTY	_RPAGE_SW3 /* software: software dirty tracking */
 #define _PAGE_SPECIAL		_RPAGE_SW2 /* software: special page */
 
@@ -51,10 +75,11 @@
  */
 #define _PAGE_NO_CACHE		_PAGE_TOLERANT
 /*
- * We support 57 bit real address in pte. Clear everything above 57, and
- * every thing below PAGE_SHIFT;
+ * We support _RPAGE_PA_MAX bit real address in pte. On the linux side
+ * we are limited by _PAGE_PA_MAX. Clear everything above _PAGE_PA_MAX
+ * and every thing below PAGE_SHIFT;
  */
-#define PTE_RPN_MASK	(((1UL << 57) - 1) & (PAGE_MASK))
+#define PTE_RPN_MASK	(((1UL << _PAGE_PA_MAX) - 1) & (PAGE_MASK))
 /*
  * set of bits not changed in pmd_modify. Even though we have hash specific bits
  * in here, on radix we expect them to be zero.