powerpc/mm/64: Document the sizes of/sizes mapped by Pxx_INDEX_SIZE

Add comments describing the size in bytes of the various levels of the
page table tree, and the size of the virtual address space mapped by
each level, to make it clear what the sizes are without having to also
look up other definitions.

The code that calculates the sizes actually uses sizeof(pgd_t) etc.,
so in theory these comments could skew vs the code, but the size of
pgd_t etc. is unlikely to change very often.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/hash-4k.h

@@ -2,10 +2,10 @@
 #ifndef _ASM_POWERPC_BOOK3S_64_HASH_4K_H
 #define _ASM_POWERPC_BOOK3S_64_HASH_4K_H
 
-#define H_PTE_INDEX_SIZE  9
-#define H_PMD_INDEX_SIZE  7
-#define H_PUD_INDEX_SIZE  9
-#define H_PGD_INDEX_SIZE  9
+#define H_PTE_INDEX_SIZE  9  // size: 8B << 9 = 4KB, maps: 2^9 x   4KB =   2MB
+#define H_PMD_INDEX_SIZE  7  // size: 8B << 7 = 1KB, maps: 2^7 x   2MB = 256MB
+#define H_PUD_INDEX_SIZE  9  // size: 8B << 9 = 4KB, maps: 2^9 x 256MB = 128GB
+#define H_PGD_INDEX_SIZE  9  // size: 8B << 9 = 4KB, maps: 2^9 x 128GB =  64TB
 
 /*
  * Each context is 512TB. But on 4k we restrict our max TASK size to 64TB

arch/powerpc/include/asm/book3s/64/hash-64k.h

@@ -2,10 +2,11 @@
 #ifndef _ASM_POWERPC_BOOK3S_64_HASH_64K_H
 #define _ASM_POWERPC_BOOK3S_64_HASH_64K_H
 
-#define H_PTE_INDEX_SIZE  8
-#define H_PMD_INDEX_SIZE  10
-#define H_PUD_INDEX_SIZE  10
-#define H_PGD_INDEX_SIZE  8
+#define H_PTE_INDEX_SIZE   8  // size: 8B <<  8 = 2KB, maps 2^8  x 64KB = 16MB
+#define H_PMD_INDEX_SIZE  10  // size: 8B << 10 = 8KB, maps 2^10 x 16MB = 16GB
+#define H_PUD_INDEX_SIZE  10  // size: 8B << 10 = 8KB, maps 2^10 x 16GB = 16TB
+#define H_PGD_INDEX_SIZE   8  // size: 8B <<  8 = 2KB, maps 2^8  x 16TB =  4PB
+
 
 /*
  * Each context is 512TB size. SLB miss for first context/default context

arch/powerpc/include/asm/book3s/64/radix-4k.h

@@ -5,10 +5,11 @@
 /*
  * For 4K page size supported index is 13/9/9/9
  */
-#define RADIX_PTE_INDEX_SIZE  9  /* 2MB huge page */
-#define RADIX_PMD_INDEX_SIZE  9  /* 1G huge page */
-#define RADIX_PUD_INDEX_SIZE	 9
-#define RADIX_PGD_INDEX_SIZE  13
+#define RADIX_PTE_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x    4K =   2MB
+#define RADIX_PMD_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x   2MB =   1GB
+#define RADIX_PUD_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x   1GB = 512GB
+#define RADIX_PGD_INDEX_SIZE  13  // size: 8B << 13 = 64KB, maps 2^13 x 512GB =   4PB
+
 /*
  * One fragment per per page
  */

arch/powerpc/include/asm/book3s/64/radix-64k.h

@@ -5,10 +5,10 @@
 /*
  * For 64K page size supported index is 13/9/9/5
  */
-#define RADIX_PTE_INDEX_SIZE  5  /* 2MB huge page */
-#define RADIX_PMD_INDEX_SIZE  9  /* 1G huge page */
-#define RADIX_PUD_INDEX_SIZE	 9
-#define RADIX_PGD_INDEX_SIZE  13
+#define RADIX_PTE_INDEX_SIZE   5  // size: 8B <<  5 = 256B, maps 2^5  x   64K =   2MB
+#define RADIX_PMD_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x   2MB =   1GB
+#define RADIX_PUD_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x   1GB = 512GB
+#define RADIX_PGD_INDEX_SIZE  13  // size: 8B << 13 = 64KB, maps 2^13 x 512GB =   4PB
 
 /*
  * We use a 256 byte PTE page fragment in radix