+ *
+ *
+ * PPC 440 core has following TLB attribute fields;
+ *
+ * TLB1:
+ * 0 1 2 3 4 ... 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
+ * RPN................................. - - - - - - ERPN.......
+ *
+ * TLB2:
+ * 0 1 2 3 4 ... 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
+ * - - - - - - U0 U1 U2 U3 W I M G E - UX UW UR SX SW SR
+ *
+ * There are some constrains and options, to decide mapping software bits
+ * into TLB entry.
+ *
+ * - PRESENT *must* be in the bottom three bits because swap cache
+ * entries use the top 29 bits for TLB2.
+ *
+ * - FILE *must* be in the bottom three bits because swap cache
+ * entries use the top 29 bits for TLB2.
+ *
+ * - CACHE COHERENT bit (M) has no effect on PPC440 core, because it
+ * doesn't support SMP. So we can use this as software bit, like
+ * DIRTY.
+ *
+ * With the PPC 44x Linux implementation, the 0-11th LSBs of the PTE are used
+ * for memory protection related functions (see PTE structure in
+ * include/asm-ppc/mmu.h). The _PAGE_XXX definitions in this file map to the
+ * above bits. Note that the bit values are CPU specific, not architecture
+ * specific.
+ *
+ * The kernel PTE entry holds an arch-dependent swp_entry structure under
+ * certain situations. In other words, in such situations some portion of
+ * the PTE bits are used as a swp_entry. In the PPC implementation, the
+ * 3-24th LSB are shared with swp_entry, however the 0-2nd three LSB still
+ * hold protection values. That means the three protection bits are
+ * reserved for both PTE and SWAP entry at the most significant three
+ * LSBs.
+ *
+ * There are three protection bits available for SWAP entry:
+ * _PAGE_PRESENT
+ * _PAGE_FILE
+ * _PAGE_HASHPTE (if HW has)
+ *
+ * So those three bits have to be inside of 0-2nd LSB of PTE.
+ *