-#define _PAGE_WT 0x001 /* WT-bit on SH-4, 0 on SH-3 */
-#define _PAGE_HW_SHARED 0x002 /* SH-bit : page is shared among processes */
-#define _PAGE_DIRTY 0x004 /* D-bit : page changed */
-#define _PAGE_CACHABLE 0x008 /* C-bit : cachable */
-#define _PAGE_SZ0 0x010 /* SZ0-bit : Size of page */
-#define _PAGE_RW 0x020 /* PR0-bit : write access allowed */
-#define _PAGE_USER 0x040 /* PR1-bit : user space access allowed */
-#define _PAGE_SZ1 0x080 /* SZ1-bit : Size of page (on SH-4) */
-#define _PAGE_PRESENT 0x100 /* V-bit : page is valid */
-#define _PAGE_PROTNONE 0x200 /* software: if not present */
-#define _PAGE_ACCESSED 0x400 /* software: page referenced */
-#define _PAGE_U0_SHARED 0x800 /* software: page is shared in user space */
-
-#define _PAGE_FILE _PAGE_WT /* software: pagecache or swap? */
+/*
+ * Linux PTEL encoding.
+ *
+ * Hardware and software bit definitions for the PTEL value:
+ *
+ * - Bits 0 and 7 are reserved on SH-3 (_PAGE_WT and _PAGE_SZ1 on SH-4).
+ *
+ * - Bit 1 is the SH-bit, but is unused on SH-3 due to an MMU bug (the
+ * hardware PTEL value can't have the SH-bit set when MMUCR.IX is set,
+ * which is the default in cpu-sh3/mmu_context.h:MMU_CONTROL_INIT).
+ *
+ * In order to keep this relatively clean, do not use these for defining
+ * SH-3 specific flags until all of the other unused bits have been
+ * exhausted.
+ *
+ * - Bit 9 is reserved by everyone and used by _PAGE_PROTNONE.
+ *
+ * - Bits 10 and 11 are low bits of the PPN that are reserved on >= 4K pages.
+ * Bit 10 is used for _PAGE_ACCESSED, bit 11 remains unused.
+ *
+ * - Bits 31, 30, and 29 remain unused by everyone and can be used for future
+ * software flags, although care must be taken to update _PAGE_CLEAR_FLAGS.
+ */
+#define _PAGE_WT 0x001 /* WT-bit on SH-4, 0 on SH-3 */
+#define _PAGE_HW_SHARED 0x002 /* SH-bit : shared among processes */
+#define _PAGE_DIRTY 0x004 /* D-bit : page changed */
+#define _PAGE_CACHABLE 0x008 /* C-bit : cachable */
+#define _PAGE_SZ0 0x010 /* SZ0-bit : Size of page */
+#define _PAGE_RW 0x020 /* PR0-bit : write access allowed */
+#define _PAGE_USER 0x040 /* PR1-bit : user space access allowed */
+#define _PAGE_SZ1 0x080 /* SZ1-bit : Size of page (on SH-4) */
+#define _PAGE_PRESENT 0x100 /* V-bit : page is valid */
+#define _PAGE_PROTNONE 0x200 /* software: if not present */
+#define _PAGE_ACCESSED 0x400 /* software: page referenced */
+#define _PAGE_FILE _PAGE_WT /* software: pagecache or swap? */