[PATCH 3/6] alpha: clarify that _PAGE_URE and _PAGE_UWE are the Executive bits

[Matt Turner]

The Alpha Architecture Reference Manual defines eight protection bits in
PTE<15:8>: KRE, ERE, SRE, URE, KWE, EWE, SWE and UWE, for Kernel,
Executive, Supervisor and User Read and Write Enable. Linux uses only
two of the four privilege modes, and its user processes run in Executive
mode, so the bits it calls _PAGE_URE and _PAGE_UWE are in fact the
architecture's ERE (bit 9) and EWE (bit 13). The true URE and UWE bits,
11 and 15, are unused.

The names have been misleading since the beginning, and the comments on
those defines said only "xxx". Spell out what the bits actually are. No
functional change.

Signed-off-by: Matt Turner <mattst88@xxxxxxxxx>
---
 arch/alpha/include/asm/pgtable.h | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/alpha/include/asm/pgtable.h b/arch/alpha/include/asm/pgtable.h
index 8a175e0c2b42..d123fd0d4091 100644
--- a/arch/alpha/include/asm/pgtable.h
+++ b/arch/alpha/include/asm/pgtable.h
@@ -65,10 +65,18 @@ struct vm_area_struct;
 #define _PAGE_FOW	0x0004	/* used for page protection (fault on write) */
 #define _PAGE_FOE	0x0008	/* used for page protection (fault on exec) */
 #define _PAGE_ASM	0x0010
-#define _PAGE_KRE	0x0100	/* xxx - see below on the "accessed" bit */
-#define _PAGE_URE	0x0200	/* xxx */
-#define _PAGE_KWE	0x1000	/* used to do the dirty bit in software */
-#define _PAGE_UWE	0x2000	/* used to do the dirty bit in software */
+/*
+ * The Alpha AXP ARM defines 8 protection bits in PTE[15:8]: KRE, ERE, SRE,
+ * URE, KWE, EWE, SWE, UWE (Kernel/Executive/Supervisor/User Read/Write
+ * Enable). Linux only uses two privilege modes: kernel (mode 0) and user.
+ * User processes run in Executive mode (mode 1), so _PAGE_URE and _PAGE_UWE
+ * correspond to the architecture's ERE (bit 9) and EWE (bit 13) bits. The
+ * true User Read/Write Enable bits (bits 11 and 15) are unused.
+ */
+#define _PAGE_KRE	0x0100	/* Kernel Read Enable (bit 8) */
+#define _PAGE_URE	0x0200	/* Executive Read Enable (bit 9); "user" in Linux */
+#define _PAGE_KWE	0x1000	/* Kernel Write Enable (bit 12) */
+#define _PAGE_UWE	0x2000	/* Executive Write Enable (bit 13); "user" in Linux */
 
 /* .. and these are ours ... */
 #define _PAGE_DIRTY	0x20000

-- 
2.54.0