MIPS: Replace some magic numbers with symbolic values in tlbex.c

The logic used to split the r4000 refill handler is liberally
sprinkled with magic numbers.  We attempt to explain what they are and
normalize them against a new symbolic value (MIPS64_REFILL_INSNS).

CC: David VomLehn <dvomlehn@cisco.com>
Reviewed-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/tlbex.c

@@ -649,6 +649,14 @@ static void __cpuinit build_update_entries(u32 **p, unsigned int tmp,
 #endif
 }
 
+/*
+ * For a 64-bit kernel, we are using the 64-bit XTLB refill exception
+ * because EXL == 0.  If we wrap, we can also use the 32 instruction
+ * slots before the XTLB refill exception handler which belong to the
+ * unused TLB refill exception.
+ */
+#define MIPS64_REFILL_INSNS 32
+
 static void __cpuinit build_r4000_tlb_refill_handler(void)
 {
 	u32 *p = tlb_handler;
@@ -702,9 +710,10 @@ static void __cpuinit build_r4000_tlb_refill_handler(void)
 	if ((p - tlb_handler) > 64)
 		panic("TLB refill handler space exceeded");
 #else
-	if (((p - tlb_handler) > 63)
-	    || (((p - tlb_handler) > 61)
-		&& uasm_insn_has_bdelay(relocs, tlb_handler + 29)))
+	if (((p - tlb_handler) > (MIPS64_REFILL_INSNS * 2) - 1)
+	    || (((p - tlb_handler) > (MIPS64_REFILL_INSNS * 2) - 3)
+		&& uasm_insn_has_bdelay(relocs,
+					tlb_handler + MIPS64_REFILL_INSNS - 3)))
 		panic("TLB refill handler space exceeded");
 #endif
 
@@ -717,16 +726,24 @@ static void __cpuinit build_r4000_tlb_refill_handler(void)
 	uasm_copy_handler(relocs, labels, tlb_handler, p, f);
 	final_len = p - tlb_handler;
 #else /* CONFIG_64BIT */
-	f = final_handler + 32;
-	if ((p - tlb_handler) <= 32) {
+	f = final_handler + MIPS64_REFILL_INSNS;
+	if ((p - tlb_handler) <= MIPS64_REFILL_INSNS) {
 		/* Just copy the handler. */
 		uasm_copy_handler(relocs, labels, tlb_handler, p, f);
 		final_len = p - tlb_handler;
 	} else {
-		u32 *split = tlb_handler + 30;
+		/*
+		 * Split two instructions before the end.  One for the
+		 * branch and one for the instruction in the delay
+		 * slot.
+		 */
+		u32 *split = tlb_handler + MIPS64_REFILL_INSNS - 2;
 
 		/*
-		 * Find the split point.
+		 * Find the split point.  If the branch would fall in
+		 * a delay slot, we must back up an additional
+		 * instruction so that it is no longer in a delay
+		 * slot.
 		 */
 		if (uasm_insn_has_bdelay(relocs, split - 1))
 			split--;
@@ -749,7 +766,8 @@ static void __cpuinit build_r4000_tlb_refill_handler(void)
 
 		/* Copy the rest of the handler. */
 		uasm_copy_handler(relocs, labels, split, p, final_handler);
-		final_len = (f - (final_handler + 32)) + (p - split);
+		final_len = (f - (final_handler + MIPS64_REFILL_INSNS)) +
+			    (p - split);
 	}
 #endif /* CONFIG_64BIT */