MIPS: c-r4k.c: Fix the 74K D-cache alias erratum workaround

Fix the 74K D-cache alias erratum workaround so that it actually works.
Our current code sets MIPS_CACHE_VTAG for the D-cache, but that flag
only has any effect for the I-cache.  Additionally MIPS_CACHE_PINDEX is
set for the D-cache if CP0.Config7.AR is also set for an affected
processor, leading to confusing information in the bootstrap log (the
flag isn't used beyond that).

So delete the setting of MIPS_CACHE_VTAG and rely on MIPS_CACHE_ALIASES,
set in a common place, removing I-cache coherency issues seen in GDB
testing with software breakpoints, gdbserver and ptrace(2), on affected
systems.

While at it add a little piece of explanation of what CP0.Config6.SYND
is so that people do not have to chase documentation.
Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8507/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/c-r4k.c

@@ -889,33 +889,39 @@ static inline void rm7k_erratum31(void)
 	}
 }
 
-static inline void alias_74k_erratum(struct cpuinfo_mips *c)
+static inline int alias_74k_erratum(struct cpuinfo_mips *c)
 {
 	unsigned int imp = c->processor_id & PRID_IMP_MASK;
 	unsigned int rev = c->processor_id & PRID_REV_MASK;
+	int present = 0;
 
 	/*
 	 * Early versions of the 74K do not update the cache tags on a
 	 * vtag miss/ptag hit which can occur in the case of KSEG0/KUSEG
 	 * aliases.  In this case it is better to treat the cache as always
-	 * having aliases.
+	 * having aliases.  Also disable the synonym tag update feature
+	 * where available.  In this case no opportunistic tag update will
+	 * happen where a load causes a virtual address miss but a physical
+	 * address hit during a D-cache look-up.
 	 */
 	switch (imp) {
 	case PRID_IMP_74K:
 		if (rev <= PRID_REV_ENCODE_332(2, 4, 0))
-			c->dcache.flags |= MIPS_CACHE_VTAG;
+			present = 1;
 		if (rev == PRID_REV_ENCODE_332(2, 4, 0))
 			write_c0_config6(read_c0_config6() | MIPS_CONF6_SYND);
 		break;
 	case PRID_IMP_1074K:
 		if (rev <= PRID_REV_ENCODE_332(1, 1, 0)) {
-			c->dcache.flags |= MIPS_CACHE_VTAG;
+			present = 1;
 			write_c0_config6(read_c0_config6() | MIPS_CONF6_SYND);
 		}
 		break;
 	default:
 		BUG();
 	}
+
+	return present;
 }
 
 static void b5k_instruction_hazard(void)
@@ -939,6 +945,7 @@ static void probe_pcache(void)
 	struct cpuinfo_mips *c = &current_cpu_data;
 	unsigned int config = read_c0_config();
 	unsigned int prid = read_c0_prid();
+	int has_74k_erratum = 0;
 	unsigned long config1;
 	unsigned int lsize;
 
@@ -1247,7 +1254,7 @@ static void probe_pcache(void)
 
 	case CPU_74K:
 	case CPU_1074K:
-		alias_74k_erratum(c);
+		has_74k_erratum = alias_74k_erratum(c);
 		/* Fall through. */
 	case CPU_M14KC:
 	case CPU_M14KEC:
@@ -1262,7 +1269,7 @@ static void probe_pcache(void)
 		if (!(read_c0_config7() & MIPS_CONF7_IAR) &&
 		    (c->icache.waysize > PAGE_SIZE))
 			c->icache.flags |= MIPS_CACHE_ALIASES;
-		if (read_c0_config7() & MIPS_CONF7_AR) {
+		if (!has_74k_erratum && (read_c0_config7() & MIPS_CONF7_AR)) {
 			/*
 			 * Effectively physically indexed dcache,
 			 * thus no virtual aliases.
@@ -1271,7 +1278,7 @@ static void probe_pcache(void)
 			break;
 		}
 	default:
-		if (c->dcache.waysize > PAGE_SIZE)
+		if (has_74k_erratum || c->dcache.waysize > PAGE_SIZE)
 			c->dcache.flags |= MIPS_CACHE_ALIASES;
 	}