MIPS: Tell R4k SC and MC variations apart

There is no reliable way to tell R4000/R4400 SC and MC variations apart,
however simple heuristic should give good results.  Only the MC version
supports coherent caching so we can rely on such a mode having been set
for KSEG0 by the power-on firmware to reliably indicate an MC processor.
SC processors reportedly hang on coherent cached memory accesses and Linux
is linked to a cached load address so the firmware has to use the correct
caching mode to download the kernel image in a cached mode successfully.

OTOH if the firmware chooses to use either the non-coherent cached or the
uncached mode for KSEG0 on an MC processor, then the SC variant will be
reported, just as we currently do, so no regression here.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Jonas Gorski <jogo@openwrt.org>
Cc: MIPS Mailing List <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/5882/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/cpu-probe.c

@@ -376,13 +376,33 @@ static inline void cpu_probe_legacy(struct cpuinfo_mips *c, unsigned int cpu)
 				__cpu_name[cpu] = "R4000PC";
 			}
 		} else {
+			int cca = read_c0_config() & CONF_CM_CMASK;
+			int mc;
+
+			/*
+			 * SC and MC versions can't be reliably told apart,
+			 * but only the latter support coherent caching
+			 * modes so assume the firmware has set the KSEG0
+			 * coherency attribute reasonably (if uncached, we
+			 * assume SC).
+			 */
+			switch (cca) {
+			case CONF_CM_CACHABLE_CE:
+			case CONF_CM_CACHABLE_COW:
+			case CONF_CM_CACHABLE_CUW:
+				mc = 1;
+				break;
+			default:
+				mc = 0;
+				break;
+			}
 			if ((c->processor_id & PRID_REV_MASK) >=
 			    PRID_REV_R4400) {
-				c->cputype = CPU_R4400SC;
-				__cpu_name[cpu] = "R4400SC";
+				c->cputype = mc ? CPU_R4400MC : CPU_R4400SC;
+				__cpu_name[cpu] = mc ? "R4400MC" : "R4400SC";
 			} else {
-				c->cputype = CPU_R4000SC;
-				__cpu_name[cpu] = "R4000SC";
+				c->cputype = mc ? CPU_R4000MC : CPU_R4000SC;
+				__cpu_name[cpu] = mc ? "R4000MC" : "R4000SC";
 			}
 		}