Commit 7f177a52 authored Sep 23, 2013 by Maciej W. Rozycki Committed by Ralf Baechle Oct 29, 2013

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>

parent 33afab80

arch/mips/kernel/cpu-probe.c

+24 −4

Original line number	Diff line number	Diff line
		@@ -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";
		}
		}

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