MIPS: Avoid pipeline stalls on some MIPS32R2 cores.

The architecture specification says that an EHB instruction is
needed to avoid a hazard when writing TLB entries. However, some
cores do not have this hazard, and thus the EHB instruction causes
a costly pipeline stall. Detect these cores and do not use the EHB
instruction.

Signed-off-by: Steven J. Hill <sjhill@mips.com>

arch/mips/mm/tlbex.c

@@ -449,8 +449,20 @@ static void __cpuinit build_tlb_write_entry(u32 **p, struct uasm_label **l,
 	}
 
 	if (cpu_has_mips_r2) {
-		if (cpu_has_mips_r2_exec_hazard)
+		/*
+		 * The architecture spec says an ehb is required here,
+		 * but a number of cores do not have the hazard and
+		 * using an ehb causes an expensive pipeline stall.
+		 */
+		switch (current_cpu_type()) {
+		case CPU_M14KC:
+		case CPU_74K:
+			break;
+
+		default:
 			uasm_i_ehb(p);
+			break;
+		}
 		tlbw(p);
 		return;
 	}