[MIPS] Workaround for 4Kc machine check exception

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/tlbex.c

@@ -57,6 +57,21 @@ static __init int __maybe_unused r10000_llsc_war(void)
 	return R10000_LLSC_WAR;
 }
 
+/*
+ * Found by experiment: At least some revisions of the 4kc throw under
+ * some circumstances a machine check exception, triggered by invalid
+ * values in the index register.  Delaying the tlbp instruction until
+ * after the next branch,  plus adding an additional nop in front of
+ * tlbwi/tlbwr avoids the invalid index register values. Nobody knows
+ * why; it's not an issue caused by the core RTL.
+ *
+ */
+static __init int __attribute__((unused)) m4kc_tlbp_war(void)
+{
+	return (current_cpu_data.processor_id & 0xffff00) ==
+	       (PRID_COMP_MIPS | PRID_IMP_4KC);
+}
+
 /*
  * A little micro-assembler, intended for TLB refill handler
  * synthesizing. It is intentionally kept simple, does only support
@@ -894,6 +909,8 @@ static __init void build_tlb_write_entry(u32 **p, struct label **l,
 	case CPU_20KC:
 	case CPU_25KF:
 	case CPU_LOONGSON2:
+		if (m4kc_tlbp_war())
+			i_nop(p);
 		tlbw(p);
 		break;
 
@@ -1705,6 +1722,7 @@ build_r4000_tlbchange_handler_head(u32 **p, struct label **l,
 	l_smp_pgtable_change(l, *p);
 # endif
 	iPTE_LW(p, l, pte, ptr); /* get even pte */
+	if (!m4kc_tlbp_war())
 		build_tlb_probe_entry(p);
 }
 
@@ -1747,6 +1765,8 @@ static void __init build_r4000_tlb_load_handler(void)
 
 	build_r4000_tlbchange_handler_head(&p, &l, &r, K0, K1);
 	build_pte_present(&p, &l, &r, K0, K1, label_nopage_tlbl);
+	if (m4kc_tlbp_war())
+		build_tlb_probe_entry(&p);
 	build_make_valid(&p, &r, K0, K1);
 	build_r4000_tlbchange_handler_tail(&p, &l, &r, K0, K1);
 
@@ -1781,6 +1801,8 @@ static void __init build_r4000_tlb_store_handler(void)
 
 	build_r4000_tlbchange_handler_head(&p, &l, &r, K0, K1);
 	build_pte_writable(&p, &l, &r, K0, K1, label_nopage_tlbs);
+	if (m4kc_tlbp_war())
+		build_tlb_probe_entry(&p);
 	build_make_write(&p, &r, K0, K1);
 	build_r4000_tlbchange_handler_tail(&p, &l, &r, K0, K1);
 
@@ -1815,6 +1837,8 @@ static void __init build_r4000_tlb_modify_handler(void)
 
 	build_r4000_tlbchange_handler_head(&p, &l, &r, K0, K1);
 	build_pte_modifiable(&p, &l, &r, K0, K1, label_nopage_tlbm);
+	if (m4kc_tlbp_war())
+		build_tlb_probe_entry(&p);
 	/* Present and writable bits set, set accessed and dirty bits. */
 	build_make_write(&p, &r, K0, K1);
 	build_r4000_tlbchange_handler_tail(&p, &l, &r, K0, K1);