MIPS: microMIPS: Fix improper definition of ISA exception bit.

The ISA exception bit selects whether exceptions are taken in classic
or microMIPS mode. This bit is Config3.ISAOnExc and was improperly
defined as bits 16 and 17 instead of just bit 16. A new function was
added so that platforms could set this bit when running a kernel
compiled with only microMIPS instructions.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5377/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/mipsregs.h

@@ -596,7 +596,7 @@
 #define MIPS_CONF3_RXI		(_ULCAST_(1) << 12)
 #define MIPS_CONF3_ULRI		(_ULCAST_(1) << 13)
 #define MIPS_CONF3_ISA		(_ULCAST_(3) << 14)
-#define MIPS_CONF3_ISA_OE	(_ULCAST_(3) << 16)
+#define MIPS_CONF3_ISA_OE	(_ULCAST_(1) << 16)
 #define MIPS_CONF3_VZ		(_ULCAST_(1) << 23)
 
 #define MIPS_CONF4_MMUSIZEEXT	(_ULCAST_(255) << 0)

arch/mips/kernel/cpu-probe.c

@@ -269,9 +269,6 @@ static inline unsigned int decode_config3(struct cpuinfo_mips *c)
 		c->options |= MIPS_CPU_ULRI;
 	if (config3 & MIPS_CONF3_ISA)
 		c->options |= MIPS_CPU_MICROMIPS;
-#ifdef CONFIG_CPU_MICROMIPS
-	write_c0_config3(read_c0_config3() | MIPS_CONF3_ISA_OE);
-#endif
 	if (config3 & MIPS_CONF3_VZ)
 		c->ases |= MIPS_ASE_VZ;
 

arch/mips/kernel/traps.c

@@ -1878,6 +1878,15 @@ void __init trap_init(void)
 			ebase += (read_c0_ebase() & 0x3ffff000);
 	}
 
+	if (cpu_has_mmips) {
+		unsigned int config3 = read_c0_config3();
+
+		if (IS_ENABLED(CONFIG_CPU_MICROMIPS))
+			write_c0_config3(config3 | MIPS_CONF3_ISA_OE);
+		else
+			write_c0_config3(config3 & ~MIPS_CONF3_ISA_OE);
+	}
+
 	if (board_ebase_setup)
 		board_ebase_setup();
 	per_cpu_trap_init(true);