MIPS: kernel: cpu-probe: Detect unique RI/XI exceptions

Detect if the core supports unique exception codes for the
Read-Inhibit and Execute-Inhibit exceptions and set the
option accordingly. The RI/XI exception support is detected
by setting the 27th bit (IEC) of the PageGrain C0 register
and reading back the value of that register to verify the
bit is enabled.

Signed-off-by: Leonid Yegoshin <[e-mail]>
Signed-off-by: Markos Chandras <[e-mail]>
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/7340/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/include/asm/mipsregs.h

···
 #define PG_XIE		(_ULCAST_(1) <<	 30)
 #define PG_ELPA		(_ULCAST_(1) <<	 29)
 #define PG_ESP		(_ULCAST_(1) <<	 28)
+#define PG_IEC		(_ULCAST_(1) <<  27)
 
 /*
  * R4x00 interrupt enable / cause bits

arch/mips/kernel/cpu-probe.c

···
 
 	mips_probe_watch_registers(c);
 
+	if (cpu_has_rixi) {
+		/* Enable the RIXI exceptions */
+		write_c0_pagegrain(read_c0_pagegrain() | PG_IEC);
+		back_to_back_c0_hazard();
+		/* Verify the IEC bit is set */
+		if (read_c0_pagegrain() & PG_IEC)
+			c->options |= MIPS_CPU_RIXIEX;
+	}
+
 #ifndef CONFIG_MIPS_CPS
 	if (cpu_has_mips_r2) {
 		c->core = get_ebase_cpunum();