MIPS: Allow kernel to use coprocessor 2

Kernel threads should be able to use COP2 if the platform needs it.
Do not call die_if_kernel() for a coprocessor unusable exception if
the exception due to COP2 usage.  Instead, the default notifier for
COP2 exceptions is updated to call die_if_kernel.
Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Cc: ddaney.cavm@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/5415/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/traps.c

@@ -1056,15 +1056,9 @@ static int default_cu2_call(struct notifier_block *nfb, unsigned long action,
 {
 	struct pt_regs *regs = data;
 
-	switch (action) {
-	default:
-		die_if_kernel("Unhandled kernel unaligned access or invalid "
+	die_if_kernel("COP2: Unhandled kernel unaligned access or invalid "
 			      "instruction", regs);
-		/* Fall through	 */
-
-	case CU2_EXCEPTION:
-		force_sig(SIGILL, current);
-	}
+	force_sig(SIGILL, current);
 
 	return NOTIFY_OK;
 }
@@ -1080,10 +1074,11 @@ asmlinkage void do_cpu(struct pt_regs *regs)
 	unsigned long __maybe_unused flags;
 
 	prev_state = exception_enter();
-	die_if_kernel("do_cpu invoked from kernel context!", regs);
-
 	cpid = (regs->cp0_cause >> CAUSEB_CE) & 3;
 
+	if (cpid != 2)
+		die_if_kernel("do_cpu invoked from kernel context!", regs);
+
 	switch (cpid) {
 	case 0:
 		epc = (unsigned int __user *)exception_epc(regs);