MIPS: Use dins to simplify __write_64bit_c0_split()

The code in __write_64bit_c0_split() is used by MIPS32 kernels running
on MIPS64 CPUs to write a 64-bit value to a 64-bit coprocessor 0
register using a single 64-bit dmtc0 instruction. It does this by
combining the 2x 32-bit registers used to hold the 64-bit value into a
single register, which in the existing code involves three steps:

  1) Zero extend register A which holds bits 31:0 of our data, since it
     may have previously held a sign-extended value.

  2) Shift register B which holds bits 63:32 of our data in bits 31:0
     left by 32 bits, such that the bits forming our data are in the
     position they'll be in the final 64-bit value & bits 31:0 of the
     register are zero.

  3) Or the two registers together to form the 64-bit value in one
     64-bit register.

From MIPS r2 onwards we have a dins instruction which can effectively
perform all 3 of those steps using a single instruction.

Add a path for MIPS r2 & beyond which uses dins to take bits 31:0 from
register B & insert them into bits 63:32 of register A, giving us our
full 64-bit value in register A with one instruction.

Since we know that MIPS r2 & above support the sel field for the dmtc0
instruction, we don't bother special casing sel==0. Omiting the sel
field would assemble to exactly the same instruction as when we
explicitly specify that it equals zero.
Signed-off-by: Paul Burton <paul.burton@mips.com>

arch/mips/include/asm/mipsregs.h

@@ -16,6 +16,7 @@
 #include <linux/linkage.h>
 #include <linux/types.h>
 #include <asm/hazards.h>
+#include <asm/isa-rev.h>
 #include <asm/war.h>
 
 /*
@@ -1489,7 +1490,15 @@ do {									\
 	unsigned long __flags;						\
 									\
 	local_irq_save(__flags);					\
-	if (sel == 0)							\
+	if (MIPS_ISA_REV >= 2)						\
+		__asm__ __volatile__(					\
+			".set\tpush\n\t"				\
+			".set\t" MIPS_ISA_LEVEL "\n\t"			\
+			"dins\t%L0, %M0, 32, 32\n\t"			\
+			"dmtc0\t%L0, " #source ", " #sel "\n\t"		\
+			".set\tpop"					\
+			: "+r" (__tmp));				\
+	else if (sel == 0)						\
 		__asm__ __volatile__(					\
 			".set\tmips64\n\t"				\
 			"dsll\t%L0, %L0, 32\n\t"			\