MIPS: R12000: Enable branch prediction global history

The R12000 added a new feature to enhance branch prediction called
"global history".  Per the Vr10000 Series User Manual (U10278EJ4V0UM),
Coprocessor 0, Diagnostic Register (22):

"""
If bit 26 is set, branch prediction uses all eight bits of the global
history register.  If bit 26 is not set, then bits 25:23 specify a count
of the number of bits of global history to be used. Thus if bits 26:23
are all zero, global history is disabled.

The global history contains a record of the taken/not-taken status of
recently executed branches, and when used is XOR'ed with the PC of a
branch being predicted to produce a hashed value for indexing the BPT.
Some programs with small "working set of conditional branches" benefit
significantly from the use of such hashing, some see slight performance
degradation.
"""

This patch enables global history on R12000 CPUs and up by setting bit
26 in the branch prediction diagnostic register (CP0 $22) to '1'.  Bits
25:23 are left alone so that all eight bits of the global history
register are available for branch prediction.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/mips/include/asm/cpu.h b/arch/mips/include/asm/cpu.h
index 73dd35787d1a..e46e40602af3 100644
--- a/arch/mips/include/asm/cpu.h
+++ b/arch/mips/include/asm/cpu.h
@@ -381,6 +381,7 @@ enum cpu_type_enum {
 #define MIPS_CPU_RW_LLB		0x1000000000ull /* LLADDR/LLB writes are allowed */
 #define MIPS_CPU_XPA		0x2000000000ull /* CPU supports Extended Physical Addressing */
 #define MIPS_CPU_CDMM		0x4000000000ull	/* CPU has Common Device Memory Map */
+#define MIPS_CPU_BP_GHIST	0x8000000000ull /* R12K+ Branch Prediction Global History */
 
 /*
  * CPU ASE encodings