MIPS: cpu-probe: Set the FTLB probability bit on supported cores

Make use of the Config6/FLTBP bit to set the probability of a TLBWR
instruction to hit the FTLB or the VTLB. A value of 0 (which may be
the default value on certain cores, such as proAptiv or P5600)
means that a TLBWR instruction will never hit the VTLB which
leads to performance limitations since it effectively decreases
the number of available TLB slots.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: <stable@vger.kernel.org> # v3.15+
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8368/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

2 files changed, 34 insertions, 1 deletions

diff --git a/arch/mips/include/asm/mipsregs.h b/arch/mips/include/asm/mipsregs.h
index b46cd220a018..22a135ac91de 100644
--- a/arch/mips/include/asm/mipsregs.h
+++ b/arch/mips/include/asm/mipsregs.h
@@ -661,6 +661,8 @@
 #define MIPS_CONF6_SYND         (_ULCAST_(1) << 13)
 /* proAptiv FTLB on/off bit */
 #define MIPS_CONF6_FTLBEN       (_ULCAST_(1) << 15)
+/* FTLB probability bits */
+#define MIPS_CONF6_FTLBP_SHIFT  (16)
 
 #define MIPS_CONF7_WII          (_ULCAST_(1) << 31)
 
diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
index d5a4f380b019..dc49cf30c2db 100644
--- a/arch/mips/kernel/cpu-probe.c
+++ b/arch/mips/kernel/cpu-probe.c
@@ -193,6 +193,32 @@ static void set_isa(struct cpuinfo_mips *c, unsigned int isa)
 static char unknown_isa[] = KERN_ERR \
         "Unsupported ISA type, c0.config0: %d.";
 
+static unsigned int calculate_ftlb_probability(struct cpuinfo_mips *c)
+{
+
+        unsigned int probability = c->tlbsize / c->tlbsizevtlb;
+
+        /*
+         * 0 = All TLBWR instructions go to FTLB
+         * 1 = 15:1: For every 16 TBLWR instructions, 15 go to the
+         * FTLB and 1 goes to the VTLB.
+         * 2 = 7:1: As above with 7:1 ratio.
+         * 3 = 3:1: As above with 3:1 ratio.
+         *
+         * Use the linear midpoint as the probability threshold.
+         */
+        if (probability >= 12)
+                return 1;
+        else if (probability >= 6)
+                return 2;
+        else
+                /*
+                 * So FTLB is less than 4 times bigger than VTLB.
+                 * A 3:1 ratio can still be useful though.
+                 */
+                return 3;
+}
+
 static void set_ftlb_enable(struct cpuinfo_mips *c, int enable)
 {
         unsigned int config6;
@@ -203,9 +229,14 @@ static void set_ftlb_enable(struct cpuinfo_mips *c, int enable)
         case CPU_P5600:
                 /* proAptiv & related cores use Config6 to enable the FTLB */
                 config6 = read_c0_config6();
+                /* Clear the old probability value */
+                config6 &= ~(3 << MIPS_CONF6_FTLBP_SHIFT);
                 if (enable)
                         /* Enable FTLB */
-                        write_c0_config6(config6 | MIPS_CONF6_FTLBEN);
+                        write_c0_config6(config6 |
+                                         (calculate_ftlb_probability(c)
+                                          << MIPS_CONF6_FTLBP_SHIFT)
+                                         | MIPS_CONF6_FTLBEN);
                 else
                         /* Disable FTLB */
                         write_c0_config6(config6 &  ~MIPS_CONF6_FTLBEN);