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>

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)
 

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);