MIPS: Stop setting I6400 FTLBP

The FTLBP field in Config7 for the I6400 is intended as chicken bits for
debugging rather than as a field that software actually makes use of.
For best performance, FTLBP should be left at its default value of 0
with all TLB writes hitting the FTLB by default.

Additionally, since set_ftlb_enable is called from decode_configs before
decode_config4 which determines the size of the TLBs, this was
previously always setting FTLBP=3 for a 3:1 FTLB:VTLB write ratio which
makes abysmal use of the available FTLB resources.

This effectively reverts b0c4e1b79d8a ("MIPS: Set up FTLB probability
for I6400").
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Fixes: b0c4e1b79d8a ("MIPS: Set up FTLB probability for I6400")
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14021/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/mipsregs.h

@@ -660,8 +660,6 @@
 
 #define MIPS_CONF7_IAR		(_ULCAST_(1) << 10)
 #define MIPS_CONF7_AR		(_ULCAST_(1) << 16)
-/* FTLB probability bits for R6 */
-#define MIPS_CONF7_FTLBP_SHIFT	(18)
 
 /* WatchLo* register definitions */
 #define MIPS_WATCHLO_IRW	(_ULCAST_(0x7) << 0)

arch/mips/kernel/cpu-probe.c

@@ -555,13 +555,8 @@ static int set_ftlb_enable(struct cpuinfo_mips *c, int enable)
 			write_c0_config6(config &  ~MIPS_CONF6_FTLBEN);
 		break;
 	case CPU_I6400:
-		/* I6400 & related cores use Config7 to configure FTLB */
-		config = read_c0_config7();
-		/* Clear the old probability value */
-		config &= ~(3 << MIPS_CONF7_FTLBP_SHIFT);
-		write_c0_config7(config | (calculate_ftlb_probability(c)
-					   << MIPS_CONF7_FTLBP_SHIFT));
-		break;
+		/* There's no way to disable the FTLB */
+		return !enable;
 	case CPU_LOONGSON3:
 		/* Flush ITLB, DTLB, VTLB and FTLB */
 		write_c0_diag(LOONGSON_DIAG_ITLB | LOONGSON_DIAG_DTLB |