LoongArch: Refine the init_hw_perf_events() function

(1) Use the existing CPUCFG6_PMNUM_SHIFT macro definition instead of
the magic value 4 to get the PMU number.

(2) Detect the value of PMU bits via CPUCFG instruction according to
the ISA manual instead of hard-coded as 64, because the value may be
different for various micro-architectures.

Link: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#_cpucfg
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: seehearfeel

arch/loongarch/include/asm/loongarch.h

@@ -128,6 +128,7 @@
 #define  CPUCFG6_PMNUM			GENMASK(7, 4)
 #define  CPUCFG6_PMNUM_SHIFT		4
 #define  CPUCFG6_PMBITS			GENMASK(13, 8)
+#define  CPUCFG6_PMBITS_SHIFT		8
 #define  CPUCFG6_UPM			BIT(14)
 
 #define LOONGARCH_CPUCFG16		0x10

arch/loongarch/kernel/perf_event.c

@@ -845,13 +845,14 @@ static const struct loongarch_perf_event *loongarch_pmu_map_raw_event(u64 config
 
 static int __init init_hw_perf_events(void)
 {
-	int counters;
+	int bits, counters;
 
 	if (!cpu_has_pmp)
 		return -ENODEV;
 
 	pr_info("Performance counters: ");
-	counters = ((read_cpucfg(LOONGARCH_CPUCFG6) & CPUCFG6_PMNUM) >> 4) + 1;
+	bits = ((read_cpucfg(LOONGARCH_CPUCFG6) & CPUCFG6_PMBITS) >> CPUCFG6_PMBITS_SHIFT) + 1;
+	counters = ((read_cpucfg(LOONGARCH_CPUCFG6) & CPUCFG6_PMNUM) >> CPUCFG6_PMNUM_SHIFT) + 1;
 
 	loongarch_pmu.num_counters = counters;
 	loongarch_pmu.max_period = (1ULL << 63) - 1;
@@ -867,7 +868,7 @@ static int __init init_hw_perf_events(void)
 	on_each_cpu(reset_counters, NULL, 1);
 
 	pr_cont("%s PMU enabled, %d %d-bit counters available to each CPU.\n",
-			loongarch_pmu.name, counters, 64);
+			loongarch_pmu.name, counters, bits);
 
 	perf_pmu_register(&pmu, "cpu", PERF_TYPE_RAW);