arm64: kernel: Add min_field_value and use '>=' for feature detection

When a new cpu feature is available, the cpu feature bits will have some
initial value, which is incremented when the feature is updated.
This patch changes 'register_value' to be 'min_field_value', and checks
the feature bits value (interpreted as a signed int) is greater than this
minimum.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/cpufeature.h

@@ -42,8 +42,8 @@ struct arm64_cpu_capabilities {
 		};
 
 		struct {	/* Feature register checking */
-			u64 register_mask;
-			u64 register_value;
+			int field_pos;
+			int min_field_value;
 		};
 	};
 };

arch/arm64/kernel/cpufeature.c

@@ -22,13 +22,21 @@
 #include <asm/cpu.h>
 #include <asm/cpufeature.h>
 
+static bool
+feature_matches(u64 reg, const struct arm64_cpu_capabilities *entry)
+{
+	int val = cpuid_feature_extract_field(reg, entry->field_pos);
+
+	return val >= entry->min_field_value;
+}
+
 static bool
 has_id_aa64pfr0_feature(const struct arm64_cpu_capabilities *entry)
 {
 	u64 val;
 
 	val = read_cpuid(id_aa64pfr0_el1);
-	return (val & entry->register_mask) == entry->register_value;
+	return feature_matches(val, entry);
 }
 
 static const struct arm64_cpu_capabilities arm64_features[] = {
@@ -36,8 +44,8 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.desc = "GIC system register CPU interface",
 		.capability = ARM64_HAS_SYSREG_GIC_CPUIF,
 		.matches = has_id_aa64pfr0_feature,
-		.register_mask = (0xf << 24),
-		.register_value = (1 << 24),
+		.field_pos = 24,
+		.min_field_value = 1,
 	},
 	{},
 };