arm64: Read system wide CPUID value

Add an API for reading the safe CPUID value across the
system from the new infrastructure.
Signed-off-by: NSuzuki K. Poulose <suzuki.poulose@arm.com>
Tested-by: NDave Martin <Dave.Martin@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/cpufeature.h

@@ -144,6 +144,8 @@ void check_local_cpu_features(void);
 bool cpu_supports_mixed_endian_el0(void);
 bool system_supports_mixed_endian_el0(void);
 
+u64 read_system_reg(u32 id);
+
 #endif /* __ASSEMBLY__ */
 
 #endif

arch/arm64/kernel/cpufeature.c

@@ -590,6 +590,15 @@ void update_cpu_features(int cpu,
 	update_mixed_endian_el0_support(info);
 }
 
+u64 read_system_reg(u32 id)
+{
+	struct arm64_ftr_reg *regp = get_arm64_ftr_reg(id);
+
+	/* We shouldn't get a request for an unsupported register */
+	BUG_ON(!regp);
+	return regp->sys_val;
+}
+
 static bool
 feature_matches(u64 reg, const struct arm64_cpu_capabilities *entry)
 {