arm64: cpufeature: Warn if we attempt to read a zero width field

Add a WARN_ON_ONCE() when extracting a field if no width is specified. This
should never happen outside of development since it will be triggered with
or without the feature so long as the relevant ID register is present.  If
the warning triggers hope that the field was the standard 4 bits wide and
soldier on.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220307180900.3045812-1-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

Author: broonie

arch/arm64/include/asm/cpufeature.h

@@ -577,6 +577,8 @@ static inline u64 arm64_ftr_reg_user_value(const struct arm64_ftr_reg *reg)
 static inline int __attribute_const__
 cpuid_feature_extract_field_width(u64 features, int field, int width, bool sign)
 {
+	if (WARN_ON_ONCE(!width))
+		width = 4;
 	return (sign) ?
 		cpuid_feature_extract_signed_field_width(features, field, width) :
 		cpuid_feature_extract_unsigned_field_width(features, field, width);