[PATCH 4/9] arm64: cpufeature: Document the rules of safe value for features

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Document the rules for choosing the safe value for different types
of features.

Cc: Dave Martin <dave.martin@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
 arch/arm64/include/asm/cpufeature.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 0bc0b1d..59508a9 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -29,7 +29,21 @@
 
 #include <linux/kernel.h>
 
-/* CPU feature register tracking */
+/*
+ * CPU feature register tracking
+ *
+ * The safe value of a CPUID feature field is dependent on the implications
+ * of the values assigned to it by the architecture. Based on the relationship
+ * between the values, the features are classified into 3 types.
+ *
+ * a) LOWER_SAFE - The value 'n+1' indicates, value 'n' and some
+ *    additional features. (where n >= 0). The smaller value (n) is
+ *    considered safer in this case.
+ * b) HIGHER_SAFE - The value 'n+1' is safer than 'n' (for n>= 0).
+ * c) EXACT - If the values of the feature don't have any relationship,
+ *    a predefined safe value is used.
+ */
+
 enum ftr_type {
 	FTR_EXACT,	/* Use a predefined safe value */
 	FTR_LOWER_SAFE,	/* Smaller value is safe */
-- 
2.7.4