From 156e0d57f8bc9a4c0f09f4f79af7b51c6d7d0600 Mon Sep 17 00:00:00 2001
From: Suzuki K Poulose <suzuki.poulose@arm.com>
Date: Mon, 9 Jan 2017 17:28:27 +0000
Subject: [PATCH] arm64: cpufeature: Document the rules of safe value for
features
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>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
arch/arm64/include/asm/cpufeature.h | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index b4989df486701..10d56242e6491 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -29,7 +29,20 @@
#include <linux/jump_label.h>
#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 - LOWER_SAFE,
+ * HIGHER_SAFE and EXACT.
+ *
+ * The lowest value of all the CPUs is chosen for LOWER_SAFE and highest
+ * for HIGHER_SAFE. It is expected that all CPUs have the same value for
+ * a field when EXACT is specified, failing which, the safe value specified
+ * in the table is chosen.
+ */
+
enum ftr_type {
FTR_EXACT, /* Use a predefined safe value */
FTR_LOWER_SAFE, /* Smaller value is safe */
--
2.30.2