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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>
This commit is contained in:
Suzuki K Poulose 2017-01-09 17:28:27 +00:00 committed by Will Deacon
parent eab43e8873
commit 156e0d57f8
1 changed files with 14 additions and 1 deletions
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15
arch/arm64/include/asm/cpufeature.h
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@ -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 */