linux-kernel - Re: [PATCH v3 4/9] arm64: cpufeature: Document the rules of safe value for features

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Message-ID: <2338875a-6a5b-ba9e-59fe-d681091c2923@arm.com>
Date:   Mon, 9 Jan 2017 10:43:07 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ryan.arnold@...aro.org, sid@...erved-bit.com, aph@...hat.com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        adhemerval.zanella@...aro.org, dave.martin@....com
Subject: Re: [PATCH v3 4/9] arm64: cpufeature: Document the rules of safe
 value for features

On 06/01/17 12:30, Catalin Marinas wrote:
> On Wed, Jan 04, 2017 at 05:49:02PM +0000, Suzuki K. Poulose wrote:
>> --- a/arch/arm64/include/asm/cpufeature.h
>> +++ b/arch/arm64/include/asm/cpufeature.h
>> @@ -29,7 +29,21 @@
>>  #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.
>> + *
>> + * 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.
>> + */
>
> I don't think this text fully describes what is actually compared. You
> could say something that the lowest value of all the CPUs is chosen for
> LOWER_SAFE, highest for HIGHER_SAFE and it is expected that all CPUs
> have the same value for a field when EXACT is specified.

Ok. I have changed it as below :

/*
  * 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.
  */


Suzuki