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

[Suzuki K Poulose]

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