Re: [PATCH v3 2/2] arm64: Support systems without FP/ASIMD

[Suzuki K Poulose]

On 14/11/16 11:48, Catalin Marinas wrote:
> Hi Suzuki,

> > +static inline bool system_supports_fpsimd(void)
> > +{
> > +	return !cpus_have_const_cap(ARM64_HAS_NO_FPSIMD);
> > +}
>
> Any particular reason why using negation instead of a ARM64_HAS_FPSIMD?
> A potential problem would be the default cpus_have_const_cap()
> implementation and the default static key having a slight performance
> impact.

The negation was chosen to avoid hotpatching in the most common case.
But as you said, it has an impact on the other side. I think doing
a one time hotpatching at boot time is more optimal than penalising
a bunch of other users throughout the execution. I will take a look
at changing it back to a ARM64_HAS_FPSIMD.

> >  	},
> > +	{
> > +		/* FP/SIMD is not implemented */
> > +		.capability = ARM64_HAS_NO_FPSIMD,
> > +		.def_scope = SCOPE_SYSTEM,
> > +		.min_field_value = 0,
> > +		.matches = has_no_fpsimd,
> > +	},
>
> If we go for negation, I don't think we need a min_field_value at all,
> the matching is done by the has_no_fpsimd() function.

You're right.

Suzuki