Re: [PATCH v3 16/28] arm64/sve: Probe SVE capabilities and usable vector lengths

Dave Martin
2017-10-16T16:44:07Z

On Mon, Oct 16, 2017 at 05:27:59PM +0100, Suzuki K Poulose wrote:
> On 16/10/17 16:46, Dave Martin wrote:
> >On Thu, Oct 12, 2017 at 01:56:51PM +0100, Suzuki K Poulose wrote:
> >>On 10/10/17 19:38, Dave Martin wrote:

[...]

> >>>@@ -670,6 +689,14 @@ void update_cpu_features(int cpu,
> >>> info->reg_mvfr2, boot->reg_mvfr2);
> >>> }
> >>>+ if (id_aa64pfr0_sve(info->reg_id_aa64pfr0)) {
> >>>+ taint |= check_update_ftr_reg(SYS_ZCR_EL1, cpu,
> >>>+ info->reg_zcr, boot->reg_zcr);
> >>>+
> >>>+ if (!sys_caps_initialised)
> >>>+ sve_update_vq_map();
> >>>+ }
> >>
> >>nit: I am not sure if we should also check if the "current" sanitised value
> >>of the id_aa64pfr0 also supports sve and skip the update if it isn't. The code
> >>is as such fine without the check, its just that we can avoid computing the
> >>map. It is in the CPU boot up path and hence is not performance critical.
> >>So, either way we are fine.
> >>
> >>Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> >
> >I think I prefer to avoid adding extra code to optimise the "broken SoC
> >design" case.
> >
>
> Sure.
>
> >Maybe we could revisit this later if needed.
> >
> >Can you suggest some code? Maybe the check is simpler than I think.
>
> Something like :
>
> if (id_aa64pfr0_sve(read_sanitised_ftr_reg(SYS_IDAA64PFR0)) &&
> id_aa64pfr0_sve(id_aa64pfr0)) {
> ...
> }
>
> should be enough.
>
> Or even we could hack it to :
>
> if (id_aa64pfr0_sve(id_aa64pfr0 | read_sanitised_ftr_reg(SYS_IDAA64PFR0)))
>
> As I mentioned, the code as such is fine. Its just that we try to detect
> if the SVE is already moot and skip the steps for this CPU.

How about the following, keeping the outer
if(id_aa64pfr0_sve(int->reg_id_aa64pfr0)) from my current code:

- if (!sys_caps_initialised)
+ /* Probe vector lengths, unless we already gave up on SVE */
+ if (id_aa64pfr0_sve(read_sanitised_ftr_reg(ID_AA64PFR0_SVE)) &&
+ !sys_caps_initialised)
sve_update_vq_map();

Cheers
---Dave