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

[Dave Martin <Dave.Martin@arm.com>]

On Mon, Oct 16, 2017 at 05:47:16PM +0100, Suzuki K Poulose wrote:
> On 16/10/17 17:44, Dave Martin wrote:
> >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();
> 
> Yep, that looks neater.

Sorry, that should have been

	if (id_aa64pfr0_sve(read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1)) &&

(Disturbingly, the original does build and then hits a BUG(), because
ID_AA64PFR0_SVE happens to be defined).


With the above, are you happy for me to apply your Reviewed-by, or would
you prefer to wait for the respin?

Cheers
---Dave