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 05:27:59PM +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