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

[Suzuki K Poulose]

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 at 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.

Cheers
Suzuki