linux-kernel - Re: [PATCH v4 02/26] arm64: cpufeature: Add cpufeature for IRQ priority masking

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Message-ID: <54ff6127-928d-99a3-a6e9-59799628ca87@arm.com>
Date:   Fri, 25 May 2018 11:39:16 +0100
From:   Julien Thierry <julien.thierry@....com>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, daniel.thompson@...aro.org,
        joel@...lfernandes.org, marc.zyngier@....com, mark.rutland@....com,
        christoffer.dall@....com, james.morse@....com,
        catalin.marinas@....com, will.deacon@....com
Subject: Re: [PATCH v4 02/26] arm64: cpufeature: Add cpufeature for IRQ
 priority masking



On 25/05/18 11:36, Suzuki K Poulose wrote:
> On 25/05/18 11:17, Julien Thierry wrote:
>>
>>
>> On 25/05/18 11:04, Suzuki K Poulose wrote:
>>> On 25/05/18 10:49, Julien Thierry wrote:
>>>> Add a cpufeature indicating whether a cpu supports masking interrupts
>>>> by priority.
>>>
>>> How is this different from the SYSREG_GIC_CPUIF cap ? Is it just
>>> the description ?
>>
>> More or less.
>>
>> It is just to have an easier condition in the rest of the series. 
>> Basically the PRIO masking feature is enabled if we have a GICv3 CPUIF 
>> working *and* the option was selected at build time. Before this meant 
>> that I was checking for the GIC_CPUIF cap inside #ifdefs (and putting 
>> alternatives depending on that inside #ifdefs as well).
>>
>> Having this as a separate feature feels easier to manage in the code. 
>> It also makes it clearer at boot time that the kernel will be using 
>> irq priorities (although I admit it was not the initial intention):
>>
>> [    0.000000] CPU features: detected: IRQ priority masking
>>
>>
>> But yes that new feature will be detected only if SYSREG_GIC_CPUIF 
>> gets detected as well.
> 
> Well, you could always wrap the check like :
> 
> static inline bool system_has_irq_priority_masking(void)
> {
>      return (IS_ENABLED(CONFIG_YOUR_CONFIG) && 
> cpus_have_const_cap(HWCAP_SYSREG_GIC_CPUIF));
> }
> 
> and use it everywhere.
> 

Yes, but I can't use that in the asm parts that use alternatives and 
would need to surround them in #ifdef... :\

> The description could be statically changed to reflect based on the #ifdef.
> 
> 
> static const struct arm64_cpu_capabilities arm64_features[] = {
>          {
> #ifdef CONFIG_YOUR_CONFIG
>          .desc = "GIC System register CPU interface with IRQ priority 
> masking"
> #else
>                  .desc = "GIC system register CPU interface",
> #endif
>                  .capability = ARM64_HAS_SYSREG_GIC_CPUIF,
>                  .type = ARM64_CPUCAP_SYSTEM_FEATURE,
>                  .matches = has_useable_gicv3_cpuif,
>                  .sys_reg = SYS_ID_AA64PFR0_EL1,
>                  .field_pos = ID_AA64PFR0_GIC_SHIFT,
>                  .sign = FTR_UNSIGNED,
>                  .min_field_value = 1,
> 
> Cheers
> Suzuki

-- 
Julien Thierry