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

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Message-ID: <c4e5f8ff-ecde-7b88-8a37-39e47ef22917@arm.com>
Date:   Fri, 25 May 2018 11:36:25 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Julien Thierry <julien.thierry@....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 11:17, 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.

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