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Subject: Re: [PATCH v4 02/26] arm64: cpufeature: Add cpufeature for IRQ
 priority masking
To:     Suzuki K Poulose <Suzuki.Poulose@arm.com>,
        linux-arm-kernel@lists.infradead.org
Cc:     linux-kernel@vger.kernel.org, daniel.thompson@linaro.org,
        joel@joelfernandes.org, marc.zyngier@arm.com, mark.rutland@arm.com,
        christoffer.dall@arm.com, james.morse@arm.com,
        catalin.marinas@arm.com, will.deacon@arm.com
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From:   Julien Thierry <julien.thierry@arm.com>
Message-ID: <acf4b44f-d5c7-c31a-7f47-cfbe1049eadc@arm.com>
Date:   Fri, 25 May 2018 11:48:10 +0100
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On 25/05/18 11:41, Suzuki K Poulose wrote:
> On 25/05/18 11:39, Julien Thierry wrote:
>>
>>
>> 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... :\
> 
> I thought there is _ALTERNATIVE_CFG() to base the alternative depend on 
> a CONFIG_xxx ?
> Doesn't that solve the problem ?

Right, I didn't see that one. It should work yes.

I'll try that when working on the next version.

Thanks,

-- 
Julien Thierry