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Subject: Re: [PATCH v4 02/26] arm64: cpufeature: Add cpufeature for IRQ
 priority masking
From:   Julien Thierry <julien.thierry@arm.com>
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,
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        catalin.marinas@arm.com, will.deacon@arm.com
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Date:   Tue, 12 Jun 2018 14:46:38 +0100
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On 25/05/18 11:48, Julien Thierry wrote:
> 
> 
> 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.

I've been trying to use this now, but I can't figure out how.

The _ALTERNATIVE_CFG does not seem to work in assembly code (despite 
having its own definition for __ASSEMBLY__), and the alternative_insn 
does not seem to be suited for instructions that take operands (or more 
than one operand)

If I am mistaken, can you provide an example of how to use this in 
assembly with instructions having more than 1 operand?

Cheers,

-- 
Julien Thierry