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

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Date:   Fri, 15 Jun 2018 11:16:45 +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 12/06/18 14:46, Julien Thierry wrote:
> 
> 
> 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?

I am sorry, but I think the ALTERNATIVE_CFG is not the right one, as it
omits the entire block, if the CONFIG is not enabled. So you are left with
only three choices :

1) Use alternative call back
2) Stick to two separate caps.
3) Use #ifdef

Cheers
Suzuki