Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v4 02/26] arm64: cpufeature: Add cpufeature for IRQ
 priority masking
To:     Julien Thierry <julien.thierry@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
References: <1527241772-48007-1-git-send-email-julien.thierry@arm.com>
 <1527241772-48007-3-git-send-email-julien.thierry@arm.com>
 <e44df12f-9868-ae94-4623-ec221d72d783@arm.com>
 <b0673d7e-9a1e-7dcb-5322-c05fe0821349@arm.com>
 <c4e5f8ff-ecde-7b88-8a37-39e47ef22917@arm.com>
 <54ff6127-928d-99a3-a6e9-59799628ca87@arm.com>
 <3f0afa54-8e98-798e-68dc-bbbc74bd9e19@arm.com>
 <acf4b44f-d5c7-c31a-7f47-cfbe1049eadc@arm.com>
 <bc8bb69d-8db4-87b7-adfb-589e1dd905c0@arm.com>
From:   Suzuki K Poulose <Suzuki.Poulose@arm.com>
Message-ID: <9b87c045-e925-a412-f750-ef2c94e50de5@arm.com>
Date:   Fri, 15 Jun 2018 11:16:45 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.8.0
MIME-Version: 1.0
In-Reply-To: <bc8bb69d-8db4-87b7-adfb-589e1dd905c0@arm.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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