linux-kernel - Re: [PATCH v3] irqchip/tango: Don't use incorrect irq_mask

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Message-ID: <dc85dbbe-3f2f-8630-3acd-9b11183383c7@gmail.com>
Date:   Fri, 18 Aug 2017 11:24:28 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Marc Zyngier <marc.zyngier@....com>,
        Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        Doug Berger <opendmb@...il.com>
Cc:     Mans Rullgard <mans@...sr.com>, Mason <slash.tmp@...e.fr>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3] irqchip/tango: Don't use incorrect irq_mask_ack
 callback

On 08/07/2017 05:56 AM, Marc Zyngier wrote:
> On 28/07/17 15:06, Marc Gonzalez wrote:
>> On 27/07/2017 20:17, Florian Fainelli wrote:
>>
>>> On 07/26/2017 12:13 PM, Måns Rullgård wrote:
>>>
>>>> Florian Fainelli writes:
>>>>
>>>>> On 07/25/2017 06:29 AM, Måns Rullgård wrote:
>>>>>
>>>>>> Marc Gonzalez <marc_gonzalez@...madesigns.com> writes:
>>>>>>
>>>>>>> On 25/07/2017 15:16, Måns Rullgård wrote:
>>>>>>>
>>>>>>>> What happened to the patch adding the proper combined function?
>>>>>>>
>>>>>>> It appears you're not CCed on v2.
>>>>>>>
>>>>>>> https://patchwork.kernel.org/patch/9859799/
>>>>>>>
>>>>>>> Doug wrote:
>>>>>>>> Yes, you understand correctly.  The irq_mask_ack method is entirely
>>>>>>>> optional and I assume that is why this issue went undetected for so
>>>>>>>> long; however, it is slightly more efficient to combine the functions
>>>>>>>> (even if the ack is unnecessary) which is why I chose to do so for my
>>>>>>>> changes to the irqchip-brcmstb-l2 driver where I first discovered this
>>>>>>>> issue.  How much value the improved efficiency has is certainly
>>>>>>>> debatable, but interrupt handling is one area where people might care
>>>>>>>> about such a small difference.  As the irqchip-tango driver maintainer
>>>>>>>> you are welcome to decide whether or not the irq_mask_ack method makes
>>>>>>>> sense to you.
>>>>>>>
>>>>>>> My preference goes to leaving the irq_mask_ack callback undefined,
>>>>>>> and let the irqchip framework use irq_mask and irq_ack instead.
>>>>>>
>>>>>> Why would you prefer the less efficient way?
>>>>>>
>>>>>
>>>>> Same question here, that does not really make sense to me.
>>>>>
>>>>> The whole point of this patch series is to have a set of efficient and
>>>>> bugfree (or nearly) helper functions that drivers can rely on, are you
>>>>> saying that somehow using irq_mask_and_ack is exposing a bug in the
>>>>> tango irqchip driver and using the separate functions does not expose
>>>>> this bug?
>>>>
>>>> There is currently a bug in that the function used doesn't do what its
>>>> name implies which can't be good.  Using the separate mask and ack
>>>> functions obviously works, but combining them saves a lock/unlock
>>>> sequence.  The correct combined function has already been written, so I
>>>> see no reason not to use it.
>>>
>>> Marc/Mason, are you intending to get this patch accepted in order to
>>> provide a quick bugfix targeting earlier kernels with the tango irqchip
>>> driver or is this how you think the correct fix for the tango irqchip
>>> driver is as opposed to using Doug's fix?
>>
>> Hello Florian,
>>
>> I am extremely grateful for you and Doug bringing the defect to
>> my attention, as it was indeed causing an issue which I had not
>> found the time to investigate.
>>
>> The reason I proposed an alternate patch is that
>> 1) Doug didn't seem to mind, 2) simpler code leads to fewer bugs
>> and less maintenance IME, and 3) I didn't see many drivers using
>> the irq_mask_ack() callback (9 out of 86) with a few misusing it,
>> by defining irq_mask = irq_mask_ack.
>>
>> As you point out, my patch might be slightly easier to backport
>> than Doug's (TBH, I hadn't considered that aspect until you
>> mentioned it).
>>
>> Has anyone ever quantified the performance improvement of
>> mask_ack over mask + ack?
> 
> Aren't you the one who is in position of measuring this effect on the
> actual HW that uses this?

What do we do with this patch series to move forward? Can we get Doug's
changes queued up for 4.14?
-- 
Florian