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SubjectRe: [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 <> 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.
>>>>>>> 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?

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