Re: [PATCH v3] irqchip/tango: Don't use incorrect irq_mask_ack callback

Marc Zyngier
2017-08-07T12:56:24Z

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@sigmadesigns.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?

Thanks,

M.
--
Jazz is not dead. It just smells funny...