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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional
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 <57af1851-9341-985e-7b28-d2ba86770ecb@omp.ru>
 <20220117084732.cdy2sash5hxp4lwo@pengutronix.de>
From:   Sergey Shtylyov <s.shtylyov@omp.ru>
Organization: Open Mobile Platform
Message-ID: <68d3bb7a-7572-7495-d295-e1d512ef509e@omp.ru>
Date:   Tue, 18 Jan 2022 23:21:45 +0300
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Hello!

On 1/17/22 11:47 AM, Uwe Kleine-K?nig wrote:

[...]
>>>>>>>>> To me it sounds much more logical for the driver to check if an
>>>>>>>>> optional irq is non-zero (available) or zero (not available), than to
>>>>>>>>> sprinkle around checks for -ENXIO. In addition, you have to remember
>>>>>>>>> that this one returns -ENXIO, while other APIs use -ENOENT or -ENOSYS
>>>>>>>>> (or some other error code) to indicate absence. I thought not having
>>>>>>>>> to care about the actual error code was the main reason behind the
>>>>>>>>> introduction of the *_optional() APIs.
>>>>>>>
>>>>>>>> No, the main benefit of gpiod_get_optional() (and clk_get_optional()) is
>>>>>>>> that you can handle an absent GPIO (or clk) as if it were available.
>>>>>>
>>>>>>    Hm, I've just looked at these and must note that they match 1:1 with
>>>>>> platform_get_irq_optional(). Unfortunately, we can't however behave the
>>>>>> same way in request_irq() -- because it has to support IRQ0 for the sake
>>>>>> of i8253 drivers in arch/...
>>>>>
>>>>> Let me reformulate your statement to the IMHO equivalent:
>>>>>
>>>>> 	If you set aside the differences between
>>>>> 	platform_get_irq_optional() and gpiod_get_optional(),
>>>>
>>>>    Sorry, I should make it clear this is actually the diff between a would-be
>>>> platform_get_irq_optional() after my patch, not the current code...
>>>
>>> The similarity is that with your patch both gpiod_get_optional() and
>>> platform_get_irq_optional() return NULL and 0 on not-found. The relevant
>>> difference however is that for a gpiod NULL is a dummy value, while for
>>> irqs it's not. So the similarity is only syntactically, but not
>>> semantically.
>>
>>    I have noting to say here, rather than optional IRQ could well have a different
>> meaning than for clk/gpio/etc.
>>
>> [...]
>>>>> However for an interupt this cannot work. You will always have to check
>>>>> if the irq is actually there or not because if it's not you cannot just
>>>>> ignore that. So there is no benefit of an optional irq.
>>>>>
>>>>> Leaving error message reporting aside, the introduction of
>>>>> platform_get_irq_optional() allows to change
>>>>>
>>>>> 	irq = platform_get_irq(...);
>>>>> 	if (irq < 0 && irq != -ENXIO) {
>>>>> 		return irq;
>>>>> 	} else if (irq >= 0) {
>>>>
>>>>    Rather (irq > 0) actually, IRQ0 is considered invalid (but still returned).
>>>
>>> This is a topic I don't feel strong for, so I'm sloppy here. If changing
>>> this is all that is needed to convince you of my point ...
>>
>>    Note that we should absolutely (and first of all) stop returning 0 from platform_get_irq()
>> on a "real" IRQ0. Handling that "still good" zero absolutely doesn't scale e.g. for the subsystems
>> (like libata) which take 0 as an indication that the polling mode should be used... We can't afford
>> to be sloppy here. ;-)
> 
> Then maybe do that really first?

   I'm doing it first already:

https://lore.kernel.org/all/5e001ec1-d3f1-bcb8-7f30-a6301fd9930c@omp.ru/

   This series is atop of the above patch...

> I didn't recheck, but is this what the
> driver changes in your patch is about?

   Partly, yes. We can afford to play with the meaning of 0 after the above patch.

> After some more thoughts I wonder if your focus isn't to align
> platform_get_irq_optional to (clk|gpiod|regulator)_get_optional, but to
> simplify return code checking. Because with your change we have:
> 
>  - < 0 -> error
>  - == 0 -> no irq
>  - > 0 -> irq

   Mainly, yes. That's why the code examples were given in the description.

> For my part I'd say this doesn't justify the change, but at least I
> could better life with the reasoning. If you start at:
> 
> 	irq = platform_get_irq_optional(...)
> 	if (irq < 0 && irq != -ENXIO)
> 		return irq
> 	else if (irq > 0)
> 		setup_irq(irq);
> 	else
> 		setup_polling()
> 
> I'd change that to
> 
> 	irq = platform_get_irq_optional(...)
> 	if (irq > 0) /* or >= 0 ? */

   Not >= 0, no...

> 		setup_irq(irq)
> 	else if (irq == -ENXIO)
> 		setup_polling()
> 	else
> 		return irq
> 
> This still has to mention -ENXIO, but this is ok and checking for 0 just
> hardcodes a different return value.

   I think comparing with 0 is simpler (and shorter) than with -ENXIO, if you
consider the RISC CPUs, like e.g. MIPS...

> Anyhow, I think if you still want to change platform_get_irq_optional
> you should add a few patches converting some drivers which demonstrates
> the improvement for the callers.

   Mhm, I did include all the drivers where the IRQ checks have to be modified,
not sure what else you want me to touch...

> Best regards
> Uwe

MBR, Sergey