Date:   Mon, 17 Jan 2022 09:47:32 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Sergey Shtylyov <s.shtylyov@....ru>
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

Hello,

On Sun, Jan 16, 2022 at 09:15:20PM +0300, Sergey Shtylyov wrote:
> On 1/14/22 11:22 PM, Uwe Kleine-König 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 didn't recheck, but is this what the
driver changes in your patch is about?

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

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 ? */
		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.

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.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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