Message-ID: <20220118120806.pbjsat4ulg3vnhsh@pengutronix.de>
Date:   Tue, 18 Jan 2022 13:08:06 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

Hello Geert,

On Tue, Jan 18, 2022 at 10:37:25AM +0100, Geert Uytterhoeven wrote:
> On Tue, Jan 18, 2022 at 10:09 AM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> > For the (clk|gpiod|regulator)_get_optional() you don't have to check
> > against the magic not-found value (so no implementation detail magic
> > leaks into the caller code) and just pass it to the next API function.
> > (And my expectation would be that if you chose to represent not-found by
> > (void *)66 instead of NULL, you won't have to adapt any user, just the
> > framework internal checks. This is a good thing!)
> 
> Ah, there is the wrong assumption: drivers sometimes do need to know
> if the resource was found, and thus do need to know about (void *)66,
> -ENODEV, or -ENXIO.  I already gave examples for IRQ and clk before.
> I can imagine these exist for gpiod and regulator, too, as soon as
> you go beyond the trivial "enable" and "disable" use-cases.

My premise is that every user who has to check for "not found"
explicitly should not use (clk|gpiod)_get_optional() but
(clk|gpiod)_get() and do proper (and explicit) error handling for
-ENODEV. (clk|gpiod)_get_optional() is only for these trivial use-cases.

> And 0/NULL vs. > 0 is the natural check here: missing, but not
> an error.

For me it it 100% irrelevant if "not found" is an error for the query
function or not. I just have to be able to check for "not found" and
react accordingly.

And adding a function

	def platform_get_irq_opional():
		ret = platform_get_irq()
		if ret == -ENXIO:
			return 0
		return ret

it's not a useful addition to the API if I cannot use 0 as a dummy
because it doesn't simplify the caller enough to justify the additional
function.

The only thing I need to be able is to distinguish the cases "there is
an irq", "there is no irq" and anything else is "there is a problem I
cannot handle and so forward it to my caller". The semantic of
platform_get_irq() is able to satisfy this requirement[1], so why introduce
platform_get_irq_opional() for the small advantage that I can check for
not-found using

	if (!irq)

instead of

	if (irq != -ENXIO)

? The semantic of platform_get_irq() is easier ("Either a usable
non-negative irq number or a negative error number") compared to
platform_get_irq_optional() ("Either a usable positive irq number or a
negative error number or 0 meaning not found"). Usage of
platform_get_irq() isn't harder or more expensive (neither for a human
reader nor for a maching running the resulting compiled code).
For a human reader

	if (irq != -ENXIO)

is even easier to understand because for

	if (!irq)

they have to check where the value comes from, see it's
platform_get_irq_optional() and understand that 0 means not-found.

This function just adds overhead because as a irq framework user I have
to understand another function. For me the added benefit is too small to
justify the additional function. And you break out-of-tree drivers.
These are all no major counter arguments, but as the advantage isn't
major either, they still matter.

Best regards
Uwe

[1] the only annoying thing is the error message.

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

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