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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 18 Jan 2022 13:49:15 +0100
Message-ID: <CAMuHMdWkwV9XE_R5FZ=jPtDwLpDbEngG6+X2JmiDJCZJZvUjYA@mail.gmail.com>
Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional
To:     =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= 
        <u.kleine-koenig@pengutronix.de>
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Hi Uwe,

On Tue, Jan 18, 2022 at 1:08 PM Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
> 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@pengutronix.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.

"vIRQ zero does not exist."

> 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.

So there's still a need for two functions.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds