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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 12 Jan 2022 11:27:02 +0100
Message-ID: <CAMuHMdWsMGPiQaPS0-PJ_+Mc5VQ37YdLfbHr_aS40kB+SfW-aw@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 Wed, Jan 12, 2022 at 9:51 AM Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
> On Wed, Jan 12, 2022 at 09:33:48AM +0100, Geert Uytterhoeven wrote:
> > On Mon, Jan 10, 2022 at 10:20 PM Andrew Lunn <andrew@lunn.ch> wrote:
> > > On Mon, Jan 10, 2022 at 09:10:14PM +0100, Uwe Kleine-König wrote:
> > > > On Mon, Jan 10, 2022 at 10:54:48PM +0300, Sergey Shtylyov wrote:
> > > > > This patch is based on the former Andy Shevchenko's patch:
> > > > >
> > > > > https://lore.kernel.org/lkml/20210331144526.19439-1-andriy.shevchenko@linux.intel.com/
> > > > >
> > > > > Currently platform_get_irq_optional() returns an error code even if IRQ
> > > > > resource simply has not been found. It prevents the callers from being
> > > > > error code agnostic in their error handling:
> > > > >
> > > > >     ret = platform_get_irq_optional(...);
> > > > >     if (ret < 0 && ret != -ENXIO)
> > > > >             return ret; // respect deferred probe
> > > > >     if (ret > 0)
> > > > >             ...we get an IRQ...
> > > > >
> > > > > All other *_optional() APIs seem to return 0 or NULL in case an optional
> > > > > resource is not available. Let's follow this good example, so that the
> > > > > callers would look like:
> > > > >
> > > > >     ret = platform_get_irq_optional(...);
> > > > >     if (ret < 0)
> > > > >             return ret;
> > > > >     if (ret > 0)
> > > > >             ...we get an IRQ...
> > > >
> > > > The difference to gpiod_get_optional (and most other *_optional) is that
> > > > you can use the NULL value as if it were a valid GPIO.
> > > >
> > > > As this isn't given with for irqs, I don't think changing the return
> > > > value has much sense.
> > >
> > > We actually want platform_get_irq_optional() to look different to all
> > > the other _optional() methods because it is not equivalent. If it
> > > looks the same, developers will assume it is the same, and get
> > > themselves into trouble.
> >
> > Developers already assume it is the same, and thus forget they have
> > to check against -ENXIO instead of zero.
>
> Is this an ack for renaming platform_get_irq_optional() to
> platform_get_irq_silent()?

No it isn't ;-)

If an optional IRQ is not present, drivers either just ignore it (e.g.
for devices that can have multiple interrupts or a single muxed IRQ),
or they have to resort to polling. For the latter, fall-back handling
is needed elsewhere in the driver.
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.

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