Message-ID: <20220112213121.5ruae5mxwj6t3qiy@pengutronix.de>
Date:   Wed, 12 Jan 2022 22:31:21 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Andrew Lunn <andrew@...n.ch>, Ulf Hansson <ulf.hansson@...aro.org>,
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

On Wed, Jan 12, 2022 at 11:27:02AM +0100, Geert Uytterhoeven wrote:
> Hi Uwe,
> 
> On Wed, Jan 12, 2022 at 9:51 AM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.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@...n.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.

I agree that -ENXIO is unfortunate and -ENOENT would be more in line
with other functions. I assume it's insane to want to change that.

> > 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.

I think irq are not suitable for such a dummy handling. For clocks or
GPIOs there are cases where just doing nothing in the absence of a
certain optional clock or GPIO is fine.

I checked a few users of platform_get_irq_optional() and I didn't find a
single one that doesn't need to differentiate the irq and the no-irq
case later. Do you know one? If you do, isn't that so exceptional that
it doesn't justify the idea of a dummy irq value? So until proven
otherwise I think platform_get_irq_optional() just isn't in the spirit
of clk_get_optional() and gpiod_get_optional() because there are no use
cases where a dummy value would be good enough. (Even if request_irq
would be a noop for a dummy irq value.)

The motivation why platform_get_irq_optional() was introduced was just
that platform_get_irq() started to emit an error message (in commit
7723f4c5ecdb8d832f049f8483beb0d1081cedf6) and the (proportional) few
drivers where the error message was bad needed a variant that doesn't
emit the error message. Look at
31a8d8fa84c51d3ab00bf059158d5de6178cf890, the motivation to use
platform_get_irq_optional() wasn't that it simplifies handling in the
driver, but that it doesn't emit an error message. Or
8f5783ad9eb83747471f61f94dbe209fb9fb8a7d, or
2fd276c3ee4bd42eb034f8954964a5ae74187c6b, or
55cc33fab5ac9f7e2a97aa7c564e8b35355886d5. Just look at the output of git
log -Splatform_get_irq_optional to find some more.

That convinces me, that platform_get_irq_optional() is a bad name. The
only difference to platform_get_irq is that it's silent. And returning
a dummy irq value (which would make it aligned with the other _optional
functions) isn't possible.

> 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.

Best regards
Uwe

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

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