linux-kernel - Re: [PATCH] gpio: return NULL from gpiod_get

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Date:	Fri, 20 Feb 2015 19:54:02 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Alexandre Courbot <gnurou@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpio: return NULL from gpiod_get_optional when GPIOLIB
 is disabled

Hello,

On Thu, Feb 19, 2015 at 09:33:44PM -0800, Dmitry Torokhov wrote:
> On Fri, Feb 20, 2015 at 01:59:43PM +0900, Alexandre Courbot wrote:
> > On Fri, Feb 20, 2015 at 9:30 AM, Dmitry Torokhov
> > <dmitry.torokhov@...il.com> wrote:
> > > Given the intent behind gpiod_get_optional() and friends it does not make
> > > sense to return -ENOSYS when GPIOLIB is disabled: the driver is expected to
> > > work just fine without gpio so let's behave as if gpio was not found.
> > > Otherwise we have to special-case -ENOSYS in drivers.
> > 
> > Interestingly Uwe sent a RFC for this one week ago:
> > 
> > http://patchwork.ozlabs.org/patch/439135/
> > 
> > Maybe credit him with a Suggested-by.?
> 
> I certainly am fine with crediting him with Suggested-by even though I did not
> see that Uwe's e-mail but this patch was prompted by his other patch changing a
> few input drivers to use gpiod_get_optional() and me recalling that I
> explicitly did not use it as it made no difference from gpiod_get() since I had
> to handle -ENOSYS anyway.
Note that I'm not convinced any more this is a good idea. Consider you
have a device tree entry specifying 

	reset-gpio = <&gpio5 4 0>;

for your device.

With

	gpiod_get_optional(dev, "reset", GPIO_OUT_LOW);

the drivers tells that some of the devices it can handle have a reset
gpio. If the device in question does have such a gpio the driver must
know and do something with it. If the device doesn't have such a gpio
that's fine, too.

But if GPIOLIB is off and the device has a reset-gpio specified you
certainly want to error out, right?

So IMHO the right thing to do is to return NULL iff there is no
reset-gpio specified. Otherwise -ENOSYS is the right thing to return.

Best regards
Uwe

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