linux-kernel - Re: [PATCH 1/4] gpiolib: introduce descriptor-based GPIO interface

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Date:	Wed, 9 Jan 2013 10:44:14 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Alexandre Courbot <acourbot@...dia.com>,
	linux-arch@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH 1/4] gpiolib: introduce descriptor-based GPIO interface

On Wed, Jan 09, 2013 at 10:35:22AM +0000, Arnd Bergmann wrote:
> On Wednesday 09 January 2013, Alexandre Courbot wrote:
> > On Tue, Jan 8, 2013 at 9:59 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > > Please avoid the use of IS_ERR_OR_NULL(), especially on interfaces you
> > > introduce yourself. AFAICT, gpiod_get cannot return NULL, so you
> > > should not check for that.
> > 
> > Sure - you sound like IS_ERR_OR_NULL() is generally considered evil,
> 
> Correct.
> 
> > may I ask why this is the case?
> 
> It's very hard to get right: either you are interested in the error code,
> and then you don't have one in some cases, or you don't care but have
> to check for it anyway. When you define a function, just make it clear
> what the expected return values are, either NULL for error or a negative
> ERR_PTR value, but not both.

Indeed, and any code which does this:

	if (IS_ERR_OR_NULL(ptr))
		return PTR_ERR(ptr);

is buggy because on NULL it returns 0, which is generally accepted as being
"success".
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