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

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Date:	Wed, 9 Jan 2013 10:25:48 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Alexandre Courbot <acourbot@...dia.com>
Cc:	Arnd Bergmann <arnd@...db.de>, 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 10:06:16AM +0900, 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,
> may I ask why this is the case?

I think I've explained that in the past; many people just do not think.
They just use whatever macro they feel like is the right one.  We keep
seeing this, and this is a persistent problem. It's getting to be more
of a problem because people are starting to argue back when you point
out that they're wrong.

People are even starting to believe that documentation which specifies
explicitly "values where IS_ERR() is true are considered errors,
everything else is valid" means that the use of IS_ERR_OR_NULL() in such
cases is permissible.  (I've had such an argument with two people
recently.)

So, interfaces which have well defined return values and even interfaces
which specify _how_ errors should be checked end up being checked with
the wrong macros.  People constantly translate IS_ERR() to IS_ERR_OR_NULL()
even when it's inappropriate.

People don't think and people don't read documentation.  People don't
remember this level of detail.  Whatever the excuse, the problem remains.
IS_ERR_OR_NULL() always gets used inappropriately and without any regard
to whether it's correct or not.

So yes, IS_ERR_OR_NULL() _is_ pure evil.  IMHO this macro is doing more
harm than good.
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