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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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@arndb.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.