linux-kernel - Re: [PATCH 6/9] gpiolib: use descriptors internally

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Date:	Sat, 09 Feb 2013 09:17:29 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Alexandre Courbot <acourbot@...dia.com>,
	Linus Walleij <linus.walleij@...aro.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arch <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alexandre Courbot <gnurou@...il.com>
Subject: Re: [PATCH 6/9] gpiolib: use descriptors internally

On Thu, 7 Feb 2013 15:57:32 +0900, Alexandre Courbot <acourbot@...dia.com> wrote:
> On Wed, Feb 6, 2013 at 2:53 AM, Linus Walleij <linus.walleij@...aro.org> wrote:
> > On Sat, Feb 2, 2013 at 5:29 PM, Alexandre Courbot <acourbot@...dia.com> wrote:
> >> +/**
> >> + * Convert a GPIO number to its descriptor
> >> + */
> >> +static struct gpio_desc *gpio_to_desc(unsigned gpio)
> >> +{
> >> +       if (WARN(!gpio_is_valid(gpio), "invalid GPIO %d\n", gpio))
> >> +               return NULL;
> >
> > Don't we want to return ERR_PTR(-EINVAL); here?
> >
> > Then you can use IS_ERR() on the pointers later.
> >
> > This is the approach taken by the external API for clk
> > and pins.
> 
> Yes, that completely makes sense.
> 

No, it does not. The ERR_PTR()/IS_ERR() is a horrible pattern for code
readability because it breaks the expectations that programmers have for
what is and is not a bad pointer. There are decades of history where the
test for a bad pointer is 'if (!ptr)'. Not only does ERR_PTR make make
that test not work, but the compiler won't tell you when you get it
wrong.

There are places where ERR_PTR makes sense. Particularly when
communicating with userspace where error codes have very specific
meanings, but I don't want it in the GPIO subsystem.

g.

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