Re: [PATCH] checkpatch: Warn on comparisons to true and false

From: BjÃrn Mork
Date: Thu Apr 11 2013 - 07:57:22 EST

Dave Jones <davej@xxxxxxxxxx> writes:
> On Wed, Apr 10, 2013 at 03:57:51PM -0700, Andrew Morton wrote:
> > On Tue, 09 Apr 2013 20:17:14 -0700 Joe Perches <joe@xxxxxxxxxxx> wrote:
> >
> > > Comparisons of A to true and false are better written
> > > as A and !A.
> > >
> > > Bleat a message on use.
> >
> > hm. I'm counting around 1,100 instances of "== true" and "== false".
> >
> > That's a lot of people to shout at. Is it really worthwhile?
> > "foo==true" is a bit of a waste of space but I can't say that I find it
> > terribly offensive.
> It would be interesting to see how many people have historically screwed
> up and used (!a) when they mean (a) and vice versa, versus spelling
> it out longform. I'd be surprised if the results weren't skewed
> in favour of the more verbose form.

You have to consider that it is still possible to reverse the operator
even if spelling it out, so you don't really gain anything:

bjorn@nemi:/usr/local/src/git/linux$ git grep -E '!=\s*(true|false)'|wc -l

and since most of these compare to true, they are also at risk wrt

bjorn@nemi:/usr/local/src/git/linux$ git grep -E '!=\s*true'|wc -l

Based on a quick look at a few of these I guess they are mostly OK,
testing against bool values. But I felt I had to share this little gem
which showed up in drivers/gpu/drm/exynos/exynos_drm_vidi.c:

static int vidi_power_on(struct vidi_context *ctx, bool enable)
struct exynos_drm_subdrv *subdrv = &ctx->subdrv;
struct device *dev = subdrv->dev;

DRM_DEBUG_KMS("%s\n", __FILE__);

if (enable != false && enable != true)
return -EINVAL;

That's taking failsafe to the next step :)

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