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

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Date:	Thu, 11 Apr 2013 13:56:54 +0200
From:	Bjørn Mork <bjorn@...k.no>
To:	Dave Jones <davej@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>,
	Andy Whitcroft <apw@...dowen.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: Re: [PATCH] checkpatch: Warn on comparisons to true and false

Dave Jones <davej@...hat.com> 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@...ches.com> 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@...i:/usr/local/src/git/linux$ git grep -E '!=\s*(true|false)'|wc -l
 63

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

 bjorn@...i:/usr/local/src/git/linux$ git grep -E '!=\s*true'|wc -l
 54

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 :)


Bjørn
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