Re: [PATCH v2] staging: media: sunxi: Add bool cast to value

[Paul Kocialkowski <paul.kocialkowski@bootlin.com>]

Hi,

On Mon 22 Jul 19, 12:12, Jeremy Sowden wrote:
> On 2019-07-22, at 11:36:51 +0530, Nishka Dasgupta wrote:
> > Typecast as bool the return value of cedrus_find_format in
> > cedrus_check_format as the return value of cedrus_check_format is
> > always treated like a boolean value.
> >
> > Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
> > ---
> > Changes in v2:
> > - Add !! to the returned pointer to ensure that the return value is
> >   always either true or false, and never a non-zero value other than
> >   true.
> >
> >  drivers/staging/media/sunxi/cedrus/cedrus_video.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/staging/media/sunxi/cedrus/cedrus_video.c b/drivers/staging/media/sunxi/cedrus/cedrus_video.c
> > index e2b530b1a956..b731745f21f8 100644
> > --- a/drivers/staging/media/sunxi/cedrus/cedrus_video.c
> > +++ b/drivers/staging/media/sunxi/cedrus/cedrus_video.c
> > @@ -86,7 +86,7 @@ static struct cedrus_format *cedrus_find_format(u32 pixelformat, u32 directions,
> >  static bool cedrus_check_format(u32 pixelformat, u32 directions,
> >  				unsigned int capabilities)
> >  {
> > -	return cedrus_find_format(pixelformat, directions, capabilities);
> > +	return !!(bool)cedrus_find_format(pixelformat, directions, capabilities);
> >  }
> 
> I think the original was fine.  The return value of cedrus_find_format
> will be automatically converted to bool before being returned from
> cedrus_check_format since that is the return-type of the function, and
> the result of converting any non-zero value to bool is 1.

Okay I was a bit unsure about that and wanted to play it on the safe side
without really looking it up, but that gave me the occasion to verify.

From what I could find (from my GNU system's /usr/include/unistring/stdbool.h):

   Limitations of this substitute, when used in a C89 environment:

       - In C99, casts and automatic conversions to '_Bool' or 'bool' are
         performed in such a way that every nonzero value gets converted
         to 'true', and zero gets converted to 'false'.  This doesn't work
         with this substitute.  With this substitute, only the values 0 and 1
         give the expected result when converted to _Bool' or 'bool'.

So since the kernel is built for C89 (unless I'm mistaken), I don't think the
compiler provides any guarantee about bool values being converted to 1 when
they are non-zero. As a result, I think it's best to be careful.

However, I'm not sure I really see what cocinelle was unhappy about. You
mentionned single-line functions, but I don't see how that can be a problem.

So in the end, I think we should keep the !! and drop the (bool) cast if there's
no particular warning about it.

What do you think?

Cheers,

Paul

> >  static void cedrus_prepare_format(struct v4l2_pix_format *pix_fmt)
> > --
> > 2.19.1

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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