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

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

Hi,

On Mon 22 Jul 19, 19:14, Nishka Dasgupta wrote:
> On 22/07/19 5:54 PM, Paul Kocialkowski wrote:
> > 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.
> 
> It's not a problem per se. I'm just working on a cleanup project for which I
> went through all of staging replacing single-line functions with what they
> were calling. In some cases that makes it easier to figure out what a
> particular function call does, since the called function actually does
> something itself instead of just calling a different function?
> This function was also flagged as one such potentially-removable function by
> Coccinelle; but in order to do the same replacement that I'd done in other
> staging drivers, I thought I would do something about the type mismatch
> first, especially since find_format doesn't appear to be used anywhere else.
> However, now I won't remove check_format and replace it with find_format as
> I'd originally planned, since you've said that isn't necessary here. That
> leaves the return type issue.

Is that an issue reported by coccinelle?
If so, could you provide details about it?
 
> > So in the end, I think we should keep the !! and drop the (bool) cast if there's
> > no particular warning about it.
> 
> Should I send a version 3 that does this?

Yes I think !! would work out fine either way. Apparently we don't need any
explicit (bool) cast from a pointer anyway.

Cheers,

Paul

> Thanking you,
> Nishka
> 
> > 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