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

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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@xxxxxxxxx>
---
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.


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?

Thanking you,
Nishka

What do you think?

Cheers,

Paul

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


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