Hi Laurent,
On Thu, May 18, 2017 at 11:54:46PM +0300, Laurent Pinchart wrote:
> Hi Sakari,
>
> On Thursday 18 May 2017 23:50:34 Sakari Ailus wrote:
> > On Thu, May 18, 2017 at 07:08:00PM +0300, Laurent Pinchart wrote:
> > > On Wednesday 17 May 2017 22:20:57 Sakari Ailus wrote:
> > >> On Wed, May 17, 2017 at 04:38:14PM +0100, Kieran Bingham wrote:
> > >>> From: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>
> > >>>
> > >>> Return NULL, if a null entity is parsed for it's v4l2_subdev
> > >>>
> > >>> Signed-off-by: Kieran Bingham
> > >>> <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>
> > >>> ---
> > >>>
> > >>> include/media/v4l2-subdev.h | 2 +-
> > >>> 1 file changed, 1 insertion(+), 1 deletion(-)
> > >>>
> > >>> diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> > >>> index 5f1669c45642..72d7f28f38dc 100644
> > >>> --- a/include/media/v4l2-subdev.h
> > >>> +++ b/include/media/v4l2-subdev.h
> > >>> @@ -829,7 +829,7 @@ struct v4l2_subdev {
> > >>> };
> > >>>
> > >>> #define media_entity_to_v4l2_subdev(ent) \
> > >>> - container_of(ent, struct v4l2_subdev, entity)
> > >>> + (ent ? container_of(ent, struct v4l2_subdev, entity) : NULL)
> > >>> #define vdev_to_v4l2_subdev(vdev) \
> > >>> ((struct v4l2_subdev *)video_get_drvdata(vdev))
> > >>
> > >> The problem with this is that ent is now referenced twice. If the ent
> > >> macro argument has side effect, this would introduce bugs. It's
> > >> unlikely, but worth avoiding. Either use a macro or a function.
> > >>
> > >> I think I'd use function for there's little use for supporting for const
> > >> and non-const arguments presumably. A simple static inline function
> > >> should do.
> > >
> > > Note that, if we want to keep using a macro, this could be written as
> > >
> > > #define media_entity_to_v4l2_subdev(ent) ({ \
> > >
> > > typeof(ent) __ent = ent; \
>
> I just realized that this should be written
>
> typeof(ent) __ent = (ent);
I don't think that really makes much of a difference. It's a little bit
safer still perhaps. I don't remember having seen a case where the function
argument would have required parentheses there though.
>
> > > __ent ? container_of(__ent, struct v4l2_subdev, entity) : NULL; \
> > >
> > > })
> > >
> > > Bonus point if you can come up with a way to return a const struct
> > > v4l2_subdev pointer when then ent argument is const.
> >
> > I can't think of a use case for that. I've never seen a const struct
> > v4l2_subdev anywhere. I could be just oblivious though. :-)
>
> I agree with you, it's overkill, at least for now. Although I'd like to see
> how it could be done, for other similar constructs where both const and non-
> const versions are useful.
Yes, that approach is fine. Another example here (not merged yet):
<URL:https://git.linuxtv.org/sailus/media_tree.git/commit/?h=of&id=1461f5172d40c1c4632bcb457e5f580836922879>
>
> > Better give a __ent a name that someone will not accidentally come up with.
> > That can lead to problems that are difficult to debug --- for the code
> > compiles, it just doesn't do what's expected.
>
> Won't it generate a compilation error as the variable would be redefined by
> the macro ?
It's perfectly fine redefine local variables. The compiler could just
generate a warning and not an error.
--
Regards,
Sakari Ailus
e-mail: sakari.ailus@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx
