Hi Sakari,
On Friday 19 May 2017 00:05:17 Sakari Ailus wrote:
> On Thu, May 18, 2017 at 11:54:46PM +0300, Laurent Pinchart wrote:
> > 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+rene...@ideasonboard.com>
> >>>>>
> >>>>> Return NULL, if a null entity is parsed for it's v4l2_subdev
> >>>>>
> >>>>> Signed-off-by: Kieran Bingham
> >>>>> <kieran.bingham+rene...@ideasonboard.com>
> >>>>> ---
> >>>>>
> >>>>> 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=1461f5172
> d40c1c4632bcb457e5f580836922879>
The problem here is that the container_of() macro ends up casting the argument
to a struct v4l2_subdev *, regardless of whether the original pointer was
const or not.
> > > 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.
Of course, my bad. As the local __ent variable would shadow any variable of
the same name declared in a parent context, the only case where this would
cause a problem is if the ent argument to the macro references the __ent
variable. In the simplest case, if the ent argument is __ent, the construct
would lead to
typeof(__ent) __ent = (__ent);
which interestingly enough compiles without generating a warning, but
generates incorrect code. Various macros in kernel.h seem to be subject to the
same problem.
Do you have a suggestion for a better variable name ?
> The compiler could just generate a warning and not an error.
--
Regards,
Laurent Pinchart
