Hi Laurent,
On Fri, May 19, 2017 at 12:59:12AM +0300, Laurent Pinchart wrote:
> 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+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=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.
Uh, good point. Hmm.
>
> > > > 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 ?
Documentation/process/coding-style.rst around line 760 is helpful:
5) namespace collisions when defining local variables in macros resembling
functions:
.. code-block:: c
#define FOO(x) \
({ \
typeof(x) ret; \
ret = calc_ret(x); \
(ret); \
})
ret is a common name for a local variable - __foo_ret is less likely
to collide with an existing variable.
--
Regards,
Sakari Ailus
e-mail: sakari.ailus@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx
