Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit controls from a pipeline

[Steve Longerbeam]

On 03/03/2017 11:17 AM, Sakari Ailus wrote:
> Hi Steve,
>
> On Thu, Mar 02, 2017 at 06:12:43PM -0800, Steve Longerbeam wrote:
> > On 03/02/2017 03:48 PM, Steve Longerbeam wrote:
> > > On 03/02/2017 08:02 AM, Sakari Ailus wrote:
> > > > Hi Steve,
> > > >
> > > > On Wed, Feb 15, 2017 at 06:19:16PM -0800, Steve Longerbeam wrote:
> > > > > v4l2_pipeline_inherit_controls() will add the v4l2 controls from
> > > > > all subdev entities in a pipeline to a given video device.
> > > > >
> > > > > Signed-off-by: Steve Longerbeam <steve_longerbeam@xxxxxxxxxx>
> > > > > ---
> > > > > drivers/media/v4l2-core/v4l2-mc.c | 48
> > > > > +++++++++++++++++++++++++++++++++++++++
> > > > > include/media/v4l2-mc.h           | 25 ++++++++++++++++++++
> > > > > 2 files changed, 73 insertions(+)
> > > > >
> > > > > diff --git a/drivers/media/v4l2-core/v4l2-mc.c
> > > > > b/drivers/media/v4l2-core/v4l2-mc.c
> > > > > index 303980b..09d4d97 100644
> > > > > --- a/drivers/media/v4l2-core/v4l2-mc.c
> > > > > +++ b/drivers/media/v4l2-core/v4l2-mc.c
> > > > > @@ -22,6 +22,7 @@
> > > > > #include <linux/usb.h>
> > > > > #include <media/media-device.h>
> > > > > #include <media/media-entity.h>
> > > > > +#include <media/v4l2-ctrls.h>
> > > > > #include <media/v4l2-fh.h>
> > > > > #include <media/v4l2-mc.h>
> > > > > #include <media/v4l2-subdev.h>
> > > > > @@ -238,6 +239,53 @@ int v4l_vb2q_enable_media_source(struct
> > > > > vb2_queue *q)
> > > > > }
> > > > > EXPORT_SYMBOL_GPL(v4l_vb2q_enable_media_source);
> > > > >
> > > > > +int __v4l2_pipeline_inherit_controls(struct video_device *vfd,
> > > > > +                     struct media_entity *start_entity)
> > > >
> > > > I have a few concerns / questions:
> > > >
> > > > - What's the purpose of this patch? Why not to access the sub-device node
> > > >  directly?
> > >
> > >
> > > I don't really understand what you are trying to say.<snip>
> >
> > Actually I think I understand what you mean now. Yes, the user can
> > always access a subdev's control directly from its /dev/v4l-subdevXX.
> > I'm only providing this feature as a convenience to the user, so that
> > all controls in a pipeline can be accessed from one place, i.e. the
> > main capture device node.
>
> No other MC based V4L2 driver does this. You'd be creating device specific
> behaviour that differs from what the rest of the drivers do. The purpose of
> MC is to provide the user with knowledge of what devices are there, and the
> V4L2 sub-devices interface is used to access them in this case.

Well, again, I don't mind removing this. As I said it is only a
convenience (although quite a nice one in my opinion). I'd like
to hear from others whether this is worth keeping though.


> It does matter where a control is implemented, too. If the pipeline contains
> multiple sub-devices that implement the same control, only one of them may
> be accessed. The driver calling the function (or even less the function)
> would not know which one of them should be ignored.

Yes the pipeline should not have any duplicate controls. On imx-media no
pipelines that can be configured have duplicate controls.

Steve

> If you need such functionality, it should be implemented in the user space
> instead.