Re: [PATCH] media/v4l2-ctrls: Always run s_ctrl on volatile ctrls

[Jacek Anaszewski]

On 02/17/2015 03:06 PM, Sakari Ailus wrote:
> Hi Jacek,
>
> Jacek Anaszewski wrote:
> > Hi Hans, Sakari,
> >
> > On 02/17/2015 12:32 PM, Sakari Ailus wrote:
> > > Hi Hans,
> > >
> > > Hans Verkuil wrote:
> > > ...
> > > > > Unfortunately, it only works one time, because the next time the
> > > > > user writes
> > > > > a zero to the control cluster_changed returns false.
> > > > >
> > > > > I think on volatile controls it is safer to run s_ctrl twice than
> > > > > missing a
> > > > > valid s_ctrl.
> > > > >
> > > > > I know I am abusing a bit the API for this :P, but I also believe
> > > > > that the
> > > > > semantic here is a bit confusing.
> > > >
> > > > The reason for that is that I have yet to see a convincing argument for
> > > > allowing s_ctrl for a volatile control.
> > >
> > > Well, one example are LED flash class devices which implement V4L2 flash
> > > API through a wrapper. The user may use the LED flash class API to
> > > change the values of the controls, and V4L2 framework has no clue about
> > > this. The V4L2 controls are volatile, and the real values of the
> > > settings are stored in the LED flash class.
> > >
> > > This is the current implementation (not merged yet); an alternative, a
> > > more correct one, would be to use callbacks to tell about the changes in
> > > control values. I haven't pushed for that, primarily because the
> > > patchset is already quite complex and I've seen this as something that
> > > can be always implemented later if it bothers someone.
> > >
> > > Cc Jacek.
> >
> > Actually this will be not an issue for v4l2-flash sub-device anymore.
> > In the next version of the patch set the v4l2-flash sub-device
> > will be synchronizing the flash device registers with the
> > state of the controls on open.
>
> Ah, right --- you're preventing the use of the LED flash class whilst
> the V4L2 sub-device is opened?

Yes.

> I'm not fully certain whether that'd be
> really useful, as the V4L2 sub-device can also be opened by multiple
> users at the same time.

We also prevent from this using v4l2_fh_is_singular on open.

> However the applications that would access the
> LED class API are mostly different ones and for different purposes; I
> don't really have a strong opinion either way here.


--
Best Regards,
Jacek Anaszewski
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