On Tuesday, September 06, 2011 13:45:48 Sakari Ailus wrote:
> Hi Hans,
>
> On Tue, Sep 06, 2011 at 01:20:26PM +0200, Hans Verkuil wrote:
> > On Tuesday, September 06, 2011 13:07:42 Sakari Ailus wrote:
> > > Hi,
> > >
> > > I remember being in a discussion a while ago regarding the requirement of
> > > having all the controls belonging to the same class in
> > > VIDIOC_{TRY,S,G}_EXT_CTRLS. The answer I remember was that there was a
> > > historical reason for this and it no longer exists.
> >
> > The original rule was that all controls have to belong to the same class. This was
> > done to simplify drivers. Drivers that use the control framework can handle a class
> > of 0, which means that the controls can be of any class.
> >
> > But we still have drivers that implement S_EXT_CTRLS but do not use the control
> > framework, and for those this restriction is still valid. Usually such drivers will only
> > handle MPEG class controls through that API.
> >
> > So I don't think this restriction can be lifted as long as there are drivers that do not
> > use the control framework.
>
> All the drivers which implement *_EXT_CTRLS and check for ctrl_class do the
> check for a single class. All the references for ctrl_class in individual
> drivers (which actually were only checks that the user has set the field
> correctly) are removed by the patch I posted.
>
> So I don't see a reason why we couldn't just say "please set this to zero
> from now on".
>
>
>From what I remember (and I may be wrong by now) the drivers that implement S_EXT_CTRLS
by themselves typically only support ext_ctrls for controls of a specific class (MPEG usually).
Dropping the check means that: 1) applications may think they can use any control when they
can't for a certain group of drivers, and 2) applications can no longer detect up front whether
a driver supports mixing of control classes or not.
The way you can do 2) is by setting the control class to 0 and calling G/S/TRY_EXT_CTRLS with
0 controls.
Once everything is converted I don't mind dropping this check, but until then I believe it should
stay.
Regards,
Hans
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