Re: [RFC v3 02/19] media/v4l2-core: add new ioctl VIDIOC_G_DEF_EXT_CTRLS

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

I like your proposal, and especially how it would integrate with the requests 
API. Should we give the requests API a try to make sure your proposal works 
fine with it ?

As a side note, I'll need to requests API for Renesas. The current schedule is 
to have a first RFC implementation by the end of October.

On Monday 20 July 2015 16:31:22 Hans Verkuil wrote:
> On 07/20/2015 03:52 PM, Ricardo Ribalda Delgado wrote:
> > Hello
> > 
> > I have no preference over the two implementations, but I see an issue
> > with this suggestion.
> > 
> > What happens to out out tree drivers, or drivers that don't support
> > this functionality?
> > 
> > With the ioctl, the user receives a -ENOTTY. So he knows there is
> > something wrong with the driver.
> > 
> > With this class, the driver might interpret this a simple G_VAL and
> > return he current value with no way for the user to know what is going
> > on.
> 
> Drivers that implement the current API correctly will return an error
> if V4L2_CTRL_WHICH_DEF_VAL was specified. Such drivers will interpret
> the value as a control class, and no control classes in that range exist.
> See also class_check() in v4l2-ctrls.c.
> 
> The exception here is uvc which doesn't have this class check and it will
> just return the current value :-(
> 
> I don't see a way around this, unfortunately.

The rationale for implementing VIDIOC_G_DEF_EXT_CTRLS was to get the default 
value of controls that don't fit in 32 bits. uvcvideo doesn't have any such 
control, so I don't think we really need to care. Of course newer versions of 
the uvcvideo driver should implement the new API.

> Out-of-tree drivers that use the control framework are fine, and I don't
> really care about drivers (out-of-tree or otherwise) that do not use the
> control framework.
> 
> > Regarding the new implementation.... I can make some code next week,
> > this week I am 120% busy :)
> 
> Wait until there is a decision first :-)
> 
> It's not a lot of work, I think.

I think I like your proposal better than VIDIOC_G_DEF_EXT_CTRLS, so seeing an 
implementation would be nice.

-- 
Regards,

Laurent Pinchart