Re: [RFC] New class for low level sensors controls?

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

Hi Hans,

On Monday 26 September 2011 11:51:05 Hans Verkuil wrote:
> On Tuesday, September 06, 2011 13:36:53 Sakari Ailus wrote:
> > Hi all,
> > 
> > We are beginning to have raw bayer image sensor drivers in the mainline.
> > Typically such sensors are not controlled by general purpose applications
> > but e.g. require a camera control algorithm framework in user space. This
> > needs to be implemented in libv4l for general purpose applications to
> > work properly on this kind of hardware.
> > 
> > These sensors expose controls such as
> > 
> > - Per-component gain controls. Red, blue, green (blue) and green (red)
> > 
> >   gains.
> > 
> > - Link frequency. The frequency of the data link from the sensor to the
> > 
> >   bridge.
> > 
> > - Horizontal and vertical blanking.
> > 
> > None of these controls are suitable for use of general purpose
> > applications (let alone the end user!) but for the camera control
> > algorithms.
> > 
> > We have a control class called V4L2_CTRL_CLASS_CAMERA for camera
> > controls. However, the controls in this class are relatively high level
> > controls which are suitable for end user. The algorithms in the libv4l
> > or a webcam could implement many of these controls whereas I see that
> > only
> > V4L2_CID_EXPOSURE_ABSOLUTE might be implemented by raw bayer sensors.
> > 
> > My question is: would it make sense to create a new class of controls for
> > the low level sensor controls in a similar fashion we have a control
> > class for the flash controls?
> 
> I'm plowing through all the mails on this list that piled up during the
> last 3-4 weeks, so my reply is a bit late :-)
> 
> I don't believe a new class is the right approach. If such controls are
> part of a sub-device, then they can be marked 'private', which means they
> are only accessible through the subdev device node.

We're not trying to hide controls, but to standardize low-level sensor 
controls that are common across many sensors. They don't really belong to any 
of the existing classes, hence the idea of creating a new class for them.

> For low-level controls that are part of the bridge chip there is currently
> no suitable way of hiding them.
> 
> In that case the best approach would be to add a new control flag called
> V4L2_CTRL_FLAG_HIDE (or _INVISIBLE, or _LOW_LEVEL, or something similar).
> Applications can filter such controls and not show them.
> 
> I've toyed with this idea before, but of course it has the disadvantage of
> requiring application support.
> 
> One alternative to this is to let QUERYCTRL skip such hidden controls
> unless instructed otherwise (e.g. by adding a V4L2_CTRL_FLAG_SHOW_ALL to
> the control's id). But I think that's getting a bit too complex.
> 
> I think adding a 'HIDE' flag of some sort is a good idea. It's simple and
> does the job.

-- 
Regards,

Laurent Pinchart