linux-kernel - Re: [PATCH v2] V4L: uvcvideo: Add support for relative pan/tilt controls

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Date:	Wed, 25 Jun 2014 11:54:30 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Pawel Osciak <posciak@...omium.org>
Cc:	Vincent Palatin <vpalatin@...omium.org>,
	Hans de Goede <hdegoede@...hat.com>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	linux-kernel@...r.kernel.org, Olof Johansson <olofj@...omium.org>,
	Zach Kuznia <zork@...omium.org>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>
Subject: Re: [PATCH v2] V4L: uvcvideo: Add support for relative pan/tilt controls

Hi Pawel,

On Wednesday 25 June 2014 11:46:24 Pawel Osciak wrote:
> On Tue, Jun 17, 2014 at 11:45 PM, Vincent Palatin wrote:
> > Map V4L2_CID_TILT_RELATIVE and V4L2_CID_PAN_RELATIVE to the standard UVC
> > CT_PANTILT_RELATIVE_CONTROL terminal control request.
> > 
> > Tested by plugging a Logitech ConferenceCam C3000e USB camera
> > and controlling pan/tilt from the userspace using the VIDIOC_S_CTRL ioctl.
> > Verified that it can pan and tilt at the same time in both directions.
> > 
> > Signed-off-by: Vincent Palatin <vpalatin@...omium.org>
> > 
> > Change-Id: I7b70b228e5c0126683f5f0be34ffd2807f5783dc
> > ---
> > 
> > Changes
> > v2: fix control request name in description.
> 
> The patch looks good, but I have a more general comment for everyone to
> consider. This doesn't match the expected functionality of
> controls V4L2_CID_PAN/TILT_RELATIVE. This is basically an on/off switch for
> pan/tilt, which once enabled will keep going until turned off (or I'm
> guessing until the maximum pan/tilt is reached), while the controls are
> supposed to expose an ability to turn the camera by a specified amount.
> Here the amount will also be ignored...

I agree with you here, and this mismatch between the V4L and UVC controls is 
the reason why I haven't implemented relative pan/tilt support.

> Given that this is a standard UVC control, perhaps we need new V4L2
> controls for it, as I'm assuming we can't change the meaning of existing
> controls?

We could extend the meaning of the controls to cover the UVC behaviour in a 
device-specific fashion, but that would be confusing for applications, so new 
controls might be a better idea.

-- 
Regards,

Laurent Pinchart

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