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Date: Mon, 20 Jul 2015 16:31:22 +0200
From: Hans Verkuil <hverkuil@xs4all.nl>
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To: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
CC: Hans Verkuil <hans.verkuil@cisco.com>,
	Sakari Ailus <sakari.ailus@linux.intel.com>,
	Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>,
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	Guennadi Liakhovetski <g.liakhovetski@gmx.de>,
	linux-media <linux-media@vger.kernel.org>
Subject: Re: [RFC v3 02/19] media/v4l2-core: add new ioctl VIDIOC_G_DEF_EXT_CTRLS
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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.

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.

Regards,

	Hans

> What do you think?
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