linux-kernel - Re: [PATCH] media/v4l2-ctrls: Always run s

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Date:	Tue, 17 Feb 2015 13:32:16 +0200
From:	Sakari Ailus <sakari.ailus@...ux.intel.com>
To:	Hans Verkuil <hansverk@...co.com>,
	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Antti Palosaari <crope@....fi>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jacek Anaszewski <j.anaszewski@...sung.com>
Subject: Re: [PATCH] media/v4l2-ctrls: Always run s_ctrl on volatile ctrls

Hi Hans,

Hans Verkuil wrote:
...
>> Unfortunately, it only works one time, because the next time the user writes
>> a zero to the control cluster_changed returns false.
>>
>> I think on volatile controls it is safer to run s_ctrl twice than missing a
>> valid s_ctrl.
>>
>> I know I am abusing a bit the API for this :P, but I also believe that the
>> semantic here is a bit confusing.
> 
> The reason for that is that I have yet to see a convincing argument for
> allowing s_ctrl for a volatile control.

Well, one example are LED flash class devices which implement V4L2 flash
API through a wrapper. The user may use the LED flash class API to
change the values of the controls, and V4L2 framework has no clue about
this. The V4L2 controls are volatile, and the real values of the
settings are stored in the LED flash class.

This is the current implementation (not merged yet); an alternative, a
more correct one, would be to use callbacks to tell about the changes in
control values. I haven't pushed for that, primarily because the
patchset is already quite complex and I've seen this as something that
can be always implemented later if it bothers someone.

Cc Jacek.

-- 
Kind regards,

Sakari Ailus
sakari.ailus@...ux.intel.com
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