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

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Date:	Tue, 17 Feb 2015 15:22:13 +0100
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:	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
Subject: Re: [PATCH] media/v4l2-ctrls: Always run s_ctrl on volatile ctrls

On 02/17/2015 03:06 PM, Sakari Ailus wrote:
> Hi Jacek,
>
> Jacek Anaszewski wrote:
>> Hi Hans, Sakari,
>>
>> On 02/17/2015 12:32 PM, Sakari Ailus wrote:
>>> 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.
>>>
>>
>> Actually this will be not an issue for v4l2-flash sub-device anymore.
>> In the next version of the patch set the v4l2-flash sub-device
>> will be synchronizing the flash device registers with the
>> state of the controls on open.
>
> Ah, right --- you're preventing the use of the LED flash class whilst
> the V4L2 sub-device is opened?

Yes.

> I'm not fully certain whether that'd be
> really useful, as the V4L2 sub-device can also be opened by multiple
> users at the same time.

We also prevent from this using v4l2_fh_is_singular on open.

> However the applications that would access the
> LED class API are mostly different ones and for different purposes; I
> don't really have a strong opinion either way here.


-- 
Best Regards,
Jacek Anaszewski
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