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

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Message-ID: <54E351B5.8040608@linux.intel.com>
Date:	Tue, 17 Feb 2015 16:35:33 +0200
From:	Sakari Ailus <sakari.ailus@...ux.intel.com>
To:	Jacek Anaszewski <j.anaszewski@...sung.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

Jacek Anaszewski wrote:
> 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.

I'm not fully certain if I'd do that --- no other flash chip driver
does. It might be good to think about how does one acquire the ownership
of media devices or parts of media devices, or whether it's something
that's needed at all.

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