Re: [PATCH] media/v4l2-ctrls: Always run s_ctrl on volatile ctrls

[Sakari Ailus <sakari.ailus@linux.intel.com>]

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? 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. 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.

-- 
Regards,

Sakari Ailus
sakari.ailus@linux.intel.com