Re: S_CTRL must be called twice to set volatile controls

[Jacek Anaszewski <j.anaszewski@samsung.com>]

Hi Hans,

On 04/29/2015 09:53 AM, Hans Verkuil wrote:
> Hi Jacek,
>
> On 04/29/15 09:33, Jacek Anaszewski wrote:
>> Hi,
>>
>> After testing my v4l2-flash helpers patch [1] with the recent patches
>> for v4l2-ctrl.c ([2] and [3]) s_ctrl op isn't called despite setting
>> the value that should be aligned to the other step than default one.
>>
>> This happens for V4L2_CID_FLASH_TORCH_INTENSITY control with
>> V4L2_CTRL_FLAG_VOLATILE flag.
>>
>> The situation improves after setting V4L2_CTRL_FLAG_EXECUTE_ON_WRITE
>> flag for the control. Is this flag required now for volatile controls
>> to be writable?
>
> Yes, you need that if you want to be able to write to a volatile control.
>
> It was added for exactly that purpose.

Thanks for the explanation.

> Why is V4L2_CID_FLASH_TORCH_INTENSITY volatile? Volatile typically only
> makes sense if the hardware itself is modifying the value without the
> software knowing about it.

This can be the case for the flash LED devices that can reduce torch
current when battery voltage level falls below predefined threshold.

-- 
Best Regards,
Jacek Anaszewski