Re: S_CTRL must be called twice to set volatile controls

[Hans Verkuil <hansverk@cisco.com>]

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.

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.

Regards,

	Hans

> 
> [1] http://www.spinics.net/lists/linux-media/msg89004.html
> [2] 45f014c5 [media] media/v4l2-ctrls: Always execute EXECUTE_ON_WRITE ctrls
> [3] b08d8d26 [media] media/v4l2-ctrls: volatiles should not generate CH_VALUE