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SubjectRe: [PATCH] media/v4l2-ctrls: Always run s_ctrl on volatile ctrls
Hi Ricardo,

I've thought about this some more and I agree that this should be allowed.

But I have some comments, see below.

On 02/17/15 12:02, Ricardo Ribalda Delgado wrote:
> Volatile controls can change their value outside the v4l-ctrl framework.
> We should ignore the cached written value of the ctrl when evaluating if
> we should run s_ctrl.
> Signed-off-by: Ricardo Ribalda Delgado <>
> ---
> I have a control that tells the user when there has been a external trigger
> overrun. (Trigger while processing old image). This is a volatile control.
> The user writes 0 to the control, to ack the error condition, and clear the
> hardware flag.
> 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.
> drivers/media/v4l2-core/v4l2-ctrls.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
> index 45c5b47..3d0c7f4 100644
> --- a/drivers/media/v4l2-core/v4l2-ctrls.c
> +++ b/drivers/media/v4l2-core/v4l2-ctrls.c
> @@ -1605,7 +1605,7 @@ static int cluster_changed(struct v4l2_ctrl *master)
> for (i = 0; i < master->ncontrols; i++) {
> struct v4l2_ctrl *ctrl = master->cluster[i];
> - bool ctrl_changed = false;
> + bool ctrl_changed = ctrl->flags & V4L2_CTRL_FLAG_VOLATILE;

Should be done after the 'ctrl == NULL' check.

> if (ctrl == NULL)
> continue;

There is one more change that has to be made: setting a volatile control
should never generate a V4L2_EVENT_CTRL_CH_VALUE event since that makes
no sense. The way to prevent that is to ensure that ctrl->has_changed is
always false for volatile controls. The new_to_cur function looks at that
field to decide whether to send an event.

The documentation should also be updated: that of V4L2_CTRL_FLAG_VOLATILE



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