Hi Hans,
On Tuesday 28 June 2011 17:18:07 Hans Verkuil wrote:
> Originally no control events would go to the filehandle that called the
> VIDIOC_S_CTRL/VIDIOC_S_EXT_CTRLS ioctls. This was to prevent a feedback
> loop.
>
> This is now only done if the new V4L2_EVENT_SUB_FL_NO_FEEDBACK flag is
> set.
What about doing it the other way around ? Most applications won't want that
feedback, you could disable it by default.
> Based on a suggestion from Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>.
>
> This goes on top of the patch series I posted today titled:
> "Allocate events per-event-type, v4l2-ctrls cleanup"
>
> Signed-off-by: Hans Verkuil <hans.verkuil@xxxxxxxxx>
> ---
> .../DocBook/media/v4l/vidioc-subscribe-event.xml | 31
> ++++++++++++++------ drivers/media/video/v4l2-ctrls.c |
> 3 +-
> include/linux/videodev2.h | 3 +-
> 3 files changed, 26 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
> b/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml index
> 039a969..0d4465f 100644
> --- a/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
> +++ b/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
> @@ -114,25 +114,20 @@
> <row>
> <entry><constant>V4L2_EVENT_CTRL</constant></entry>
> <entry>3</entry>
> - <entry>This event requires that the <structfield>id</structfield>
> + <entry><para>This event requires that the
> <structfield>id</structfield> matches the control ID from which you want
> to receive events.
> This event is triggered if the control's value changes, if a
> button control is pressed or if the control's flags change.
> This event has a &v4l2-event-ctrl; associated with it. This struct
> contains much of the same information as &v4l2-queryctrl; and
> - &v4l2-control;.
> + &v4l2-control;.</para>
>
> - If the event is generated due to a call to &VIDIOC-S-CTRL; or
> - &VIDIOC-S-EXT-CTRLS;, then the event will not be sent to
> - the file handle that called the ioctl function. This prevents
> - nasty feedback loops.
> -
> - This event type will ensure that no information is lost when
> + <para>This event type will ensure that no information is lost when
> more events are raised than there is room internally. In that
> case the &v4l2-event-ctrl; of the second-oldest event is kept,
> but the <structfield>changes</structfield> field of the
> second-oldest event is ORed with the <structfield>changes</structfield>
> - field of the oldest event.
> + field of the oldest event.</para>
> </entry>
> </row>
> <row>
> @@ -157,6 +152,24 @@
> that are triggered by a status change such as
> <constant>V4L2_EVENT_CTRL</constant>. Other events will ignore this
> flag.</entry>
> </row>
> + <row>
> + <entry><constant>V4L2_EVENT_SUB_FL_NO_FEEDBACK</constant></entry>
> + <entry>0x0002</entry>
> + <entry><para>If set, then events directly caused by an ioctl will not
> be sent to + the filehandle that called that ioctl. For example,
changing
> a control using + &VIDIOC-S-CTRL; will not cause a V4L2_EVENT_CTRL to be
> sent back to that + same filehandle. All other filehandles that are
> subscribed to that event + will still receive it. This prevents feedback
> loops where an application + changes a control to a one value and then
> another, and then receives an + event telling it that that control has
> changed to the first value.</para> +
> + <para>Since it can't tell whether that event was caused by another
> application + or by the first value change it is hard to decide whether
> to set the + control to the value in the event, or ignore it.</para>
> +
> + <para>This flag will prevent this situation from happening.</para>
> + </entry>
> + </row>
> </tbody>
> </tgroup>
> </table>
> diff --git a/drivers/media/video/v4l2-ctrls.c
> b/drivers/media/video/v4l2-ctrls.c index bc08f86..bac793a 100644
> --- a/drivers/media/video/v4l2-ctrls.c
> +++ b/drivers/media/video/v4l2-ctrls.c
> @@ -590,7 +590,8 @@ static void send_event(struct v4l2_fh *fh, struct
> v4l2_ctrl *ctrl, u32 changes) fill_event(&ev, ctrl, changes);
>
> list_for_each_entry(sev, &ctrl->ev_subs, node)
> - if (sev->fh && sev->fh != fh)
> + if (sev->fh && (sev->fh != fh ||
> + !(sev->flags & V4L2_EVENT_SUB_FL_NO_FEEDBACK)))
> v4l2_event_queue_fh(sev->fh, &ev);
> }
>
> diff --git a/include/linux/videodev2.h b/include/linux/videodev2.h
> index baafe2f..00bae77 100644
> --- a/include/linux/videodev2.h
> +++ b/include/linux/videodev2.h
> @@ -1832,7 +1832,8 @@ struct v4l2_event {
> __u32 reserved[8];
> };
>
> -#define V4L2_EVENT_SUB_FL_SEND_INITIAL (1 << 0)
> +#define V4L2_EVENT_SUB_FL_SEND_INITIAL (1 << 0)
> +#define V4L2_EVENT_SUB_FL_NO_FEEDBACK (1 << 1)
>
> struct v4l2_event_subscription {
> __u32 type;
--
Regards,
Laurent Pinchart
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