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. Based on a suggestion from Mauro Carvalho Chehab <[email protected]>. 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 <[email protected]> --- .../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; -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
