From 60c0732244164f14e376cfae493dba368f761514 Mon Sep 17 00:00:00 2001 From: Hans Verkuil Date: Wed, 29 Jun 2011 08:56:22 -0300 Subject: [media] v4l2-ctrls.c: add support for V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK Normally no control events will go to the filehandle that called the VIDIOC_S_CTRL/VIDIOC_S_EXT_CTRLS ioctls. This is to prevent a feedback loop. This can now be overridden by setting the new V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK flag. Based on suggestions from Mauro Carvalho Chehab and Laurent Pinchart . Signed-off-by: Hans Verkuil Signed-off-by: Mauro Carvalho Chehab --- .../DocBook/media/v4l/vidioc-subscribe-event.xml | 36 +++++++++++++++++----- 1 file changed, 29 insertions(+), 7 deletions(-) (limited to 'Documentation/DocBook/media/v4l') diff --git a/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml b/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml index 039a9694fd1..25471e8e5da 100644 --- a/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml +++ b/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml @@ -114,25 +114,28 @@ V4L2_EVENT_CTRL 3 - This event requires that the id + This event requires that the id 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;. - 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 + 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. + nasty feedback loops. If you do want to get the + event, then set the V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK + flag. + - This event type will ensure that no information is lost when + 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 changes field of the second-oldest event is ORed with the changes - field of the oldest event. + field of the oldest event. @@ -157,6 +160,25 @@ that are triggered by a status change such as V4L2_EVENT_CTRL. Other events will ignore this flag. + + V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK + 0x0002 + If set, then events directly caused by an ioctl will also be sent to + the filehandle that called that ioctl. For example, changing a control using + &VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same + filehandle. Normally such events are suppressed to prevent 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. + + Since it can't tell whether that event was caused by another application + or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the + control to the value in the event, or ignore it. + + Think carefully when you set this flag so you won't get into situations + like that. + + -- cgit v1.2.3