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 <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;
--
1.7.1
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