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authorYunhong Jiang <yunhong.jiang@intel.com>2015-08-04 12:17:53 -0700
committerYunhong Jiang <yunhong.jiang@intel.com>2015-08-04 15:44:42 -0700
commit9ca8dbcc65cfc63d6f5ef3312a33184e1d726e00 (patch)
tree1c9cafbcd35f783a87880a10f85d1a060db1a563 /kernel/Documentation/DocBook/media/v4l/dev-event.xml
parent98260f3884f4a202f9ca5eabed40b1354c489b29 (diff)
Add the rt linux 4.1.3-rt3 as base
Import the rt linux 4.1.3-rt3 as OPNFV kvm base. It's from git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git linux-4.1.y-rt and the base is: commit 0917f823c59692d751951bf5ea699a2d1e2f26a2 Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Date: Sat Jul 25 12:13:34 2015 +0200 Prepare v4.1.3-rt3 Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> We lose all the git history this way and it's not good. We should apply another opnfv project repo in future. Change-Id: I87543d81c9df70d99c5001fbdf646b202c19f423 Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
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+ <title>Event Interface</title>
+
+ <para>The V4L2 event interface provides a means for a user to get
+ immediately notified on certain conditions taking place on a device.
+ This might include start of frame or loss of signal events, for
+ example. Changes in the value or state of a V4L2 control can also be
+ reported through events.
+ </para>
+
+ <para>To receive events, the events the user is interested in first must
+ be subscribed using the &VIDIOC-SUBSCRIBE-EVENT; ioctl. Once an event is
+ subscribed, the events of subscribed types are dequeueable using the
+ &VIDIOC-DQEVENT; ioctl. Events may be unsubscribed using
+ VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event type V4L2_EVENT_ALL may
+ be used to unsubscribe all the events the driver supports.</para>
+
+ <para>The event subscriptions and event queues are specific to file
+ handles. Subscribing an event on one file handle does not affect
+ other file handles.</para>
+
+ <para>The information on dequeueable events is obtained by using select or
+ poll system calls on video devices. The V4L2 events use POLLPRI events on
+ poll system call and exceptions on select system call.</para>
+
+ <para>Starting with kernel 3.1 certain guarantees can be given with
+ regards to events:<orderedlist>
+ <listitem>
+ <para>Each subscribed event has its own internal dedicated event queue.
+This means that flooding of one event type will not interfere with other
+event types.</para>
+ </listitem>
+ <listitem>
+ <para>If the internal event queue for a particular subscribed event
+becomes full, then the oldest event in that queue will be dropped.</para>
+ </listitem>
+ <listitem>
+ <para>Where applicable, certain event types can ensure that the payload
+of the oldest event that is about to be dropped will be merged with the payload
+of the next oldest event. Thus ensuring that no information is lost, but only an
+intermediate step leading up to that information. See the documentation for the
+event you want to subscribe to whether this is applicable for that event or not.</para>
+ </listitem>
+ </orderedlist></para>