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diff --git a/Documentation/userspace-api/media/v4l/vidioc-subscribe-event.rst b/Documentation/userspace-api/media/v4l/vidioc-subscribe-event.rst new file mode 100644 index 000000000000..ae3ed73c0a9e --- /dev/null +++ b/Documentation/userspace-api/media/v4l/vidioc-subscribe-event.rst @@ -0,0 +1,123 @@ +.. Permission is granted to copy, distribute and/or modify this +.. document under the terms of the GNU Free Documentation License, +.. Version 1.1 or any later version published by the Free Software +.. Foundation, with no Invariant Sections, no Front-Cover Texts +.. and no Back-Cover Texts. A copy of the license is included at +.. Documentation/userspace-api/media/fdl-appendix.rst. +.. +.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections + +.. _VIDIOC_SUBSCRIBE_EVENT: +.. _VIDIOC_UNSUBSCRIBE_EVENT: + +****************************************************** +ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT +****************************************************** + +Name +==== + +VIDIOC_SUBSCRIBE_EVENT - VIDIOC_UNSUBSCRIBE_EVENT - Subscribe or unsubscribe event + + +Synopsis +======== + +.. c:function:: int ioctl( int fd, VIDIOC_SUBSCRIBE_EVENT, struct v4l2_event_subscription *argp ) + :name: VIDIOC_SUBSCRIBE_EVENT + +.. c:function:: int ioctl( int fd, VIDIOC_UNSUBSCRIBE_EVENT, struct v4l2_event_subscription *argp ) + :name: VIDIOC_UNSUBSCRIBE_EVENT + + +Arguments +========= + +``fd`` + File descriptor returned by :ref:`open() <func-open>`. + +``argp`` + Pointer to struct :c:type:`v4l2_event_subscription`. + + +Description +=========== + +Subscribe or unsubscribe V4L2 event. Subscribed events are dequeued by +using the :ref:`VIDIOC_DQEVENT` ioctl. + + +.. tabularcolumns:: |p{4.6cm}|p{4.4cm}|p{8.7cm}| + +.. c:type:: v4l2_event_subscription + +.. flat-table:: struct v4l2_event_subscription + :header-rows: 0 + :stub-columns: 0 + :widths: 1 1 2 + + * - __u32 + - ``type`` + - Type of the event, see :ref:`event-type`. + + .. note:: + + ``V4L2_EVENT_ALL`` can be used with + :ref:`VIDIOC_UNSUBSCRIBE_EVENT <VIDIOC_SUBSCRIBE_EVENT>` for + unsubscribing all events at once. + * - __u32 + - ``id`` + - ID of the event source. If there is no ID associated with the + event source, then set this to 0. Whether or not an event needs an + ID depends on the event type. + * - __u32 + - ``flags`` + - Event flags, see :ref:`event-flags`. + * - __u32 + - ``reserved``\ [5] + - Reserved for future extensions. Drivers and applications must set + the array to zero. + + + +.. tabularcolumns:: |p{6.8cm}|p{2.2cm}|p{8.5cm}| + +.. _event-flags: + +.. flat-table:: Event Flags + :header-rows: 0 + :stub-columns: 0 + :widths: 3 1 4 + + * - ``V4L2_EVENT_SUB_FL_SEND_INITIAL`` + - 0x0001 + - When this event is subscribed an initial event will be sent + containing the current status. This only makes sense for events + 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 :ref:`VIDIOC_S_CTRL <VIDIOC_G_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 :ref:`VIDIOC_S_CTRL <VIDIOC_G_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. + + +Return Value +============ + +On success 0 is returned, on error -1 and the ``errno`` variable is set +appropriately. The generic error codes are described at the +:ref:`Generic Error Codes <gen-errors>` chapter. |