diff options
| -rw-r--r-- | Documentation/video4linux/v4l2-framework.txt | 51 |
1 files changed, 35 insertions, 16 deletions
diff --git a/Documentation/video4linux/v4l2-framework.txt b/Documentation/video4linux/v4l2-framework.txt index 312a0e2f9d66..f8dcabf7852c 100644 --- a/Documentation/video4linux/v4l2-framework.txt +++ b/Documentation/video4linux/v4l2-framework.txt @@ -897,14 +897,38 @@ V4L2 events The V4L2 events provide a generic way to pass events to user space. The driver must use v4l2_fh to be able to support V4L2 events. -Useful functions: +Events are defined by a type and an optional ID. The ID may refer to a V4L2 +object such as a control ID. If unused, then the ID is 0. + +When the user subscribes to an event the driver will allocate a number of +kevent structs for that event. So every (type, ID) event tuple will have +its own set of kevent structs. This guarantees that if a driver is generating +lots of events of one type in a short time, then that will not overwrite +events of another type. + +But if you get more events of one type than the number of kevents that were +reserved, then the oldest event will be dropped and the new one added. + +Furthermore, the internal struct v4l2_subscribed_event has merge() and +replace() callbacks which drivers can set. These callbacks are called when +a new event is raised and there is no more room. The replace() callback +allows you to replace the payload of the old event with that of the new event, +merging any relevant data from the old payload into the new payload that +replaces it. It is called when this event type has only one kevent struct +allocated. The merge() callback allows you to merge the oldest event payload +into that of the second-oldest event payload. It is called when there are two +or more kevent structs allocated. -- v4l2_event_alloc() +This way no status information is lost, just the intermediate steps leading +up to that state. - To use events, the driver must allocate events for the file handle. By - calling the function more than once, the driver may assure that at least n - events in total have been allocated. The function may not be called in - atomic context. +A good example of these replace/merge callbacks is in v4l2-event.c: +ctrls_replace() and ctrls_merge() callbacks for the control event. + +Note: these callbacks can be called from interrupt context, so they must be +fast. + +Useful functions: - v4l2_event_queue() @@ -916,7 +940,9 @@ Useful functions: The video_device->ioctl_ops->vidioc_subscribe_event must check the driver is able to produce events with specified event id. Then it calls - v4l2_event_subscribe() to subscribe the event. + v4l2_event_subscribe() to subscribe the event. The last argument is the + size of the event queue for this event. If it is 0, then the framework + will fill in a default value (this depends on the event type). - v4l2_event_unsubscribe() @@ -931,14 +957,8 @@ Useful functions: Returns the number of pending events. Useful when implementing poll. -Drivers do not initialise events directly. The events are initialised -through v4l2_fh_init() if video_device->ioctl_ops->vidioc_subscribe_event is -non-NULL. This *MUST* be performed in the driver's -v4l2_file_operations->open() handler. - Events are delivered to user space through the poll system call. The driver -can use v4l2_fh->events->wait wait_queue_head_t as the argument for -poll_wait(). +can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait(). There are standard and private events. New standard events must use the smallest available event type. The drivers must allocate their events from @@ -948,5 +968,4 @@ The first event type in the class is reserved for future use, so the first available event type is 'class base + 1'. An example on how the V4L2 events may be used can be found in the OMAP -3 ISP driver available at <URL:http://gitorious.org/omap3camera> as of -writing this. +3 ISP driver (drivers/media/video/omap3isp). |