Merge branch 'sh-latest' of git://git.kernel.org/pub/scm/linux/kernel/git/lethal...

[pandora-kernel.git] / Documentation / video4linux / v4l2-framework.txt

diff --git a/Documentation/video4linux/v4l2-framework.txt b/Documentation/video4linux/v4l2-framework.txt
index cf21f7a..f8dcabf 100644 (file)
--- a/Documentation/video4linux/v4l2-framework.txt
+++ b/Documentation/video4linux/v4l2-framework.txt
@@ -817,11 +817,7 @@ int my_open(struct file *file)
 
        ...
 
 
        ...
 

-       ret = v4l2_fh_init(&my_fh->fh, vfd);
-       if (ret) {
-               kfree(my_fh);
-               return ret;
-       }
+       v4l2_fh_init(&my_fh->fh, vfd);

 
        ...
 
 
        ...
 


@@ -844,7 +840,7 @@ int my_release(struct file *file)
 
 Below is a short description of the v4l2_fh functions used:
 
 
 Below is a short description of the v4l2_fh functions used:
 

-int v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
+void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)

 
   Initialise the file handle. This *MUST* be performed in the driver's
   v4l2_file_operations->open() handler.
 
   Initialise the file handle. This *MUST* be performed in the driver's
   v4l2_file_operations->open() handler.


@@ -901,14 +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.
 
 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()
 
 
 - v4l2_event_queue()
 


@@ -920,7 +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
 
   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()
 
 
 - v4l2_event_unsubscribe()
 


@@ -935,14 +957,8 @@ Useful functions:
 
   Returns the number of pending events. Useful when implementing poll.
 
 
   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
 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
 
 There are standard and private events. New standard events must use the
 smallest available event type. The drivers must allocate their events from


@@ -952,5 +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
 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).