[media] v4l2-event.h: add overview documentation to the header
It's getting confusing who is linking to what, so add an overview at the start of the header. Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>wifi-calibration
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@ -29,6 +29,40 @@
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#include <linux/videodev2.h>
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#include <linux/videodev2.h>
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#include <linux/wait.h>
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#include <linux/wait.h>
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/*
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* Overview:
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*
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* Events are subscribed per-filehandle. An event specification consists of a
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* type and is optionally associated with an object identified through the
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* 'id' field. So an event is uniquely identified by the (type, id) tuple.
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*
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* The v4l2-fh struct has a list of subscribed events. The v4l2_subscribed_event
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* struct is added to that list, one for every subscribed event.
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*
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* Each v4l2_subscribed_event struct ends with an array of v4l2_kevent structs.
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* This array (ringbuffer, really) is used to store any events raised by the
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* driver. The v4l2_kevent struct links into the 'available' list of the
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* v4l2_fh struct so VIDIOC_DQEVENT will know which event to dequeue first.
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*
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* Finally, if the event subscription is associated with a particular object
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* such as a V4L2 control, then that object needs to know about that as well
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* so that an event can be raised by that object. So the 'node' field can
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* be used to link the v4l2_subscribed_event struct into a list of that
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* object.
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*
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* So to summarize:
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*
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* struct v4l2_fh has two lists: one of the subscribed events, and one of the
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* pending events.
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*
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* struct v4l2_subscribed_event has a ringbuffer of raised (pending) events of
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* that particular type.
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*
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* If struct v4l2_subscribed_event is associated with a specific object, then
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* that object will have an internal list of struct v4l2_subscribed_event so
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* it knows who subscribed an event to that object.
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*/
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struct v4l2_fh;
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struct v4l2_fh;
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struct v4l2_subscribed_event;
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struct v4l2_subscribed_event;
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struct video_device;
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struct video_device;
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