• Hans Verkuil's avatar
    [media] v4l2-event/ctrls/fh: allocate events per fh and per type instead of just per-fh · f1e393de
    Hans Verkuil authored
    The driver had to decide how many events to allocate when the v4l2_fh struct
    was created. It was possible to add more events afterwards, but there was no
    way to ensure that you wouldn't miss important events if the event queue
    would fill up for that filehandle.
    
    In addition, once there were no more free events, any new events were simply
    dropped on the floor.
    
    For the control event in particular this made life very difficult since
    control status/value changes could just be missed if the number of allocated
    events and the speed at which the application read events was too low to keep
    up with the number of generated events. The application would have no idea
    what the latest state was for a control since it could have missed the latest
    control change.
    
    So this patch makes some major changes in how events are allocated. Instead
    of allocating events per-filehandle they are now allocated when subscribing an
    event. So for that particular event type N events (determined by the driver)
    are allocated. Those events are reserved for that particular event type.
    This ensures that you will not miss events for a particular type altogether.
    
    In addition, if there are N events in use and a new event is raised, then
    the oldest event is dropped and the new one is added. So the latest event
    is always available.
    
    This can be further improved by adding the ability to merge the state of
    two events together, ensuring that no data is lost at all. This will be
    added in the next patch.
    
    This also makes it possible to allow the user to determine the number of
    events that will be allocated. This is not implemented at the moment, but
    would be trivial.
    Signed-off-by: default avatarHans Verkuil <hans.verkuil@cisco.com>
    Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@redhat.com>
    f1e393de
uvc_v4l2.c 8.05 KB