On Thursday 15 October 2009 23:11:33 Laurent Pinchart wrote:
> Hi Sakari,
>
> On Wednesday 14 October 2009 19:48:33 Hans Verkuil wrote:
> > On Wednesday 14 October 2009 15:02:14 Sakari Ailus wrote:
> > > Here's the second version of the video events RFC. It's based on Laurent
> > > Pinchart's original RFC. My aim is to address the issues found in the
> > > old RFC during the V4L-DVB mini-summit in the Linux plumbers conference
> > > 2009. To get a good grasp of the problem at hand it's probably a good
> > > idea read the original RFC as well:
> > >
> > > <URL:http://www.spinics.net/lists/linux-media/msg10217.html>
>
> Thanks for the RFC update.
>
> > > Changes to version 1
> > > ----------------------------------
> > >
> > > struct video_event has been renamed to v4l2_event. The struct is used in
> > > userspace and V4L related structures appear to have v4l2 prefix so that
> > > should be better than video.
>
> In the end we will probably rename that to media_ or something similar in the
> big media controller rename (if that ever happens). For now let's keep v4l2_,
> that will be more consistent.
>
> > > The "entity" field has been removed from the struct v4l2_event since the
> > > subdevices will have their own device nodes --- the events should come
> > > from them instead of the media controller. Video nodes could be used for
> > > events, too.
>
> I would still keep the entity field. It would allow for parents to report
> children events and there could be use cases for that.
We can always convert one of the reserved fields to an entity field in the
future. Adding support in the new API for an even newer and as yet highly
experimental API is not a good idea.
> > > A few reserved fields have been added. There are new ioctls as well for
> > > enumeration and (un)subscribing.
> > >
> > >
> > > Interface description
> > > ---------------------
> > >
> > > Event type is either a standard event or private event. Standard events
> > > will be defined in videodev2.h. Private event types begin from
> > > V4L2_EVENT_PRIVATE. Some high order bits could be reserved for future
> > > use.
> > >
> > > #define V4L2_EVENT_PRIVATE_START 0x08000000
> > > #define V4L2_EVENT_RESERVED 0x10000000
> >
> > Suggestion: use the V4L2_EV_ prefix perhaps instead of the longer
> > V4L2_EVENT?
>
> EV could be confused with electron volt, exposure value, or even escape
> velocity (don't underestimate the use of V4L2 in the spaceship market ;-)).
> On
> a more serious note, while I like to keep identifiers short, is the 3
> characters gain worth it here ?
>
> > > VIDIOC_ENUM_EVENT is used to enumerate the available event types. It
> > > works a bit the same way than VIDIOC_ENUM_FMT i.e. you get the next
> > > event type by calling it with the last type in the type field. The
> > > difference is that the range is not continuous like in querying controls.
> >
> > Question: why do we need an ENUM_EVENT? I don't really see a use-case for
> > this.
> >
> > Also note that there are three methods in use for enumerating within V4L:
> >
> > 1) there is an index field in the struct that starts at 0 and that the
> > application increases by 1 until the ioctl returns an error.
> >
> > 2) old-style controls where just enumerated from CID_BASE to CID_LASTP1,
> > which is very, very ugly.
> >
> > 3) controls new-style allow one to set bit 31 on the control ID and in that
> > case the ioctl will give you the first control with an ID that is higher
> > than the specified ID.
> >
> > 1 or 3 are both valid options IMHO.
> >
> > But again, I don't see why we need it in the first place.
>
> Applications will only subscribe to the events they can handle, so I don't
> think enumeration is really required. We might want to provide "subscribe to
> all" and "subscribe to none" options though, maybe as special events
> (V4L2_EVENT_NONE, V4L2_EVENT_ALL)
Nice idea. Although we only need an EVENT_ALL. 'Subscribe to none' equals
'unsubscribe all' after all :-)
> > > VIDIOC_G_EVENT is used to get events. sequence is the event sequence
> > > number and the data is specific to driver or event type.
>
> For efficiency reasons a V4L2_G_EVENTS ioctl could also be provided to
> retrieve multiple events.
>
> struct v4l2_events {
> __u32 count;
> struct v4l2_event __user *events;
> };
>
> #define VIDIOC_G_EVENTS _IOW('V', xx, struct v4l2_events)
Hmm. Premature optimization. Perhaps as a future extension.
> > > The user will get the information that there's an event through
> > > exception file descriptors by using select(2). When an event is
> > > available the poll handler sets POLLPRI which wakes up select. -EINVAL
> > > will be returned if there are no pending events.
> > >
> > > VIDIOC_SUBSCRIBE_EVENT and VIDIOC_UNSUBSCRIBE_EVENT are used to
> > > subscribe and unsubscribe from events. The argument is event type.
> >
> > Two event types can be defined already (used by ivtv):
> >
> > #define V4L2_EVENT_DECODER_STOPPED 1
> > #define V4L2_EVENT_OUTPUT_VSYNC 2
> >
> > > struct v4l2_eventdesc {
> > > __u32 type;
> > > __u8 description[64];
> > > __u32 reserved[4];
> > > };
> > >
> > > struct v4l2_event {
> > > __u32 type;
> > > __u32 sequence;
> > > struct timeval timestamp;
> > > __u8 data[64];
> >
> > This should be a union:
> >
> >
> > union {
> > enum v4l2_field ev_output_vsync;
> > __u8 data[64];
> > };
>
> The union will grow pretty big and I'm scared it would soon become a mess.
But otherwise apps need to unpack the data array. That's very user-unfriendly.
I've no problem with big unions.
As an aside: I think that eventually videodev2.h should be split up. Especially
the control section should be moved to a separate header and just be included
by videodev2.h.
>
> > > __u32 reserved[4];
> > > };
> > >
> > > #define VIDIOC_ENUM_EVENT _IORW('V', 83, struct v4l2_eventdesc)
> > > #define VIDIOC_G_EVENT _IOR('V', 84, struct v4l2_event)
> > > #define VIDIOC_SUBSCRIBE_EVENT _IOW('V', 85, __u32)
> > > #define VIDIOC_UNSUBSCRIBE_EVENT _IOW('V', 86, __u32)
> >
> > For (un)subscribe I suggest that we also use a struct with the event type
> > and a few reserved fields.
>
> Agreed.
>
> > > As it was discussed in the LPC, event subscriptions should be bound to
> > > file handle. The implementation, however, is not visible to userspace.
> > > This is why I'm not specifying it in this RFC.
> > >
> > > While the number of possible standard (and probably private) events
> > > would be quite small and the implementation could be a bit field, I do
> > > see that the interface must be using types passed as numbers instead of
> > > bit fields.
> > >
> > > Is it necessary to buffer events of same type or will an event replace
> > > an older event of the same type? It probably depends on event type which
> > > is better. This is also a matter of implementation.
> > >
> > >
> > > Comments and questions are more than welcome.
> >
> > Here's a mixed bag of idea/comments:
> >
> > We need to define what to do when you unsubscribe an event and there are
> > still events of that type pending. Do we remove those pending events as
> > well? I think we should just keep them, but I'm open for other opinions.
>
> It would be easier to keep them and I don't think that would hurt.
>
> > I was wondering if a 'count' field in v4l2_event might be useful: e.g. if
> > you get multiple identical events, and that event is already registered,
> > then you can just increase the count rather than adding the same event
> > again. This might be overengineering, though. And to be honest, I can't
> > think of a use-case, but it's something to keep in mind perhaps.
>
> That's called events compression in the GUI world. The main reason to
> implement this is efficiency when dealing with events that can occur at a
> high
> frequency. For instance, when moving a window and thus exposing previously
> unexposed parts that need to be redrawn, compressing all the redraw events
> generated while the window moves make sense. There could be use cases in the
> media world as well, but I think this is a case of overengineering at the
> moment. We can always implement it later, and I don't think a count field
> would be useful anyway, as events that could be repeated will probably be
> intermixed with other events.
>
> > Would we ever need a VIDIOC_S_EVENT to let the application set an event?
> > ('software events').
>
> Using a kernel driver to pass information from one userspace application to
> another doesn't seem like a very good design IMHO. Let's not do that for now.
>
> > Rather than naming the ioctl VIDIOC_G_EVENT, perhaps VIDIOC_DQEVENT might
> > be more appropriate.
>
> No preference there.
>
> > How do we prevent the event queue from overflowing? Just hardcode a
> > watermark? Alternatively, when subscribing an event we can also pass the
> > maximum number of allowed events as an argument.
>
> We can't prevent it from overflowing if the userspace application isn't fast
> enough. In that case events will be discarded, and the application will find
> out using the sequence number.
Obviously, but my question is whether we use a fixed internal queue or
whether we make this something that the application can configure.
That said, I think the initial implementation should be that the subscribe
ioctl gets a struct with the event type and a few reserved fields so that
in the future we can use one of the reserved fields as a configuration
parameter. So for now we just have some default watermark that is set by the
driver.
Regards,
Hans
--
Hans Verkuil - video4linux developer - sponsored by TANDBERG Telecom
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