Re: [RFC] Video events, version 2
Hans Verkuil wrote:
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.
You're welcome.
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.
Then the entity field stays away for now?
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_START0x0800
#define V4L2_EVENT_RESERVED 0x1000
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 ?
I'll use V4L2_EVENT_ in the next RFC, too.
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 :-)
Ok.
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.
That *could* save one ioctl sometimes --- then you'd no there are no
more events coming right now. But just one should be supported IMO,
VIDIOC_G_EVENT or VIDIOC_G_EVENTS.
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;
Re: [RFC] Video events, version 2
On Friday 16 October 2009 09:36:51 Sakari Ailus wrote:
Hans Verkuil wrote:
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.
You're welcome.
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.
Then the entity field stays away for now?
As long as you add a reserved field that's marked as don't dare touching
this, it will be used as an entity Id later I'm fine ;-)
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 0x0800
#define V4L2_EVENT_RESERVED 0x1000
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 ?
I'll use V4L2_EVENT_ in the next RFC, too.
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 :-)
Ok.
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.
That *could* save one ioctl sometimes --- then you'd no there are no
more events coming right now. But just one should be supported IMO,
VIDIOC_G_EVENT or VIDIOC_G_EVENTS.
I forgot to mention in my last mail that we should add a flag to the
v4l2_event structure to report if more events are pending (or even possible a
pending event count).
The user will get the information that there's an event
Re: [RFC] Video events, version 2
Hans Verkuil wrote:
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.
You're welcome.
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.
Then the entity field stays away for now?
Yup.
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 0x0800
#define V4L2_EVENT_RESERVED 0x1000
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 ?
I'll use V4L2_EVENT_ in the next RFC, too.
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 :-)
Ok.
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.
That *could* save one ioctl sometimes --- then you'd no there are no
more events coming right now. But just one should be supported IMO,
VIDIOC_G_EVENT or VIDIOC_G_EVENTS.
I'm not keen on using pointers insides structures unless there is a very
good reason to do so. In practice it complicates the driver code
substantially due to all the kernel-to-userspace copies that need to be
done that are normally handled by video_ioctl2. In addition it requires
custom code in the compat-ioctl32 part as well.
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
Re: [RFC] Video events, version 2
Hans Verkuil wrote: [clip] I'm not keen on using pointers insides structures unless there is a very good reason to do so. In practice it complicates the driver code substantially due to all the kernel-to-userspace copies that need to be done that are normally handled by video_ioctl2. In addition it requires custom code in the compat-ioctl32 part as well. VIDIOC_DQEVENT then. [clip] The size of the structure is now 96 bytes. I guess we could make that around 128 to allow a bit more space for data without really affecting performance. With 'big unions' I didn't mean the memory size. I think 64 bytes (16 longs) is a decent size. I was talking about the union definition in the videodev2.h header. That was a badly placed comment, but I meant the memory size. I have currently no opinion on whether to use union or not. [clip] 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. I'd like to think a queue size defined by the driver is fine at this point. It's probably depending on the driver rather than application how long the queue should to be. At some point old events start becoming uninteresting... Question: will we drop old events or new events? Or make this configurable? Or driver dependent? This should the same than for video buffers. I guess it's undefined? I'd let the driver decide at this point. -- Sakari Ailus sakari.ai...@maxwell.research.nokia.com -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [RFC] Video events, version 2
On Friday 16 October 2009 10:55:04 Sakari Ailus wrote: Hans Verkuil wrote: [clip] I'm not keen on using pointers insides structures unless there is a very good reason to do so. In practice it complicates the driver code substantially due to all the kernel-to-userspace copies that need to be done that are normally handled by video_ioctl2. In addition it requires custom code in the compat-ioctl32 part as well. VIDIOC_DQEVENT then. [clip] The size of the structure is now 96 bytes. I guess we could make that around 128 to allow a bit more space for data without really affecting performance. With 'big unions' I didn't mean the memory size. I think 64 bytes (16 longs) is a decent size. I was talking about the union definition in the videodev2.h header. That was a badly placed comment, but I meant the memory size. I have currently no opinion on whether to use union or not. [clip] 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. I'd like to think a queue size defined by the driver is fine at this point. It's probably depending on the driver rather than application how long the queue should to be. At some point old events start becoming uninteresting... Question: will we drop old events or new events? Or make this configurable? Or driver dependent? This should the same than for video buffers. I guess it's undefined? I'd let the driver decide at this point. From the user point of view it won't make much difference. The same number of consecutive events will be lost. Which ones will depend on the events arrival time and when/how long the application fails to retrieve pending events. I agree to let the driver decide (or rather the v4l2 core, as the queue will be implemented there). -- Laurent Pinchart -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [RFC] Video events, version 2
Laurent Pinchart wrote:
On Friday 16 October 2009 09:36:51 Sakari Ailus wrote:
Hans Verkuil wrote:
On Thursday 15 October 2009 23:11:33 Laurent Pinchart wrote:
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.
That *could* save one ioctl sometimes --- then you'd no there are no
more events coming right now. But just one should be supported IMO,
VIDIOC_G_EVENT or VIDIOC_G_EVENTS.
I forgot to mention in my last mail that we should add a flag to the
v4l2_event structure to report if more events are pending (or even possible a
pending event count).
Then the V4L (or driver) would have to check the queue for that type of
events. OTOH, the queue size could be quite small and it'd never
overflow since the maximum size is number of different event types.
Can there be situations when the first or last event timestamp of
certain event would be necessary? If we put count there, then we need to
make a decision which one is useful for the userspace. The last one is
obviously useful for the AF/AEWB algorightms. I currently see no use for
the first one, but that doesn't mean there couldn't be use for it.
--
Sakari Ailus
sakari.ai...@maxwell.research.nokia.com
--
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Re: [RFC] Video events, version 2
On Friday 16 October 2009 14:34:28 Sakari Ailus wrote:
Laurent Pinchart wrote:
On Friday 16 October 2009 09:36:51 Sakari Ailus wrote:
Hans Verkuil wrote:
On Thursday 15 October 2009 23:11:33 Laurent Pinchart wrote:
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.
That *could* save one ioctl sometimes --- then you'd no there are no
more events coming right now. But just one should be supported IMO,
VIDIOC_G_EVENT or VIDIOC_G_EVENTS.
I forgot to mention in my last mail that we should add a flag to the
v4l2_event structure to report if more events are pending (or even
possible a pending event count).
Then the V4L (or driver) would have to check the queue for that type of
events. OTOH, the queue size could be quite small and it'd never
overflow since the maximum size is number of different event types.
Can there be situations when the first or last event timestamp of
certain event would be necessary? If we put count there, then we need to
make a decision which one is useful for the userspace. The last one is
obviously useful for the AF/AEWB algorightms. I currently see no use for
the first one, but that doesn't mean there couldn't be use for it.
That's not what I meant. The idea of a count field is to report the number of
events still pending after that one, type aside. If v4l2_event::count equals 0
the userspace application will know there is no need to call VIDIOC_G_EVENT
just to get a -EAGAIN.
--
Laurent Pinchart
--
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Re: [RFC] Video events, version 2
Laurent Pinchart wrote: That's not what I meant. The idea of a count field is to report the number of events still pending after that one, type aside. If v4l2_event::count equals 0 the userspace application will know there is no need to call VIDIOC_G_EVENT just to get a -EAGAIN. Thanks for the clarification. Sounds good to me, I'll put this to the next version. -- Sakari Ailus sakari.ai...@maxwell.research.nokia.com -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [RFC] Video events, version 2
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.
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_START0x0800
#define V4L2_EVENT_RESERVED 0x1000
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)
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)
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;
__u8description[64];
__u32 reserved[4];
};
struct v4l2_event {
__u32 type;
__u32 sequence;
struct timeval timestamp;
__u8data[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.
__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
Re: [RFC] Video events, version 2
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 0x0800
#define V4L2_EVENT_RESERVED 0x1000
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;
__u8description[64];
__u32 reserved[4];
};
struct v4l2_event {
Re: [RFC] Video events, version 2
On Wednesday 14 October 2009 15:02:14 Sakari Ailus wrote:
Hi,
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
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.
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.
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 0x0800
#define V4L2_EVENT_RESERVED 0x1000
Suggestion: use the V4L2_EV_ prefix perhaps instead of the longer V4L2_EVENT?
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.
VIDIOC_G_EVENT is used to get events. sequence is the event sequence
number and the data is specific to driver or event type.
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;
__u8description[64];
__u32 reserved[4];
};
struct v4l2_event {
__u32 type;
__u32 sequence;
struct timeval timestamp;
__u8data[64];
This should be a union:
union {
enum v4l2_field ev_output_vsync;
__u8 data[64];
};
__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.
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.
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
