During the Plumbers Conference a few weeks ago we had a session to resolve V4L2 ambiguities. It was very successful, but we didn't manage to tackle two of the harder topics, and a third one (timestamps) cause a lot of discussion on the mailinglist after the conference.
So here is the list I have today. Any other ambiguities or new features that should be added to the list?
Regards,
Hans
1) Make a decision how to tell userspace that the monotonic timestamp is used.
Several proposals were made, but no decision was taken AFAIK. Can someone (Sakari?) make a summary/current status of this?
2) Pixel Aspect Ratio
Pixel aspect: currently this is only available through VIDIOC_CROPCAP. It never really belonged to VIDIOC_CROPCAP IMHO. It's just not a property of cropping or composing. It really belongs to the input/output timings (STD or DV_TIMINGS). That's where the pixel aspect ratio is determined.
While it is possible to add it to the dv_timings struct, I see no way of cleanly adding it to struct v4l2_standard (mostly because VIDIOC_ENUMSTD is now handled inside the V4L2 core and doesn't call the drivers anymore).
An alternative is to add it to struct v4l2_input/output, but I don't know if it is possible to define a pixelaspect for inputs that are not the current input. What I am thinking of is just to add a new ioctl for this: VIDIOC_G_PIXELASPECT.
3) Tuner Ownership
How to handle tuner ownership if both a video and radio node share the same tuner?
Obvious rules:
- Calling S_FREQ, S_TUNER, S_MODULATOR or S_HW_FREQ_SEEK will make the filehandle the owner if possible. EBUSY is returned if someone else owns the tuner and you would need to switch the tuner mode. - Ditto for ioctls that expect a valid tuner configuration like QUERYSTD. This is likely to be driver dependent, though. - Just opening a device node should not switch ownership.
But it is not clear what to do when any of these ioctls are called:
- G_FREQUENCY: could just return the last set frequency for radio or TV: requires that that is remembered when switching ownership. This is what happens today, so G_FREQUENCY does not have to switch ownership. - G_TUNER: the rxsubchans, signal and afc fields all require ownership of the tuner. So in principle you would want to switch ownership when G_TUNER is called. On the other hand, that would mean that calling v4l2-ctl --all -d /dev/radio0 would change tuner ownership to radio for /dev/video0. That's rather unexpected.
It is possible to just set rxsubchans, signal and afc to 0 if the device node doesn't own the tuner. I'm inclined to do that. - Should closing a device node switch ownership? E.g. if nobody has a radio device open, should the tuner switch back to TV mode automatically? I don't think it should. - How about hybrid tuners?
Hi Hans,
On Monday 15 October 2012 13:35:45 Hans Verkuil wrote:
During the Plumbers Conference a few weeks ago we had a session to resolve V4L2 ambiguities. It was very successful, but we didn't manage to tackle two of the harder topics, and a third one (timestamps) cause a lot of discussion on the mailinglist after the conference.
So here is the list I have today. Any other ambiguities or new features that should be added to the list?
Small topic that we've briefly discussed on IRC: if a device doesn't tell the driver what color space it uses, should the driver guess or tell the application that the color space is unknown ? I've ran into that issue for the uvcvideo driver, while I agree with you that in that case the color space is very likely sRGB, and that the driver is probably in a better position to make that guess than the userspace application (as the driver knows it handles a webcam), what should be the rule ?
- Make a decision how to tell userspace that the monotonic timestamp is
used.
Several proposals were made, but no decision was taken AFAIK. Can someone (Sakari?) make a summary/current status of this?
- Pixel Aspect Ratio
Pixel aspect: currently this is only available through VIDIOC_CROPCAP. It never really belonged to VIDIOC_CROPCAP IMHO. It's just not a property of cropping or composing. It really belongs to the input/output timings (STD or DV_TIMINGS). That's where the pixel aspect ratio is determined.
While it is possible to add it to the dv_timings struct, I see no way of cleanly adding it to struct v4l2_standard (mostly because VIDIOC_ENUMSTD is now handled inside the V4L2 core and doesn't call the drivers anymore).
Isn't that an implementation issue instead of an API issue ?
An alternative is to add it to struct v4l2_input/output, but I don't know if it is possible to define a pixelaspect for inputs that are not the current input. What I am thinking of is just to add a new ioctl for this: VIDIOC_G_PIXELASPECT.
- Tuner Ownership
How to handle tuner ownership if both a video and radio node share the same tuner?
Obvious rules:
- Calling S_FREQ, S_TUNER, S_MODULATOR or S_HW_FREQ_SEEK will make the
filehandle the owner if possible. EBUSY is returned if someone else owns the tuner and you would need to switch the tuner mode.
- Ditto for ioctls that expect a valid tuner configuration like QUERYSTD.
This is likely to be driver dependent, though.
- Just opening a device node should not switch ownership.
But it is not clear what to do when any of these ioctls are called:
- G_FREQUENCY: could just return the last set frequency for radio or TV:
requires that that is remembered when switching ownership. This is what happens today, so G_FREQUENCY does not have to switch ownership.
- G_TUNER: the rxsubchans, signal and afc fields all require ownership of
the tuner. So in principle you would want to switch ownership when G_TUNER is called. On the other hand, that would mean that calling v4l2-ctl --all -d /dev/radio0 would change tuner ownership to radio for /dev/video0. That's rather unexpected.
It is possible to just set rxsubchans, signal and afc to 0 if the device node doesn't own the tuner. I'm inclined to do that.
- Should closing a device node switch ownership? E.g. if nobody has a radio
device open, should the tuner switch back to TV mode automatically? I don't think it should.
- How about hybrid tuners?
On Wed October 17 2012 02:25:00 Laurent Pinchart wrote:
Hi Hans,
On Monday 15 October 2012 13:35:45 Hans Verkuil wrote:
During the Plumbers Conference a few weeks ago we had a session to resolve V4L2 ambiguities. It was very successful, but we didn't manage to tackle two of the harder topics, and a third one (timestamps) cause a lot of discussion on the mailinglist after the conference.
So here is the list I have today. Any other ambiguities or new features that should be added to the list?
Small topic that we've briefly discussed on IRC: if a device doesn't tell the driver what color space it uses, should the driver guess or tell the application that the color space is unknown ? I've ran into that issue for the uvcvideo driver, while I agree with you that in that case the color space is very likely sRGB, and that the driver is probably in a better position to make that guess than the userspace application (as the driver knows it handles a webcam), what should be the rule ?
I'll add it to the list.
- Make a decision how to tell userspace that the monotonic timestamp is
used.
Several proposals were made, but no decision was taken AFAIK. Can someone (Sakari?) make a summary/current status of this?
- Pixel Aspect Ratio
Pixel aspect: currently this is only available through VIDIOC_CROPCAP. It never really belonged to VIDIOC_CROPCAP IMHO. It's just not a property of cropping or composing. It really belongs to the input/output timings (STD or DV_TIMINGS). That's where the pixel aspect ratio is determined.
While it is possible to add it to the dv_timings struct, I see no way of cleanly adding it to struct v4l2_standard (mostly because VIDIOC_ENUMSTD is now handled inside the V4L2 core and doesn't call the drivers anymore).
Isn't that an implementation issue instead of an API issue ?
True, but it will require changing all drivers. But adding it to v4l2_standard would be the best place for this, so perhaps we should think about doing all this work.
Regards,
Hans
On Mon October 15 2012 13:35:45 Hans Verkuil wrote:
During the Plumbers Conference a few weeks ago we had a session to resolve V4L2 ambiguities. It was very successful, but we didn't manage to tackle two of the harder topics, and a third one (timestamps) cause a lot of discussion on the mailinglist after the conference.
So here is the list I have today. Any other ambiguities or new features that should be added to the list?
I've got another:
Right now there are no standard ioctls that one could call for a v4l-subdev node to discover whether the device node is really a subdev node. For other v4l nodes we have QUERYCAP, but that's not available for subdev nodes.
I propose that QUERYCAP support is added (and will be required) for subdev nodes. The capabilities field will be set to V4L2_CAP_SUBDEVICE only (at least for now). All other fields of v4l2_capability are just as valid for subdevs as they are for normal v4l devices.
Regards,
Hans
-
Hans Verkuil -
Laurent Pinchart