Hi Guennadi, On Monday 28 February 2011 12:02:41 Guennadi Liakhovetski wrote: > On Mon, 28 Feb 2011, Hans Verkuil wrote: > > On Monday, February 28, 2011 11:28:58 Laurent Pinchart wrote: > > > On Saturday 26 February 2011 14:56:18 Hans Verkuil wrote: > > > > On Saturday, February 26, 2011 14:39:54 Sylwester Nawrocki wrote: > > > > > On 02/26/2011 02:03 PM, Guennadi Liakhovetski wrote: > > > > > > On Sat, 26 Feb 2011, Hans Verkuil wrote: > > > > > >> On Friday, February 25, 2011 18:08:07 Guennadi Liakhovetski > > > > > >> wrote: > > > > > >> > > > > > >> <snip> > > > > > >> > > > > > >>>>> configure the sensor to react on an external trigger provided > > > > > >>>>> by the flash controller is needed, and that could be a > > > > > >>>>> control on the flash sub-device. What we would probably miss > > > > > >>>>> is a way to issue a STREAMON with a number of frames to > > > > > >>>>> capture. A new ioctl is probably needed there. Maybe that > > > > > >>>>> would be an opportunity to create a new stream-control ioctl > > > > > >>>>> that could replace STREAMON and STREAMOFF in the long term > > > > > >>>>> (we could extend the subdev s_stream operation, and easily > > > > > >>>>> map STREAMON and STREAMOFF to the new ioctl in video_ioctl2 > > > > > >>>>> internally). > > > > > >>>> > > > > > >>>> How would this be different from queueing n frames (in total; > > > > > >>>> count dequeueing, too) and issuing streamon? --- Except that > > > > > >>>> when the last frame is processed the pipeline could be stopped > > > > > >>>> already before issuing STREAMOFF. That does indeed have some > > > > > >>>> benefits. Something else? > > > > > >>> > > > > > >>> Well, you usually see in your host driver, that the videobuffer > > > > > >>> queue is empty (no more free buffers are available), so, you > > > > > >>> stop streaming immediately too. > > > > > >> > > > > > >> This probably assumes that the host driver knows that this is a > > > > > >> special queue? Because in general drivers will simply keep > > > > > >> capturing in the last buffer and not release it to userspace > > > > > >> until a new buffer is queued. > > > > > > > > > > > > Yes, I know about this spec requirement, but I also know, that > > > > > > not all drivers do that and not everyone is happy about that > > > > > > requirement:) > > > > > > > > > > Right, similarly a v4l2 output device is not releasing the last > > > > > buffer to userland and keeps sending its content until a new > > > > > buffer is queued to the driver. But in case of capture device the > > > > > requirement is a pain, since it only causes draining the power > > > > > source, when from a user view the video capture is stopped. Also it > > > > > limits a minimum number of buffers that could be used in preview > > > > > pipeline. > > > > > > > > No, we can't change this. We can of course add some setting that will > > > > explicitly request different behavior. > > > > > > > > The reason this is done this way comes from the traditional TV/webcam > > > > viewing apps. If for some reason the app can't keep up with the > > > > capture rate, then frames should just be dropped silently. All apps > > > > assume this behavior. In a normal user environment this scenario is > > > > perfectly normal (e.g. you use a webcam app, then do a CPU intensive > > > > make run). > > > > > > Why couldn't drivers drop frames silently without a capture buffer ? If > > > the hardware can be paused, the driver could just do that when the > > > last buffer is given back to userspace, and resume the hardware when the > > > next buffer is queued. > > > > It was my understanding that the streaming would stop if no capture > > buffers are available, requiring a VIDIOC_STREAMON to get it started > > again. Of course, there is nothing wrong with stopping the hardware and > > restarting it again when a new buffer becomes available if that can be > > done efficiently enough. Just as long as userspace doesn't notice. > > > > Note that there are some problems with this anyway: often restarting DMA > > requires resyncing to the video stream, which may lead to lost frames. > > Also, the framecounter in struct v4l2_buffer will probably have failed > > to count the lost frames. > > > > In my opinion trying this might cause more problems than it solves. > > So, do I understand it right, that currently there are drivers, that > overwrite the last buffers while waiting for a new one, and ones, that > stop capture for that time. None of them violate the spec, but the former > will not work with the "snapshot mode," and the latter will. Since we do > not want / cannot enforce either way, we do need a way to tell the driver > to enter the "snapshot mode" even if only to not overwrite the last > buffer, right? > > > > > I agree that you might want different behavior in an embedded > > > > environment, but that should be requested explicitly. > > > > > > > > > In still capture mode (single shot) we might want to use only one > > > > > buffer so adhering to the requirement would not allow this, would > > > > > it? > > > > > > > > That's one of the problems with still capture mode, yes. > > > > > > > > I have not yet seen a proposal for this that I really like. Most are > > > > too specific to this use-case (snapshot) and I'd like to see > > > > something more general. > > > > > > I don't think snapshot capture is *that* special. I don't expect most > > > embedded SoCs to implement snapshot capture in hardware. What usually > > > happens is that the hardware provides some support (like two independent > > > video streams for instance, or the ability to capture a given number of > > > frames) and the scheduling is performed in userspace. Good quality > > > snapshot capture requires complex algorithms and involves several > > > hardware pieces (ISP, flash controller, lens controller, ...), so it > > > can't be implemented in the kernel. > > > > I agree. > > Right, but sensors do need it. It is not enough to just tell the sensor - > a per-frame flash is used and let the driver figure out, that it has to > switch to snapshot mode. The snapshot mode has other effects too, e.g., on > some sensors it enables the external trigger pin, which some designs might > want to use also without a flash. Maybe there are also some other side > effects of such snapshot modes on some other sensors, that I'm not aware > of.
This makes me wonder if we need a snapshot mode at all. Why should we tie flash, capture trigger (and other such options that you're not aware of yet :-)) together under a single high-level control (in the general sense, not to be strictly taken as a V4L2 CID) ? Wouldn't it be better to expose those features individually instead ? User might want to use the flash in video capture mode for a stroboscopic effect for instance. -- Regards, 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
