On Monday 28 February 2011 11:40:31 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] > > > > >>> 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. You'll loose frames when you get a buffer underrun anyway :-) > 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. Whether drivers will hold on the last buffer and keep filling it again and again until a new buffer is available, or stop the stream and resume it transparently when a new buffer is queued, should probably be left as a choice to the drivers. I'm in favour of the second option, but I understand that it might be difficult to implement for some hardware. The spec should at least not preclude it when efficient hardware support is available. > > > 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. -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
