On Mon 1 July 2013 14:42:34 Laurent Pinchart wrote:
> Hi Hans,
>
> On Monday 24 June 2013 14:48:15 Hans Verkuil wrote:
> > Hi all,
> >
> > While working on extending v4l2-compliance with cropping/selection test
> > cases I decided to add support for that to vivi as well (this would give
> > applications a good test driver to work with).
> >
> > However, I ran into problems how this should be implemented for V4L2 devices
> > (we are not talking about complex media controller devices where the video
> > pipelines are setup manually).
> >
> > There are two problems, one related to ENUM_FRAMESIZES and one to S_FMT.
> >
> > The ENUM_FRAMESIZES issue is simple: if you have a sensor that has several
> > possible frame sizes, and that can crop, compose and/or scale, then you need
> > to be able to set the frame size.
>
> You mentioned that this discussion relates to simple pipelines controlled
> through a video node only. I'd like to take a step back here and first define
> what pipelines we want to support in such a way, and what pipelines requires
> the media controller API. Based on that information we can list the use cases
> we need to support, and then decide on the S_FMT/S_SELECTION APIs behaviour.
It's fairly simple. If I have a video capture device, either using S_STD or
S_DV_TIMINGS to define the resolution of the incoming video, then I can do
cropping, composing and setting the final format without problem. I have all
the information I need to do the calculations.
On the other hand, replace the video receiver by a sensor or by a software
or hardware image generator that supports a range of resolutions and everything
falls down just because you don't have the equivalent of S_STD/S_DV_TIMINGS
for this type of device. All you need is a way to select which resolution should
be produced at the beginning/source of the video pipeline. That's exactly why
S_STD/S_DV_TIMINGS exist.
> I vaguely remember to have discussed this topic previously in a meeting but I
> can't find any related information in my notes at the moment. Would anyone
> happen to have a better memory here ?
>
> > Currently this is decided by S_FMT which maps the format size to the closest
> > valid frame size. This however makes it impossible to e.g. scale up a frame,
> > or compose the image into a larger buffer.
>
> It also makes it impossible to scale a frame down without composing it into a
> larger buffer. That's definitely a bad limitation of the API.
>
> > For video receivers this issue doesn't exist: there the size of the incoming
> > video is decided by S_STD or S_DV_TIMINGS, but no equivalent exists for
> > sensors.
> >
> > I propose that a new selection target is added: V4L2_SEL_TGT_FRAMESIZE.
>
> Just to make sure I understand you correctly, are you proposing a new
> selection target valid on video nodes only, that would control the format at
> the source pad of the sensor ?
Yes. So this would be valid for an input that:
- Does not set V4L2_IN_CAP_DV_TIMINGS or CAP_STD in ENUMINPUT
- Does support ENUM_FRAMESIZES
> > However, this leads to another problem: the current S_FMT behavior is that
> > it implicitly sets the framesize. That behavior we will have to keep,
> > otherwise applications will start to behave differently.
>
> Which frame size are you talking about ? S_FMT definitely sets the frame size
> in memory, do you mean it also implicitly sets the frame size at the sensor
> source pad ?
For such devices, yes. How else can you select today which frame size the sensor
should produce?
> > I have an idea on how to solve that, but the solution is related to the
> > second problem I found:
> >
> > When you set a new format size, then the compose rectangle must be set to
> > the new format size as well since that has always been the behavior in the
> > past that applications have come to expect.
>
> That's the behaviour applications have come to expect from devices that can't
> compose. From a quick look at the kernel source only Samsung devices
> implement
> the composition API. Does this behaviour need to be preserved there ?
I believe so. I plan on adding composing capabilities to vivi. Any existing
apps should keep working as expected.
> > But this makes certain operations impossible to execute: if a driver can't
> > scale, then you can never select a new format size larger than the current
> > (possibly cropped) frame size, even though you would want to compose the
> > unscaled image into such a larger buffer.
> >
> > So what is the behavior that I would expect from drivers?
> >
> > 1) After calling S_STD, S_DV_TIMINGS or S_SELECTION(V4L2_SEL_TGT_FRAMESIZE)
> > the cropping, composing and format parameters are all adjusted to support
> > the new input video size (typically they are all set to the new size).
> >
> > 2) After calling S_CROP/S_SELECTION(CROP) the compose and format parameters
> > are all adjusted to support the new crop rectangle.
> >
> > 3) After calling S_SEL(COMPOSE) the format parameters are adjusted.
> >
> > 4) Calling S_FMT validates the format parameters to support the compose
> > rectangle.
> >
> > However, today step 4 does something different: the compose rectangle will
> > be adjusted to the format size (and in the case of a sensor supporting
> > different framesizes the whole pipeline will be adjusted).
> >
> > The only way I see that would solve this (although it isn't perfect) is to
> > change the behavior of S_FMT only if the selection API was used before by
> > the filehandle. The core can keep easily keep track of that. When the
> > application calls S_FMT and no selection API was used in the past by that
> > filehandle, then the core will call first
> > S_SELECTION(V4L2_SEL_TGT_FRAMESIZE). If that returns -EINVAL, then it will
> > call S_SELECTION(V4L2_SEL_TGT_COMPOSE). Finally it will call S_FMT. Note
> > that a similar sequence is needed for the display case.
> >
> > This means that a driver supporting the selection API can implement the
> > logical behavior and the core will implement the historically-required
> > illogical part.
> >
> > So the fix for this would be to add a new selection target and add
> > compatibility code to the v4l2-core.
> >
> > With that in place I can easily add crop/compose support to vivi.
> >
> > One area of uncertainty is how drivers currently implement S_FMT: do they
> > reset any cropping done before? They should keep the crop rectangle
> > according to the spec (well, it is implied there). Guennadi, what does
> > soc_camera do?
> >
> > Sylwester, I am also looking at exynos4-is/fimc-lite.c. I do see that
> > setting the compose rectangle will adjust it to the format size instead of
> > the other way around, but I can't tell if setting the format size will also
> > adjust the compose rectangle if that is now out-of-bounds of the new format
> > size.
> >
> > Comments? Questions?
>
> How should we handle devices for which supported sizes (crop, compose, ...)
> are restricted by selected pixel format ?
Good question. ENUM_FRAMESIZES returns the available resolutions dependent on
the pixelformat. That means that when you select a resolution you need to
specify a pixelformat as well. So just a rectangle isn't enough.
I need to think some more about this.
Regards,
Hans
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