Re: Question: interaction between selection API, ENUM_FRAMESIZES and S_FMT?

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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.

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 ?

> 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 ?

> 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 ?

> 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 ?

-- 
Regards,

Laurent Pinchart