On Sun, 2017-03-19 at 12:08 -0700, Steve Longerbeam wrote:

On 03/19/2017 08:22 AM, Russell King - ARM Linux wrote:

quoted

On Thu, Mar 09, 2017 at 08:53:18PM -0800, Steve Longerbeam wrote:

quoted

From: Philipp Zabel <p.zabel@pengutronix.de>

The csi_try_crop call in set_fmt should compare the cropping rectangle
to the currently set input format, not to the previous input format.

Are we really sure that the cropping support is implemented correctly?

I came across this while looking at what we're doing with the
V4L2_SEL_FLAG_KEEP_CONFIG flag.

Documentation/media/uapi/v4l/dev-subdev.rst defines the behaviour of
the user API, and "Order of configuration and format propagation" says:

   The coordinates to a step always refer to the actual size of the
   previous step. The exception to this rule is the source compose
   rectangle, which refers to the sink compose bounds rectangle --- if it
   is supported by the hardware.
   
   1. Sink pad format. The user configures the sink pad format. This format
      defines the parameters of the image the entity receives through the
      pad for further processing.
   
   2. Sink pad actual crop selection. The sink pad crop defines the crop
      performed to the sink pad format.
   
   3. Sink pad actual compose selection. The size of the sink pad compose
      rectangle defines the scaling ratio compared to the size of the sink
      pad crop rectangle. The location of the compose rectangle specifies
      the location of the actual sink compose rectangle in the sink compose
      bounds rectangle.
   
   4. Source pad actual crop selection. Crop on the source pad defines crop
      performed to the image in the sink compose bounds rectangle.
   
   5. Source pad format. The source pad format defines the output pixel
      format of the subdev, as well as the other parameters with the
      exception of the image width and height. Width and height are defined
      by the size of the source pad actual crop selection.
   
   Accessing any of the above rectangles not supported by the subdev will
   return ``EINVAL``. Any rectangle referring to a previous unsupported
   rectangle coordinates will instead refer to the previous supported
   rectangle. For example, if sink crop is not supported, the compose
   selection will refer to the sink pad format dimensions instead.

Note step 3 above: scaling is defined by the ratio of the _sink_ crop
rectangle to the _sink_ compose rectangle.

The above paragraph suggests we skip any rectangles that are not
supported. In our case that would be 3. and 4., since the CSI can't
compose into a larger frame. I hadn't realised that the crop selection
currently happens on the source pad.
The hardware actually only supports cropping of the input (the crop
rectangle we write into the window registers are before downscaling). So
the crop rectangle should be moved to the sink pad.

quoted

So, lets say that the camera produces a 1280x720 image, and the sink
pad format is configured with 1280x720.  That's step 1.

The sink crop operates within that rectangle, cropping it to an area.
Let's say we're only interested in its centre, so we'd chose 640x360
with the top-left as 320,180.  This is step 2.

Then, if we want to down-scale by a factor of two, we'd set the sink
compose selection to 320x180.

Except when composing is not supported. If the sink compose and source
crop rectangles are not supported, the source pad format takes their
place in determining the scaling output resolution. At least that's how
I read the documentation.

quoted

This seems to be at odds with how the scaling is done in CSI at
present: the selection implementations all reject attempts to
configure the sink pad, instead only supporting crop rectangles on
the source,

Correct. Currently cropping is only supported at the source pad
(step 4).

Initially the CSI didn't support down-scaling, so step 3 is not supported,
so the sink pad format/crop selection rectangle/crop compose rectangle
are collapsed into the same sink pad format rectangle.

Philipp later added support for /2 downscaling, but we didn't put this in
the correct API, looks like this needs to move into the selection API at
step 3 (sink pad compose rectangle).

I am not sure about this. Wouldn't moving the input crop to the sink pad
be enough? If we added support for the sink pad compose rectangle, that
wouldn't actually allow to compose the CSI output into a larger frame.
Since the subdevice can't compose, I'd leave the sink compose rectangle
disabled.

quoted

  and we use the source crop rectangle to define the
down-scaling.

We use the source pad format to define the downscaling relative to the
source crop rectangle (which is wrong, it should be relative to the sink
crop rectangle).

Yes. And maybe there is nothing wrong with that, because scaling is also
defined by the source/sink _format_ ratios (if I'm not mistaken), so looking
at this another way, we're just defining scaling in the CSI via another
legal API.

I didn't touch the crop rectangle at all, just setting the input
resolution on the sink pad and the desired output resolution on the
source pad should work.

regards
Philipp