Using the Selection API with image sensors for arbitrary cropping

[Dave Stevenson <dave.stevenson@raspberrypi.com>]

Hi All

I'm trying to tally the selection API documentation for image sensors
with implementing it in practice, specifically over arbitrary cropping
on the sensor.

I've had a downstream PR for IMX219 that adds support for the
selection API to allow arbitrary cropping and selection of binning
mode [1].

The docs for "Writing camera sensor drivers" [2] lists the two options
as either freely configurable via multiple subdevices, or register
based. It doesn't apparently cover just cropping (there is no scaler
on IMX219), but there is the IMX274 driver that implements setting the
sensor cropping via the selection API [3].

The current IMX219 register-based modes are
- 3280x2464 up to 15fps
- 1920x1080 up to 30fps as a crop of the 3280x2464 mode
- 1640x1232 up to 40fps, 2x2 binned
- 640x480 up to 200fps, "special" 2x2 binning and cropped.

The main issue is that implementing the selection API reduces the
number of modes that can be selected directly via set_fmt to the base
3280x2464 and 1640x1232. Surely that constitutes a regression as use
cases that did work now don't, and therefore it is not acceptable.
3280x2464 can't run at 30fps, therefore we can't easily get a 1080p30
source without additional knowledge of modes and crop settings.

So how should the selection API be implemented without introducing regressions?
Is it permitted to enumerate the extra modes as before and have them
update the crop rectangle? The docs [4] say not:
"Drivers shall set the active crop rectangle to the default when the
driver is first loaded, but not later."
which leaves a bit of a quandry.

If we do drop the existing modes it just pushes the problem of which
modes to select onto the client. Most likely you end up with an
extended sensor specific helper in libcamera with a list of modes and
the framerates that each can achieve, pretty much identical to the
list of modes in the kernel at present.
Any other clients are forced to jump through similar hoops (unlikely
to happen), or we rename it to Video 4 Libcamera 2 ;-)

Making that shift also means that selecting the special binning mode
has to be done via some other heuristics. AIUI it's optimised for high
frame rates so that's possible (but not nice).

Guidance sought please.

Thanks
  Dave

[1] https://github.com/raspberrypi/linux/pull/4935
[2] https://www.kernel.org/doc/html/latest/driver-api/media/camera-sensor.html#frame-size
[3] https://github.com/torvalds/linux/blob/master/drivers/media/i2c/imx274.c
[4] https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/selection-api-configuration.html#configuration-of-video-capture