Running libcamera on KernelCI to detect regressions in the media subsystem

["Nícolas F. R. A. Prado" <nfraprado@collabora.com>]

Hello,

as part of an effort to detect regressions in the kernel's media subsystem that
affects real use cases, I want to present a proposal and ask for feedback and
ideas.

Why?
====

There's been increasing interest in catching regressions in the kernel early, to
minimize the impact on userspace, and the media subsystem is no different. The
main test tool there is v4l2-compliance [1], but its focus is to purely exercise
the uAPI. There's currently nothing in place to test real use cases.

What to do?
===========

libcamera [2] is a library that works on top of the Media Controller and V4L2
APIs and abstracts away the hardware-specific pipeline configuration from the
application. It is a real user of the v4l2 uAPI.

Just recently an initial implementation of a testing tool for libcamera, called
lc-compliance, was merged [3]. It currently has only a few tests, but they
already test the capture of images for different purposes (raw images,
high-quality video capturing, etc), which exercise different media topology
configurations and pixelformats.

Although from the point of view of libcamera lc-compliance is a compliance tool,
from v4l2's perspective it is a real use case test rather than a pure API
compliance test like v4l2-compliance.

I'm currently in the process of refactoring lc-compliance to have a better test
framework [4], and make it ready to be automatically run on a CI.

By having lc-compliance run on actual hardware at KernelCI, we can exercise real
use cases of cameras and catch any kernel regressions that impact them as soon
as they happen.

Feedback
========

So, how can we best ensure we catch real use case regressions on the media
subsystem using lc-compliance? What kind of information should be present on the
test results? Any other suggestions?

Thanks,
Nícolas


[1] https://git.linuxtv.org/v4l-utils.git/tree/utils/v4l2-compliance
[2] https://libcamera.org
[3] https://git.linuxtv.org/libcamera.git/tree/src/lc-compliance
[4] https://lists.libcamera.org/pipermail/libcamera-devel/2021-May/020382.html