panfrost/ci: Disable trace-based testing

panfrost/ci: Disable trace-based testing

Trace-based testing has not worked for Panfrost. It was a neat
experiment, and I'm glad we tried it, but the results have been mostly
negative for the driver. Disable the trace-based tests.

For testing that specific API features work correctly, we run the
conformance tests (dEQP), which are thorough for OpenGL ES. For big GL
features, we run Piglit, and if there are big GL features that we are
not testing adequately, we should extend Piglit for these. For
fine-grained driver correctness, we are already covered.

Where trace-based testing can fit in is as a smoke test, ensuring that
the overall rendering of complex scenes does not regress. In principle,
that's a lovely idea, but the current implementation has not worked out
for Panfrost thus far. The crux of the issue is that the trace based
tests are based on checksums, not fuzzy-compared reference images. That
requires updating checksums any time rendering changes. However, a
rendering change to a trace is NOT a regression. The behaviour of OpenGL
is specified very loosely. For a given trace, there are many different
valid checksums. That means that correct changes to core code frequently
fail CI after running through the rest of CI, only because a checksum
changed in a still correct way. That's a pain to deal with, exacerbated
by rebase pains, and provides negative value to the project. Some recent
examples of this I've hit in the past two weeks alone:

   panfrost: Enable rendering to 16-bit and 32-bit
   4b49241f7d7 ("panfrost: Use proper formats for pntc varying")
   ac2964dfbd1 ("nir: Be smarter fusing ffma")

The last example were virgl traces, but were especially bad: due to a
rebase fail, I had to update traces /twice/, wasting two full runs of
pre-merge CI across *all* hardware. This was extremely wasteful.

The value of trace-based testing is as a smoke test to check that traces
still render correctly. That is useful, but it turns out that checksums
are the wrong way to go about it. A better implementation would be
storing only a single reference image from a software rasterizer per
trace. No driver-specific references would be stored. That reference
image must never change, provided the trace never changes. CI would then
check rendered results against that image with tolerant fuzzy
comparisons. That tolerance matches with the fuzzy comparison that the
human eye would do when investigating a checksum change anyway. Yes, the
image comparison JavaScript will now report that
0 pixels changed within the tolerance, but there's nothing a human eye
can do with that information other than an error prone copypaste of new
checksums back in the yaml file and kicking it back to CI, itself a
waste of time.

Finally, in the time we've had trace-based testing alongside the
conformance tests, I cannot remember a single actual regression in one
of my commits the trace jobs have identified that the conformance tests
have not also identified. By contrast, the conformance test coverage has
prevented the merge of a number of actual regressions, with very few
flakes or xfail changes, and I am grateful we have that coverage. That
means the value added from the trace jobs is close to zero, while the
above checksum issues means that the cost is tremendous, even ignoring
the physical cost of the extra CI jobs.

If you work on trace-based testing and would like to understand how it
could adapted to be useful for Panfrost, see my recommendations above.
If you work on CI in general and would like to improve Panfrost's CI
coverage, what we need right now is not trace-based testing, it's
GLES3.1 conformance runs on MediaTek MT8192 or MT8195. That hardware is
already in the Collabora LAVA lab, but it's not being used for Mesa CI
as the required kernel patches haven't made their way to mainline yet
and nobody has cherry-picked them to the gfx-ci kernel. If you are a
Collaboran and interested in improving Panfrost CI, please ping
AngeloGioacchino for information on which specific patches need to be
backported or cherry-picked to our gfx-ci kernel. Thank you.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Acked-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19358>