Re: [PATCH 2/8] of: Enable DTB loading on UML for KUnit tests

David Gow
2023-03-11T06:42:42Z

On Sat, 11 Mar 2023 at 07:34, Stephen Boyd <sboyd@kernel.org> wrote:
>
> Quoting David Gow (2023-03-10 00:09:48)
> > On Fri, 10 Mar 2023 at 07:19, Stephen Boyd <sboyd@kernel.org> wrote:
> > >
> > >
> > > Hmm. I think you're suggesting that the unit test data be loaded
> > > whenever CONFIG_OF=y and CONFIG_KUNIT=y. Then tests can check for
> > > CONFIG_OF and skip if it isn't enabled?
> > >
> >
> > More of the opposite: that we should have some way of supporting tests
> > which might want to use a DTB other than the built-in one. Mostly for
> > non-UML situations where an actual devicetree is needed to even boot
> > far enough to get test output (so we wouldn't be able to override it
> > with a compiled-in test one).
>
> Ok, got it.
>
> >
> > I think moving to overlays probably will render this idea obsolete:
> > but the thought was to give test code a way to check for the required
> > devicetree nodes at runtime, and skip the test if they weren't found.
> > That way, the failure mode for trying to boot this on something which
> > required another device tree for, e.g., serial, would be "these tests
> > are skipped because the wrong device tree is loaded", not "I get no
> > output because serial isn't working".
> >
> > Again, though, it's only really needed for non-UML, and just loading
> > overlays as needed should be much more sensible anyway.
>
> I still have one niggle here. Loading overlays requires
> CONFIG_OF_OVERLAY, and the overlay loading API returns -ENOTSUPP when
> CONFIG_OF_OVERLAY=n. For now I'm checking for the config being enabled
> in each test, but I'm thinking it may be better to simply call
> kunit_skip() from the overlay loading function if the config is
> disabled. This way tests can simply call the overlay loading function
> and we'll halt the test immediately if the config isn't enabled.
>

That sounds sensible, though there is a potential pitfall. If
kunit_skip() is called directly from overlay code, might introduce a
dependency on kunit.ko from the DT overlay, which we might not want.
The solution there is either to have a kunit wrapper function (so the
call is already in kunit.ko), or to have a hook to skip the current
test (which probably makes sense to do anyway, but I think the wrapper
is the better option).

> >
> > > >
> > > > That being said, I do think that there's probably some sense in
> > > > supporting the compiled-in DTB as well (it's definitely simpler than
> > > > patching kunit.py to always pass the extra command-line option in, for
> > > > example).
> > > > But maybe it'd be nice to have the command-line option override the
> > > > built-in one if present.
> > >
> > > Got it. I need to test loading another DTB on the commandline still, but
> > > I think this won't be a problem. We'll load the unittest-data DTB even
> > > with KUnit on UML, so assuming that works on UML right now it should be
> > > unchanged by this series once I resend.
> >
> > Again, moving to overlays should render this mostly obsolete, no? Or
> > am I misunderstanding how the overlay stuff will work?
>
> Right, overlays make it largely a moot issue. The way the OF unit tests
> work today is by grafting a DTB onto the live tree. I'm reusing that
> logic to graft a container node target for kunit tests to add their
> overlays too. It will be clearer once I post v2.
>
> >
> > One possible future advantage of being able to test with custom DTs at
> > boot time would be for fuzzing (provide random DT properties, see what
> > happens in the test). We've got some vague plans to support a way of
> > passing custom data to tests to support this kind of case (though, if
> > we're using overlays, maybe the test could just patch those if we
> > wanted to do that).
>
> Ah ok. I can see someone making a fuzzer that modifies devicetree
> properties randomly, e.g. using different strings for clock-names.
>
> This reminds me of another issue I ran into. I wanted to test adding the
> same platform device to the platform bus twice to confirm that the
> second device can't be added. That prints a warning, which makes
> kunit.py think that the test has failed because it printed a warning. Is
> there some way to avoid that? I want something like
>
> KUNIT_EXPECT_WARNING(test, <call some function>)
>
> so I can test error cases.

Hmm... I'd've thought that shouldn't be a problem: kunit.py should
ignore most messages during a test, unless it can't find a valid
result line. What does the raw KTAP output look like? (You can get it
from kunit.py by passing the --raw_output option).

That being said, a KUNIT_EXPECT_LOG_MESSAGE() or similar is something
we've wanted for a while. I think that the KASAN folks have been
working on something similar using console tracepoints:
https://lore.kernel.org/all/ebf96ea600050f00ed567e80505ae8f242633640.1666113393.git.andreyknvl@google.com/

Cheers,
-- David