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

[David Gow]

On Tue, 14 Mar 2023 at 12:28, Frank Rowand <frowand.list@xxxxxxxxx> wrote:
>
> On 3/13/23 11:02, Frank Rowand wrote:
> > On 3/11/23 00:42, David Gow wrote:
> >> On Sat, 11 Mar 2023 at 07:34, Stephen Boyd <sboyd@xxxxxxxxxx> wrote:
> >>>
> >>> Quoting David Gow (2023-03-10 00:09:48)
> >>>> On Fri, 10 Mar 2023 at 07:19, Stephen Boyd <sboyd@xxxxxxxxxx> 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.
> >
> > DT unittests already have a similar concept.  A test can report that a
> > kernel warning (or any other specific text) either (1) must occur for the
> > test to pass or (2) must _not_ occur for the test to pass.  The check
> > for the kernel warning is done by the test output parsing program
> > scripts/dtc/of_unittest_expect.
> >
> > The reporting by a test of an expected error in drivers/of/unittest.c
> > is done by EXPECT_BEGIN() and EXPECT_END().  These have been in
> > unittest for a long time.
> >
> > The reporting by a test of a not expected to occur error is done
> > by EXPECT_NOT_BEGIN() and EXPECT_NOT_END().  These are added to
> > unittest in linux 6.3-rc1.
> >
> > I discussed this concept in one of the early TAP / KTAP discussion
>
> The link to the early KTAP discussion on this concept is:
>
>    https://lore.kernel.org/all/d38bf9f9-8a39-87a6-8ce7-d37e4a641675@xxxxxxxxx/T/#u
>
>

Thanks -- I'd totally forgotten about that!

I still personally would prefer a way of checking this from within the
kernel, as if we're just printing out "EXPECT: " lines, then it's not
possible to know if a test passes just from the raw results (and
things like statistics can't be updated without a separate tool like
kunit.py parsing the KTAP.

Indeed, my personal preference is that this log-based way of doing
expectations is probably best kept as a last resort. i.e.,
1. Try to add a hook to the code which prints the message, which can
then fail the test (or set a flag for the test to check later). This
probably needs some better KUnit-side helpers to be truly ergonomic,
but at least avoids too strict a dependency on the exact formatting of
the log messages.
2. If that doesn't work, use console tracepoints or similar to
implement an EXPECT_BEGIN() / EXPECT_END() or similar API entirely
within the kernel.
3. Only if we can't come up with a working way of doing the former
options, resort to adding "EXPECT:" lines and having a parser pick up
on this.

One of the downsides of doing "EXPECT" lines in KTAP is that it'll
suddenly be much more dependent on the exact layout of the tests, as
we'd need to be able to override a test result if an expectation fails
(at least, to maintain the KUnit structure). And overriding a result
which is already in the output seems really, really ugly.

There's a patch to the KASAN tests to move from doing option 1 to
option 2 above (in order to better support RCU, which didn't work with
the hook):
https://lore.kernel.org/all/ebf96ea600050f00ed567e80505ae8f242633640.1666113393.git.andreyknvl@xxxxxxxxxx/


> > threads and expect to start a discussion thread on this specific
> > topic in the KTAP Specification V2 context.  I expect the discussion
> > to result in a different implementation than what DT unittests are
> > using (bike shedding likely to ensue) but whatever is agreed to
> > should be easy for DT to switch to.
>
> The link to the KTAP Specification Version 2 process and progress is:
>
>    https://elinux.org/Test_Results_Format_Notes#KTAP_version_2
>

Thanks! We've got a few more KTAP ideas to air, so will hopefully send
those out soon!

Cheers,
-- David

> >
> >>
> >> 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@xxxxxxxxxx/
> >>
> >> Cheers,
> >> -- David
> >
>

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