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

[Frank Rowand <frowand.list@gmail.com>]

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@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.
> 
> 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@gmail.com/T/#u


> 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

-Frank

> 
>>
>> 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
>