Re: [PATCH v2 2/2] drm/tests: Split drm_test_dp_mst_sideband_msg_req_decode into parameterized tests

[David Gow]

On Fri, Sep 30, 2022 at 6:33 AM Michał Winiarski
<michal.winiarski@xxxxxxxxx> wrote:
>
> On Tue, Sep 27, 2022 at 07:12:06PM -0300, Maíra Canal wrote:
> > The drm_test_dp_mst_sideband_msg_req_decode repeats the same test
> > structure with different parameters. This could be better represented
> > by parameterized tests, provided by KUnit.
> >
> > In order to convert the tests to parameterized tests, the test case for
> > the client ID was changed: instead of using get_random_bytes to generate
> > the client ID, the client ID is now hardcoded in the test case.
>
> Generally "random" usage is not incompatible with parameterized tests, we can
> create parameterized tests that use random data.
> The idea is to pass a function that generates the actual param (where we have a
> pointer to function as one of the members in "params" struct).
>
> For example, see "random_dp_query_enc_client_id" usage here:
> https://lore.kernel.org/dri-devel/20220117232259.180459-7-michal.winiarski@xxxxxxxxx/
>
> In this case, we just compare data going in with data going out (and the data
> itself is not transformed in any way), so it doesn't really matter for coverage
> and we can hardcode.
>
> -Michał

FWIW, while the uses of randomness in DRM tests so far haven't
concerned me much, I think we'll eventually want to have some way of
ensuring the inputs to tests are deterministic.

My thoughts are that (at some point) we'll add a kunit_random()
function or similar, which will use a pseudorandom number generator
which can be set to a deterministic seed before each test case. That
way, there'd be a way to reproduce an error easily if it occurred. (Of
course, there'd be a way of setting different or random seeds to
preserve the extra coverage you'd otherwise get.)

I don't think this is something worth holding up or changing existing
tests at the moment, but having tests behave deterministically is
definitely desirable, so +1 to avoiding get_random_bytes() if it's not
giving you any real benefit.

We've also had a few requests in the past for being able to pass in a
custom set of parameters from userspace, which opens up some other
interesting possibilities, though it's not a priority at the moment.

Cheers,
-- David

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