linux-kernel - Re: [PATCH v2 2/2] drm/tests: Split drm_test_dp_mst_sideband_msg_req

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Date:   Fri, 30 Sep 2022 14:50:43 +0800
From:   David Gow <davidgow@...gle.com>
To:     Michał Winiarski <michal.winiarski@...el.com>
Cc:     Maíra Canal <mcanal@...lia.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Melissa Wen <mwen@...lia.com>,
        André Almeida <andrealmeid@...lia.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Latypov <dlatypov@...gle.com>,
        Arthur Grillo <arthur.grillo@....br>,
        Isabella Basso <isabbasso@...eup.net>, magalilemes00@...il.com,
        Tales Aparecida <tales.aparecida@...il.com>,
        dri-devel@...ts.freedesktop.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] drm/tests: Split drm_test_dp_mst_sideband_msg_req_decode
 into parameterized tests

On Fri, Sep 30, 2022 at 6:33 AM Michał Winiarski
<michal.winiarski@...el.com> 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@intel.com/
>
> 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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