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 08:37:05 -0300
From:   Maíra Canal <mcanal@...lia.com>
To:     Michał Winiarski <michal.winiarski@...el.com>,
        David Gow <davidgow@...gle.com>
Cc:     André Almeida <andrealmeid@...lia.com>,
        Arthur Grillo <arthur.grillo@....br>, magalilemes00@...il.com,
        David Airlie <airlied@...ux.ie>,
        Tales Aparecida <tales.aparecida@...il.com>,
        Daniel Latypov <dlatypov@...gle.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        dri-devel@...ts.freedesktop.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Melissa Wen <mwen@...lia.com>,
        Isabella Basso <isabbasso@...eup.net>
Subject: Re: [PATCH v2 2/2] drm/tests: Split
 drm_test_dp_mst_sideband_msg_req_decode into parameterized tests

On 9/30/22 06:11, Michał Winiarski wrote:
> On Fri, Sep 30, 2022 at 02:50:43PM +0800, David Gow wrote:
>> 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/

Although it is possible, I don't see the benefit in this case to use the
get_random_bytes instead of hardcoding. I believe it will only add more
boilerplate to the tests.

>>>
>>> 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.)
> 
> That's exactly what DRM tests do (well... most DRM tests, this one being the
> exception, and those other tests also seem to have lost a printk with seed value
> after being refactored into kunit).

I will take a look at those lost printk in drm_mm_test and
drm_buddy_test, as they are important to assure the reproducibility of
the tests.

> See the usage of DRM_RND_STATE in drm_mm_test and drm_buddy_test.
> Having kunit_random() would definitely be useful and let us remove bunch of
> boilerplate from the tests, but it doesn't prevent using reproducible random
> data in existing tests.
> 
>> 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.
> 
> Yeah - all I was refering to in my previous message was the wording of the
> commit message. We're just removing it because it is desirable in this
> particular case, not because of the fact that the test is now parameterized and
> that's somehow preventing get_random_bytes() usage.

I will send a v3 rewording the commit message to make it more clear.

Best Regards,
- Maíra Canal

> 
> -Michał
> 
>> 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
> 
>