Re: [PATCH v5 1/8] drivers: kunit: Generic helpers for test device creation

[Matti Vaittinen <mazziesaccount@gmail.com>]

Hi David, all,

On 3/23/23 09:30, David Gow wrote:
> On Wed, 22 Mar 2023 at 17:06, Matti Vaittinen <mazziesaccount@gmail.com> wrote:
>>
>> The creation of a dummy device in order to test managed interfaces is a
>> generally useful test feature. The drm test helpers
>> drm_kunit_helper_alloc_device() and drm_kunit_helper_free_device()
>> are doing exactly this. It makes no sense that each and every component
>> which intends to be testing managed interfaces will create similar
>> helpers so stole these for more generic use.
>>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
>>
>> ---
>> Changes:
>> v4 => v5:
>> - Add accidentally dropped header and email recipients
>> - do not rename + move helpers from DRM but add temporary dublicates to
>>    simplify merging. (This patch does not touch DRM and can be merged
>>    separately. DRM patch and IIO test patch still depend on this one).
>>
>> Please note that there's something similar ongoing in the CCF:
>> https://lore.kernel.org/all/20230302013822.1808711-1-sboyd@kernel.org/
>>
>> I do like the simplicity of these DRM-originated helpers so I think
>> they're worth. I do equally like the Stephen's idea of having the
>> "dummy platform device" related helpers under drivers/base and the
>> header being in include/kunit/platform_device.h which is similar to real
>> platform device under include/linux/platform_device.h
>> ---
> 
> Thanks for sending this my way.
> 
> It's clear we need some way of creating "fake" devices for KUnit
> tests. Given that there are now (at least) three different drivers
> looking to use this, we'll ultimately need something which works for
> everyone.
> 
> I think the questions we therefore need to answer are:
> - Do we specifically need a platform_device (or, any other specific
> device struct), or would any old struct device work? I can see why we
> would need a platform device for cases where we're testing things like
> device-tree properties (or, in the future, having e.g. USB-specific
> helpers which create usb_device). Most tests just use
> root_device_register() thus far, though.

Funny timing. I just found the root_device_register() while wondering 
the parent for auxiliary_devices.

I think the root_device_[un]register() meets my current needs.

> - What helpers do we need to make creating, using, and cleaning up
> these devices as simple as possible.
> 
> I think that in this particular case, we don't actually need a struct
> platform_device. Replacing these helpers with simple calls to
> root_device_register() and root_device_unregister() seems to work fine
> with this patch series for me. (It does break the
> drm_test_managed_run_action test, though.) So I don't think having
> these helpers actually help this series at the moment.

I am afraid that further work with these helpers is out of the scope for 
me (at least for now). I'll drop the DRM and the helper patches from 
this series && go with the root_device_register(), 
root_device_unregister() in the IIO tests added in this series.

> That being said, if they used the KUnit resource system to
> automatically clean up the device when the test is finished (or
> otherwise exits),

My 10 cents to this is that yes, automatic unwinding at test exit would 
be simple - and also correct for most of the time. However, I think 
there might also be tests that would like to verify the unwinding 
process has really managed to do what it was intended to do. That, I 
think would require being able to manually drop the device in the middle 
of the test.

> So, I guess we have three cases we need to look at:
> - A test just needs any old struct device. Tests thus far have
> hardcoded, or had their own wrappers around root_device_register() for
> this.

As said above, my case fits this category.

> - A test needs a device attached to a bus (but doesn't care which
> bus). Thus far, people have used struct platform_device for this (see
> the DRM helpers, which use a platform device for managed resource
> tests[2]). Maybe the right solution here is something like a struct
> kunit_device?

This sounds like, how to put it, "architecturally correct". But...

> - A test needs a device attached to a specific bus. We'll probably
> need some more detailed faking of that bus. This covers cases like
> having fake USB devices, devicetree integration, etc.

...if we in any case need this, wouldn't the kunit_device just be 
unnecessary bloat because if the test does not care which bus it is 
sitting in, then it could probably use a bus-specific device as well?

> I'd suggest that, for the majority of cases which only care about the
> first case, let's just use root_device_register() directly,

After finding the root_device_register() - I agree.

> or have a
> thin wrapper like the old root_device-based version of the DRM
> helpers[3]. This will probable serve us well enough while we work out
> how to handle the other two cases properly (which is already being
> looked at for the CLK/DeviceTree patches and the DRM stuff).
> 
> If the resulting helpers are generally useful enough, they can
> probably sit in either drivers/base or lib/kunit. I'd rather not have
> code that's really specific to certain busses sitting in lib/kunit
> rather than alongside the device/bus code in drivers/base or some
> other subsystem/driver path, but I can tolerate it for the very
> generic struct device.
> 
> Regardless, I've left a few notes on the patch itself below.

Thanks but I guess I'll just drop this one :)

> 
> Cheers,
> -- David
> 
> [1]: https://kunit-review.googlesource.com/c/linux/+/5434/3/include/kunit/resource.h
> [2]: https://lore.kernel.org/all/20221123-rpi-kunit-tests-v3-8-4615a663a84a@cerno.tech/
> [3]: https://elixir.bootlin.com/linux/v6.2/source/drivers/gpu/drm/tests/drm_kunit_helpers.c#L39

Thanks for the thorough analysis and these links! This was enlightening :)

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~