Message-ID: <CABVgOS=Tb_VsfiQZ6hE62e0LqbpKczjLxitXrzBvmcuDPxfAZw@mail.gmail.com>
Date:   Tue, 28 Mar 2023 21:38:03 +0800
From:   David Gow <davidgow@...gle.com>
To:     Matti Vaittinen <mazziesaccount@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Brendan Higgins <brendan.higgins@...ux.dev>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kunit-dev@...glegroups.com, Stephen Boyd <sboyd@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>, linux-iio@...r.kernel.org,
        Maxime Ripard <maxime@...no.tech>
Subject: Re: [PATCH v6 3/7] kunit: Add kunit wrappers for (root) device creation

On Tue, 28 Mar 2023 at 21:22, Matti Vaittinen <mazziesaccount@...il.com> wrote:
>
> Hi David & Greg and thanks for working with this!
>
> On 3/28/23 15:45, David Gow wrote:
> > Thanks, Gred and Matti.
> >
> > On Mon, 27 Mar 2023 at 20:38, Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> >>
> >> On Mon, Mar 27, 2023 at 03:20:06PM +0300, Matti Vaittinen wrote:
> >>> On 3/27/23 15:01, Greg Kroah-Hartman wrote:
> >>>> On Mon, Mar 27, 2023 at 02:34:02PM +0300, Matti Vaittinen wrote:
> >
> > I'm happy to keep working on this, but would definitely appreciate
> > your feedback.
> >
> > I've put my work-in-progress code here:
> > https://kunit.googlesource.com/linux/+/refs/heads/kunit/device-helpers%5E%21/#F0
> >
> > It creates a "kunit" bus, and adds a few helpers to create both
> > devices and drivers on that bus, and clean them up when the test
> > exits. It seems to work on all of the tests which used
> > root_device_register so far (except those -- only
> > test_iio_find_closest_gain_low so far -- which create multiple devices
> > with the same name, as the driver name won't be unique),
>
> I wouldn't worry about it for as long as it's just because an iio-gts
> test does something silly. Those tests are currently only in my personal
> playground and changing those tests should be pretty trivial.
>

Yeah: this isn't anything to worry about. It's as much my note as to
"what works with this code as-is", rather than indicative of a more
fundamental flaw.

> And right after saying that - the test_iio_find_closest_gain_low test does
>
> a) register a 'test' device
> b) perform test on devm_ API
> c) unregister the 'test' device
>
> d) register a 'test' device (same name as at step a)
> e) perform test on devm_ API
> f) unregister the 'test' device
>
> My assumption is that the test device would be gone after step c)
> because there should be no references to it anywhere. Hence, I wonder
> why registering at step d) fails? (Or did I misunderstand something?)
>

This is because I'm now creating a struct device_driver as well as a
device, and it's not being cleaned up until the end of the test.

The two solutions here would be to either:
- Add a way to clean up the driver earlier. (Shouldn't be too hard, I
just haven't got around to it yet), or:
- Use the same struct device_driver for both tests (there's a new
kunit_device_register_with_driver which allows you to pass a custom
driver in, rather than creating your own)

I think the latter's probably better, but I'll probably implement both
as I clean up the API further.

> > and the drm
> > tests work fine when ported to it as well.
> >
> > There's still a lot of cleanup to do and questions which need
> > answering, including:
> > - Working out how best to provide an owning module (it's currently
> > just kunit, but probably should be the module which contains the
> > actual tests)
>
> Maybe there is something I am not seeing but how about wrapping the
> kunit_device_register() in a macro and getting the THIS_MODULE in
> caller's context?
>

Yeah: that's probably what I'll do. The other possibility is to store
the module pointer in the struct kunit context.

> > In any case, does this seem like the right way forward?
>
> I am by no means an expert on this but this does look good to me. I
> would keep this as clean, lean and simple as possible in order to keep
> understanding / debugging the problems exposed by the tests as simple as
> possible. At some point someone is wondering why a test fails, and ends
> up looking through these helpers to ensure problem is no lurking
> there... Hence, I'd kept the code there in minimum - meaning, I might
> not add kunit class or even a driver until tests require that. (Even if
> it would not look as good in the sysfs - as far as I understand the
> kunit sysfs entries are a 'test feature' which should not be present in
> 'production systems'. This is not an excuse to make things bad - but (in
> my opinion) this is a good reason to prioritize simplicity.

I agree with you that it's best to avoid complexity for as long as we
reasonably can. I think that there are enough things which would
benefit from having the driver stuff to make it worth having
_something_ there, particularly since it makes this a more "normal"
device, so hopefully will be less surprising.

For sysfs, it's one of those things which shows up pretty rarely in
day-to-day KUnit use, as most people are using the kunit.py script
which has all of the tests built-in, and no userspace to look at sysfs
from. That being said, that doesn't cover all use cases, and I
definitely would rather not make sysfs unusably ugly for everyone
else, so I'm happy to tidy it up. (I'm not planning to do a kunit
class at the moment, though.)

I _think_ this approach (once the details of the API and
implementation are tidied up a bit) should sit in about the sweet spot
for complexity, assuming there's nothing horribly wrong with it I
haven't noticed. I suspect we're better off leaving some of the more
advanced use-cases to implement their own way of instantiating
devices, at least for now, and focus on getting these common cases
right.

Cheers,
-- David

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