Message-ID: <CABVgOS=KUg+18wJe=O29tgOB0tQghAk030kONEm5fOzJHgKLgw@mail.gmail.com>
Date:   Thu, 23 Mar 2023 15:30:34 +0800
From:   David Gow <davidgow@...gle.com>
To:     Matti Vaittinen <mazziesaccount@...il.com>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Brendan Higgins <brendan.higgins@...ux.dev>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kunit-dev@...glegroups.com, Stephen Boyd <sboyd@...nel.org>,
        Maxime Ripard <maxime@...no.tech>,
        Jonathan Cameron <jic23@...nel.org>, linux-iio@...r.kernel.org
Subject: Re: [PATCH v5 1/8] drivers: kunit: Generic helpers for test device creation

On Wed, 22 Mar 2023 at 17:06, Matti Vaittinen <mazziesaccount@...il.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@...il.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.
- 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.

That being said, if they used the KUnit resource system to
automatically clean up the device when the test is finished (or
otherwise exits), that would seem like a real advantage. The clk and
drm examples both do this, and I'm hoping to add an API to make it
even simpler going forward. (With the work-in-progress API described
here[1], the whole thing should boil down to "struct device *dev =
root_device_register(name); kunit_defer(root_device_unregister,
dev);".)

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.
- 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?
- 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.

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

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
>  drivers/base/test/Kconfig             |  5 ++
>  drivers/base/test/Makefile            |  2 +
>  drivers/base/test/test_kunit_device.c | 83 +++++++++++++++++++++++++++
>  include/kunit/platform_device.h       | 13 +++++
>  4 files changed, 103 insertions(+)
>  create mode 100644 drivers/base/test/test_kunit_device.c
>  create mode 100644 include/kunit/platform_device.h
>
> diff --git a/drivers/base/test/Kconfig b/drivers/base/test/Kconfig
> index 610a1ba7a467..642b5b183c10 100644
> --- a/drivers/base/test/Kconfig
> +++ b/drivers/base/test/Kconfig
> @@ -1,4 +1,9 @@
>  # SPDX-License-Identifier: GPL-2.0
> +
> +config TEST_KUNIT_DEVICE_HELPERS
> +       depends on KUNIT
> +       tristate
> +
>  config TEST_ASYNC_DRIVER_PROBE
>         tristate "Build kernel module to test asynchronous driver probing"
>         depends on m
> diff --git a/drivers/base/test/Makefile b/drivers/base/test/Makefile
> index 7f76fee6f989..49926524ec6f 100644
> --- a/drivers/base/test/Makefile
> +++ b/drivers/base/test/Makefile
> @@ -1,5 +1,7 @@
>  # SPDX-License-Identifier: GPL-2.0
>  obj-$(CONFIG_TEST_ASYNC_DRIVER_PROBE)  += test_async_driver_probe.o
>
> +obj-$(CONFIG_TEST_KUNIT_DEVICE_HELPERS) += test_kunit_device.o
> +
>  obj-$(CONFIG_DRIVER_PE_KUNIT_TEST) += property-entry-test.o
>  CFLAGS_property-entry-test.o += $(DISABLE_STRUCTLEAK_PLUGIN)
> diff --git a/drivers/base/test/test_kunit_device.c b/drivers/base/test/test_kunit_device.c
> new file mode 100644
> index 000000000000..75790e15b85c
> --- /dev/null
> +++ b/drivers/base/test/test_kunit_device.c
> @@ -0,0 +1,83 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * These helpers have been extracted from drm test code at
> + * drm_kunit_helpers.c which was authored by
> + * Maxime Ripard <maxime@...no.tech>
> + */
> +
> +#include <linux/device.h>
> +#include <linux/platform_device.h>
> +
> +#include <kunit/platform_device.h>
> +
> +#define KUNIT_DEVICE_NAME      "test-kunit-mock-device"

Personally, I'd really rather this be a name passed in by the test.
What if a test needs to create multiple distinct devices?

> +
> +static int fake_probe(struct platform_device *pdev)
> +{
> +       return 0;
> +}
> +
> +static int fake_remove(struct platform_device *pdev)
> +{
> +       return 0;
> +}
> +
> +static struct platform_driver fake_platform_driver = {
> +       .probe  = fake_probe,
> +       .remove = fake_remove,
> +       .driver = {
> +               .name   = KUNIT_DEVICE_NAME,
> +       },
> +};
> +
> +/**
> + * test_kunit_helper_alloc_device - Allocate a mock device for a KUnit test
> + * @test: The test context object
> + *
> + * This allocates a fake struct &device to create a mock for a KUnit
> + * test. The device will also be bound to a fake driver. It will thus be
> + * able to leverage the usual infrastructure and most notably the
> + * device-managed resources just like a "real" device.
> + *
> + * Callers need to make sure test_kunit_helper_free_device() on the
> + * device when done.
> + *
> + * Returns:
> + * A pointer to the new device, or an ERR_PTR() otherwise.
> + */
> +struct device *test_kunit_helper_alloc_device(struct kunit *test)
> +{
> +       struct platform_device *pdev;
> +       int ret;
> +
> +       ret = platform_driver_register(&fake_platform_driver);
> +       KUNIT_ASSERT_EQ(test, ret, 0);
> +
> +       pdev = platform_device_alloc(KUNIT_DEVICE_NAME, PLATFORM_DEVID_NONE);
> +       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, pdev);
> +
> +       ret = platform_device_add(pdev);
> +       KUNIT_ASSERT_EQ(test, ret, 0);
> +
> +       return &pdev->dev;
> +}
> +EXPORT_SYMBOL_GPL(test_kunit_helper_alloc_device);
> +
> +/**
> + * test_kunit_helper_free_device - Frees a mock device
> + * @test: The test context object
> + * @dev: The device to free
> + *
> + * Frees a device allocated with test_kunit_helper_alloc_device().
> + */

This really should be automatically called when the test exits,
probably using kunit reources. Ideally, there'd also be a function to
free it earlier, which can be done by calling kunit_remove_resource()
to lower the refcount.

> +void test_kunit_helper_free_device(struct kunit *test, struct device *dev)
> +{
> +       struct platform_device *pdev = to_platform_device(dev);
> +
> +       platform_device_unregister(pdev);
> +       platform_driver_unregister(&fake_platform_driver);
> +}
> +EXPORT_SYMBOL_GPL(test_kunit_helper_free_device);
> +
> +MODULE_AUTHOR("Matti Vaittinen <mazziesaccount@...il.com>");
> +MODULE_LICENSE("GPL");
> diff --git a/include/kunit/platform_device.h b/include/kunit/platform_device.h
> new file mode 100644
> index 000000000000..2a9c7bdd75eb
> --- /dev/null
> +++ b/include/kunit/platform_device.h
> @@ -0,0 +1,13 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __KUNIT_PLATFORM_DEVICE__
> +#define __KUNIT_PLATFORM_DEVICE__
> +
> +#include <kunit/test.h>
> +
> +struct device;
> +
> +struct device *test_kunit_helper_alloc_device(struct kunit *test);
> +void test_kunit_helper_free_device(struct kunit *test, struct device *dev);

If these helpers are supposed to guarantee that the resulting device
is a platform device, let's reflect that in their names. Otherwise,
let's not put this in a platform_device.h header, but maybe something
more general, like kunit/device.h.

> +
> +#endif
> --
> 2.39.2
>
>
> --
> Matti Vaittinen, Linux device drivers
> ROHM Semiconductors, Finland SWDC
> Kiviharjunlenkki 1E
> 90220 OULU
> FINLAND
>
> ~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
> Simon says - in Latin please.
> ~~~ "non cogito me" dixit Rene Descarte, deinde evanescavit ~~~
> Thanks to Simon Glass for the translation =]

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