Re: [PATCH 00/24] drm: Introduce Kunit Tests to VC4

[David Gow]

On Wed, Nov 23, 2022 at 11:28 PM Maxime Ripard <maxime@xxxxxxxxxx> wrote:
>
> Hi,
>
> This series introduce Kunit tests to the vc4 KMS driver, but unlike what we
> have been doing so far in KMS, it actually tests the atomic modesetting code.
>
> In order to do so, I've had to improve a fair bit on the Kunit helpers already
> found in the tree in order to register a full blown and somewhat functional KMS
> driver.
>
> It's of course relying on a mock so that we can test it anywhere. The mocking
> approach created a number of issues, the main one being that we need to create
> a decent mock in the first place, see patch 22. The basic idea is that I
> created some structures to provide a decent approximation of the actual
> hardware, and that would support both major architectures supported by vc4.
>
> This is of course meant to evolve over time and support more tests, but I've
> focused on testing the HVS FIFO assignment code which is fairly tricky (and the
> tests have actually revealed one more bug with our current implementation). I
> used to have a userspace implementation of those tests, where I would copy and
> paste the kernel code and run the tests on a regular basis. It's was obviously
> fairly suboptimal, so it seemed like the perfect testbed for that series.
>
> Let me know what you think,
> Maxime
>
> To: David Airlie <airlied@xxxxxxxxx>
> To: Daniel Vetter <daniel@xxxxxxxx>
> To: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> To: Maxime Ripard <mripard@xxxxxxxxxx>
> To: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Cc: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
> Cc: Javier Martinez Canillas <javierm@xxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Maíra Canal <mairacanal@xxxxxxxxxx>
> Cc: Brendan Higgins <brendan.higgins@xxxxxxxxx>
> Cc: David Gow <davidgow@xxxxxxxxxx>
> Cc: linux-kselftest@xxxxxxxxxxxxxxx
> Cc: kunit-dev@xxxxxxxxxxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-media@xxxxxxxxxxxxxxx
> Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
> Signed-off-by: Maxime Ripard <maxime@xxxxxxxxxx>
>
> ---

Hi Maxime,

Thanks very much for this! I'm really excited to see these sorts of
tests being written.

I was able to successfully run these under qemu with:
./tools/testing/kunit/kunit.py run --kunitconfig
drivers/gpu/drm/vc4/tests --arch arm64
--cross_compile=aarch64-linux-gnu-
(and also with clang, using --make_options LLVM=1 instead of the
--cross_compile flag)

On the other hand, they don't compile as a module:
ERROR: modpost: missing MODULE_LICENSE() in drivers/gpu/drm/vc4/tests/vc4_mock.o
ERROR: modpost: missing MODULE_LICENSE() in
drivers/gpu/drm/vc4/tests/vc4_mock_crtc.o
ERROR: modpost: missing MODULE_LICENSE() in
drivers/gpu/drm/vc4/tests/vc4_mock_output.o
ERROR: modpost: missing MODULE_LICENSE() in
drivers/gpu/drm/vc4/tests/vc4_mock_plane.o
ERROR: modpost: missing MODULE_LICENSE() in
drivers/gpu/drm/vc4/tests/vc4_test_pv_muxing.o
ERROR: modpost: missing MODULE_LICENSE() in
drivers/gpu/drm/tests/drm_managed_test.o
ERROR: modpost: "vc4_drm_driver"
[drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
ERROR: modpost: "vc5_drm_driver"
[drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
ERROR: modpost: "drm_kunit_helper_alloc_device"
[drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
ERROR: modpost: "__drm_kunit_helper_alloc_drm_device_with_driver"
[drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
ERROR: modpost: "__vc4_hvs_alloc"
[drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
ERROR: modpost: "vc4_dummy_plane"
[drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
ERROR: modpost: "vc4_mock_pv" [drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
ERROR: modpost: "vc4_dummy_output"
[drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
ERROR: modpost: "vc4_kms_load" [drivers/gpu/drm/vc4/tests/vc4_mock.ko]
undefined!
ERROR: modpost: "vc4_txp_crtc_data"
[drivers/gpu/drm/vc4/tests/vc4_mock.ko] undefined!
WARNING: modpost: suppressed 17 unresolved symbol warnings because
there were too many)

Most of those are just the need to export some symbols. There's some
work underway to support conditionally exporting symbols only if KUnit
is enabled, which may help:
https://lore.kernel.org/linux-kselftest/20221102175959.2921063-1-rmoar@xxxxxxxxxx/

Otherwise, I suspect the better short-term solution would just be to
require that the tests are built-in (or at least compiled into
whatever of the drm/vc4 modules makes most sense).

The only other thing which has me a little confused is the naming of
some of the functions, specifically with the __ prefix. Is it just for
internal functions (many of them aren't static, but maybe they could
use the VISIBLE_IF_KUNIT macro if that makes sense), or for versions
of functions which accept extra arguments? Not a big deal (and maybe
it's a DRM naming convention I'm ignorant of), but I couldn't quite
find a pattern on my first read through.

But on the whole, these look good from a KUnit point-of-view. It's
really to see some solid mocking and driver testing, too. There would
be ways to avoid passing the 'struct kunit' around in more places (or
to store extra data as a kunit_resource), but I think it's better
overall to pass it around like you have in this case -- it's certainly
more compatible with things which might span threads (e.g. the
workqueues).

Thanks a bunch,
-- David

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