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Date:   Wed, 25 Oct 2023 10:43:16 +0200
From:   Maxime Ripard <mripard@...nel.org>
To:     Marco Pagani <marpagan@...hat.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian Koenig <christian.koenig@....com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org
Subject: Re: [RFC PATCH] drm/test: add a test suite for GEM objects backed by
 shmem

Hi,

On Tue, Oct 24, 2023 at 07:14:25PM +0200, Marco Pagani wrote:
> >> +static void drm_gem_shmem_test_obj_create_private(struct kunit *test)
> >> +{
> >> +	struct fake_dev *fdev = test->priv;
> >> +	struct drm_gem_shmem_object *shmem;
> >> +	struct drm_gem_object *gem_obj;
> >> +	struct dma_buf buf_mock;
> >> +	struct dma_buf_attachment attach_mock;
> >> +	struct sg_table *sgt;
> >> +	char *buf;
> >> +	int ret;
> >> +
> >> +	/* Create a mock scatter/gather table */
> >> +	buf = kunit_kzalloc(test, TEST_SIZE, GFP_KERNEL);
> >> +	KUNIT_ASSERT_NOT_NULL(test, buf);
> >> +
> >> +	sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
> >> +	KUNIT_ASSERT_NOT_NULL(test, sgt);
> >> +
> >> +	ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
> >> +	KUNIT_ASSERT_EQ(test, ret, 0);
> >> +	sg_init_one(sgt->sgl, buf, TEST_SIZE);
> >> +
> >> +	/* Init a mock DMA-BUF */
> >> +	buf_mock.size = TEST_SIZE;
> >> +	attach_mock.dmabuf = &buf_mock;
> >> +
> >> +	gem_obj = drm_gem_shmem_prime_import_sg_table(&fdev->drm_dev, &attach_mock, sgt);
> >> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, gem_obj);
> >> +	KUNIT_ASSERT_EQ(test, gem_obj->size, TEST_SIZE);
> >> +	KUNIT_ASSERT_NULL(test, gem_obj->filp);
> >> +	KUNIT_ASSERT_NOT_NULL(test, gem_obj->funcs);
> >> +
> >> +	shmem = to_drm_gem_shmem_obj(gem_obj);
> >> +	KUNIT_ASSERT_PTR_EQ(test, shmem->sgt, sgt);
> >> +
> >> +	/* The scatter/gather table is freed by drm_gem_shmem_free */
> >> +	drm_gem_shmem_free(shmem);
> >> +}
> > 
> > KUNIT_ASSERT_* will stop the execution of the test on failure, you
> > should probably use a bit more of KUNIT_EXPECT_* calls otherwise you'll
> > leak resources.
> > 
> > You also probably want to use a kunit_action to clean up and avoid that
> > whole discussion
> >
> 
> You are right. I slightly prefer using KUnit expectations (unless actions
> are strictly necessary) since I feel using actions makes test cases a bit
> less straightforward to understand. Is this okay for you?

I disagree. Actions make it easier to reason about, even when comparing
assertion vs expectation

Like, for the call to sg_alloc_table and
drm_gem_shmem_prime_import_sg_table(), the reasonable use of assert vs
expect would be something like:

sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
KUNIT_ASSERT_NOT_NULL(test, sgt);

ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
KUNIT_ASSERT_EQ(test, ret, 0);

/*
 * Here, it's already not super clear whether you want to expect vs
 * assert. expect will make you handle the failure case later, assert will
 * force you to call kfree on sgt. Both kind of suck in their own ways.
 */

sg_init_one(sgt->sgl, buf, TEST_SIZE);

gem_obj = drm_gem_shmem_prime_import_sg_table(&fdev->drm_dev, &attach_mock, sgt);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, gem_obj);

/*
 * If the assert fails, we forgot to call sg_free_table(sgt) and kfree(sgt).
 */

KUNIT_EXPECT_EQ(test, gem_obj->size, TEST_SIZE);
KUNIT_EXPECT_NULL(test, gem_obj->filp);
KUNIT_EXPECT_NOT_NULL(test, gem_obj->funcs);

/*
 * And here we have to handle the case where the expectation was wrong,
 * but the test still continued.
 */

But if you're not using an action, you still have to call kfree(sgt),
which means that you might still

shmem = to_drm_gem_shmem_obj(gem_obj);
KUNIT_ASSERT_PTR_EQ(test, shmem->sgt, sgt);

/*
 * If the assertion fails, we now have to call drm_gem_shmem_free(shmem)
 */

/* The scatter/gather table is freed by drm_gem_shmem_free */
drm_gem_shmem_free(shmem);

/* everything's fine now */

The semantics around drm_gem_shmem_free make it a bit convoluted, but
doing it using goto/labels, plus handling the assertions and error
reporting would be difficult.

Using actions, we have:

sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
KUNIT_ASSERT_NOT_NULL(test, sgt);

ret = kunit_add_action_or_reset(test, kfree_wrapper, sgt);
KUNIT_ASSERT_EQ(test, ret, 0);

ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
KUNIT_ASSERT_EQ(test, ret, 0);

ret = kunit_add_action_or_reset(test, sg_free_table_wrapper, sgt);
KUNIT_ASSERT_EQ(test, ret, 0);

sg_init_one(sgt->sgl, buf, TEST_SIZE);

gem_obj = drm_gem_shmem_prime_import_sg_table(&fdev->drm_dev, &attach_mock, sgt);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, gem_obj);
KUNIT_EXPECT_EQ(test, gem_obj->size, TEST_SIZE);
KUNIT_EXPECT_NULL(test, gem_obj->filp);
KUNIT_EXPECT_NOT_NULL(test, gem_obj->funcs);

/* drm_gem_shmem_free will free the struct sg_table itself */
kunit_remove_action(test, sg_free_table_wrapper, sgt);
kunit_remove_action(test, kfree_wrapper, sgt);

shmem = to_drm_gem_shmem_obj(gem_obj);
KUNIT_ASSERT_PTR_EQ(test, shmem->sgt, sgt);

ret = kunit_add_action_or_reset(test, drm_gem_shmem_free_wrapper, shmem);
KUNIT_ASSERT_EQ(test, ret, 0);

The last one is arguable, but for the previous ones it makes error
handling much more convenient and easy to reason about.

The wrappers are also a bit inconvenient to use, but it's mostly there
to avoid a compiler warning at the moment.

This patch will help hopefully:
https://lore.kernel.org/linux-kselftest/20230915050125.3609689-1-davidgow@google.com/

Maxime

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