[PATCH v3] drm/tests: shmem: Set DMA mask to 64-bit in drm_gem_shmem

["José Expósito" <jose.exposito89@gmail.com>]

From: José Expósito <jose.exposito@redhat.com>

drm_gem_shmem_test_purge [1] and drm_gem_shmem_test_get_pages_sgt [2]
intermittently fail on ppc64le and s390x CI systems with a DMA address
overflow:

  DMA addr 0x0000000100307000+4096 overflow (mask ffffffff, bus limit 0)
  WARNING: kernel/dma/direct.h:114 dma_direct_map_sg+0x778/0x920

  drm_gem_shmem_test_purge: ASSERTION FAILED at
    drivers/gpu/drm/tests/drm_gem_shmem_test.c:330
    Expected sgt is not error, but is: -5

The call chain leading to the failure is:

  drm_gem_shmem_test_purge() / drm_gem_shmem_test_get_pages_sgt()
    drm_gem_shmem_get_pages_sgt()
      drm_gem_shmem_get_pages_sgt_locked() [drm_gem_shmem_helper.c]
        dma_map_sgtable()                  [mapping.c]
          __dma_map_sg_attrs()
            dma_direct_map_sg()            [direct.c]
              dma_direct_map_phys()        [kernel/dma/direct.h]
                dma_capable()              Checks addr against DMA mask
                  -> FAILS: addr > 0xFFFFFFFF

The root cause is that KUnit devices are initialized with a 32-bit DMA
mask (DMA_BIT_MASK(32)) in lib/kunit/device.c. On ppc64le and s390x
systems with physical memory above 4GB, page allocations can land at
addresses that exceed this mask. When drm_gem_shmem_get_pages_sgt()
attempts to DMA-map these pages via dma_map_sgtable(), the DMA layer
rejects the mapping because the physical address overflows the 32-bit
mask.

The failure is intermittent because pages may or may not be allocated
above 4GB on any given run depend on memory pressure.

Fix by setting a 64-bit DMA mask on the device before calling
drm_gem_shmem_get_pages_sgt() for all tests, following the same pattern
already used in drm_gem_shmem_test_obj_create_private().

[1] https://s3.amazonaws.com/arr-cki-prod-trusted-artifacts/trusted-artifacts/2643976103/test_s390x/15128551935/artifacts/jobwatch/logs/recipes/21561049/tasks/220716793/results/1014626315/logs/dmesg.log
[2] https://s3.amazonaws.com/arr-cki-prod-trusted-artifacts/trusted-artifacts/2643976103/test_ppc64le/15128551933/artifacts/jobwatch/logs/recipes/21561041/tasks/220716705/results/1014628163/logs/dmesg.log

Fixes: 93032ae634d4 ("drm/test: add a test suite for GEM objects backed by shmem")
Closes: https://datawarehouse.cki-project.org/issue/5345
Closes: https://datawarehouse.cki-project.org/issue/3184
Assisted-by: Claude:claude-4.6-opus
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: José Expósito <jose.exposito@redhat.com>

---

v2:

  - Move fix to drm_gem_shmem_test_init() (Thomas Zimmermann)
  - Link to v1: https://lore.kernel.org/dri-devel/20260703063517.6346-1-jose.exposito89@gmail.com/

v3:

  - Improve comments (Sashiko Bot)
  - Link to v2: https://lore.kernel.org/dri-devel/20260703142749.4099-1-jose.exposito89@gmail.com/
---
 drivers/gpu/drm/tests/drm_gem_shmem_test.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/tests/drm_gem_shmem_test.c b/drivers/gpu/drm/tests/drm_gem_shmem_test.c
index 44a190109249..5e69ff1d1ad6 100644
--- a/drivers/gpu/drm/tests/drm_gem_shmem_test.c
+++ b/drivers/gpu/drm/tests/drm_gem_shmem_test.c
@@ -95,13 +95,9 @@ static void drm_gem_shmem_test_obj_create_private(struct kunit *test)
 	sg_init_one(sgt->sgl, buf, TEST_SIZE);
 
 	/*
-	 * Set the DMA mask to 64-bits and map the sgtables
-	 * otherwise drm_gem_shmem_free will cause a warning
-	 * on debug kernels.
+	 * Map the sgtables otherwise drm_gem_shmem_free will cause a warning on
+	 * debug kernels.
 	 */
-	ret = dma_set_mask(drm_dev->dev, DMA_BIT_MASK(64));
-	KUNIT_ASSERT_EQ(test, ret, 0);
-
 	ret = dma_map_sgtable(drm_dev->dev, sgt, DMA_BIDIRECTIONAL, 0);
 	KUNIT_ASSERT_EQ(test, ret, 0);
 
@@ -352,11 +348,19 @@ static int drm_gem_shmem_test_init(struct kunit *test)
 {
 	struct device *dev;
 	struct drm_device *drm_dev;
+	int ret;
 
 	/* Allocate a parent device */
 	dev = drm_kunit_helper_alloc_device(test);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, dev);
 
+	/*
+	 * Set the DMA mask to 64-bits to avoid intermittent failures calling
+	 * drm_gem_shmem_get_pages_sgt().
+	 */
+	ret = dma_set_mask(dev, DMA_BIT_MASK(64));
+	KUNIT_ASSERT_EQ(test, ret, 0);
+
 	/*
 	 * The DRM core will automatically initialize the GEM core and create
 	 * a DRM Memory Manager object which provides an address space pool
-- 
2.54.0