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

[José Expósito]

On Fri, Jul 03, 2026 at 05:07:43PM +0200, José Expósito wrote:
> From: José Expósito <jose.exposito@xxxxxxxxxx>
> 
> 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@xxxxxxx>
> Signed-off-by: José Expósito <jose.exposito@xxxxxxxxxx>


Applied to drm-misc-fixes, thanks a lot for your review Thomas.

Jose

> ---
> 
> 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@xxxxxxxxx/
> 
> v3:
> 
>   - Improve comments (Sashiko Bot)
>   - Link to v2: https://lore.kernel.org/dri-devel/20260703142749.4099-1-jose.exposito89@xxxxxxxxx/
> ---
>  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
>