Re: [PATCH 0/2] drm/tests: shmem: Fix intermittent DMA overflow on ppc64le/s390x

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 03.07.26 um 08:34 schrieb José Expósito:
> Two drm_gem_shmem KUnit tests, drm_gem_shmem_test_get_pages_sgt() and
> drm_gem_shmem_test_purge(), intermittently fail on ppc64le and s390x CI
> with a DMA address overflow warning followed by -EIO:
>
>    DMA addr 0x00000001130b0000+65536 overflow (mask ffffffff, bus limit 0).
>    Expected sgt is not error, but is: -5
>
> Both tests call `drm_gem_shmem_get_pages_sgt()`, which internally pins
> backing pages and DMA-maps them via `dma_map_sgtable()`. The DMA mapping
> path is:
>
>    drm_gem_shmem_test_purge()
>      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
>
> KUnit devices are initialized with a 32-bit DMA mask
> (`DMA_BIT_MASK(32)`) in `lib/kunit/device.c`. On systems where the kernel
> allocates backing pages at physical addresses above 4GB, `dma_capable()`
> returns false because the address exceeds the 32-bit mask. The `dma_set_mask()`
> function updates `*dev->dma_mask` to the given value; setting it to
> `DMA_BIT_MASK(64)` allows any physical address to pass the check.
>
> The failure is intermittent because pages may or may not be allocated
> above 4GB on any given run depend on memory pressure.
>
> A third test in the same suite, `drm_gem_shmem_test_obj_create_private`,
> already calls `dma_set_mask(drm_dev->dev, DMA_BIT_MASK(64))` before its
> DMA mapping. This series applies the same fix to the two remaining tests.

Instead of doing whack-a-mole, is it possible to move the existing fix 
from drm_gem_shmem_test_obj_create_private() to _test_init at [1] ? The 
DMA mask would then be the same on all tests. [1] 
https://elixir.bootlin.com/linux/v7.1.2/source/drivers/gpu/drm/tests/drm_gem_shmem_test.c#L359 
Best regards Thomas
>
> José Expósito (2):
>    drm/tests: shmem: Set DMA mask to 64-bit in
>      drm_gem_shmem_test_get_pages_sgt
>    drm/tests: shmem: Set DMA mask to 64-bit in drm_gem_shmem_test_purge
>
>   drivers/gpu/drm/tests/drm_gem_shmem_test.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)