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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi José
Am 03.07.26 um 11:00 schrieb José Expósito:
> Hi Thomas,
>
> Thanks for your review.
>
> On Fri, Jul 03, 2026 at 08:53:30AM +0200, Thomas Zimmermann wrote:
>> 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
> Yes, I think it is possible. However, this is an intermittent failure that
> I couldn't reproduce locally and I tried to reduce as much as possible the
> risk of regression limitting the change to the 2 affected tests.
>
> I'm afraid I don't have enough knoledge about GEM's internals to know if moving
> the fix to the init function could introduce unwanted side-effects in the other
> tests. If you think it'd be safe, I'll trust you and move it.

Please do. I don't see why it should not work, and it's an easy revert 
it doesn't.

Best regards
Thomas

>
> Jose
>
> PS - Next week I'll be away from keyboard, so any change would need to wait a bit
>
>> 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)
>>
>>

-- 
--
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)