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

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

On Fri, Jul 03, 2026 at 12:32:18PM +0200, Thomas Zimmermann wrote:
> 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.

I just sent v2 with the suggested change. I run the test in several
architectures and I didn't run into any issue:

https://lore.kernel.org/dri-devel/20260703142749.4099-1-jose.exposito89@gmail.com/

Thanks,
Jose

> 
> 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)
> 
>