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

[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.

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)
> 
>