On 9/23/26 09:42, Jianfeng Liu wrote:

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Commit 143755bdabaa9 ("dma-buf: Make DMABUF_DEBUG default to y on DEBUG_KERNEL kernels") fixed a dangling reference in the DMABUF_DEBUG default, which had the side effect of enabling the option (and with it the page-stripping sg_table wrapper handed to importers) on every kernel with DEBUG_KERNEL=y - i.e. virtually every distro kernel.

drm/msm is broken by this: it maps imported dma-bufs into the GPU's own pagetables with iommu_map_sgtable(), which needs the struct page of the attachment sg_table, and it fills the GEM object's page array through drm_prime_sg_to_page_array(). With the debug wrapper in place both silently produce garbage (the wrapper zeroes sg->length,

Interesting point, we should probably change that.

so the page iterator yields nothing and an uninitialized array is kept). The VM_BIND map job then fails asynchronously after userspace has already enqueued GPU work referencing the mapping, which shows up as an arm-smmu translation fault from UCHE, e.g.:

gpu fault: ttbr0=000000088a889000 iova=000000010741c000 dir=READ type=TRANSLATION source=UCHE

This breaks hardware video decode (clapper, chromium) on Adreno systems; bisected on a Snapdragon laptop as v7.3-rc3 good, v7.3-rc4 bad, culprit 143755bdabaa9.

Revert the default until importers that legitimately need to build phys-based mappings have been converted.

Well that won't work like this, pointing those things out is exactly what DMABUF_DEBUG is made for.

What MSM is doing here is not allowed at all and can break badly. We gave drivers 5 years to get that fixed and I'm now pushing for completely deprecating that hack.

See patch 84335675f2223cbd25d0de7d38ecc7d40b95bd4a:

Author: Simona Vetter simona.vetter@ffwll.ch Date: Fri Jan 15 17:47:39 2021 +0100

dma-buf: Add debug option

When MSM needs the struct page then it must import a shmemfd and not a DMA-buf. What we could do is to either fix MSM or mark it as broken.

Regards, Christian.

Fixes: 143755bdabaa9 ("dma-buf: Make DMABUF_DEBUG default to y on DEBUG_KERNEL kernels") Signed-off-by: Jianfeng Liu liujianfeng1994@gmail.com

---

drivers/dma-buf/Kconfig | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig index e4f078a326a41..b3c581ef4c987 100644 --- a/drivers/dma-buf/Kconfig +++ b/drivers/dma-buf/Kconfig @@ -43,7 +43,14 @@ config UDMABUF config DMABUF_DEBUG bool "DMA-BUF debug checks" depends on DMA_SHARED_BUFFER

•   default y if DEBUG_KERNEL
  

•   # NOTE: keep this default n.  The page-stripping sg_table wrapper that
  

•   # this option installs for importers breaks drivers that build a
  

•   # second-stage IOMMU mapping (phys -> iova) from the attachment sg_table
  

•   # and therefore still need the struct page, e.g. drm/msm with its
  

•   # per-process GPU pagetables.  Until those importers are fixed, making
  

•   # this default y breaks hardware video decode and GPU workloads out of
  

•   # the box on affected systems.
  

•   default n
    help
      This option enables additional checks for DMA-BUF importers and
      exporters. Specifically it validates that importers do not peek at the
  

-- 2.47.3