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