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,
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.
Fixes: 143755bdabaa9 ("dma-buf: Make DMABUF_DEBUG default to y on DEBUG_KERNEL
kernels")
Signed-off-by: Jianfeng Liu <[email protected]>
---
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
