[RFC PATCH v1 1/2] dma-buf: keep DMABUF_DEBUG off by default

[Jianfeng Liu <liujianfeng1994@gmail.com>]

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 <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