[PATCH] Revert "dma-buf: Make DMABUF_DEBUG default to y on DEBUG_KERNEL kernels"

[PATCH] Revert "dma-buf: Make DMABUF_DEBUG default to y on DEBUG_KERNEL kernels"

[Jianfeng Liu]

This reverts commit 143755bdabaa96776c24f878014608e9cb44f930.

That commit fixed a dangling reference in the DMABUF_DEBUG default
and thereby enabled the option - and with it the page-stripping
sg_table wrapper that dma_buf_map_attachment() hands to importers -
on every kernel with DEBUG_KERNEL=y, i.e. virtually every distro
kernel.

drm/msm is broken by the wrapper.  Both of msm's map paths consume
sg->length and sg_phys() of the attachment sg_table:
msm_iommu_pagetable_map() for the per-process GPU pagetables, and
iommu_map_sg() (via iommu_map_sgtable()) for scanout.  The wrapper
zeroes sg->length and strips the page pointers, so mappings of
imported dma-bufs silently map nothing, and userspace observes
arm-smmu translation faults from UCHE, e.g. during hardware video
decode (clapper, chromium) on Adreno systems:

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

Bisected on a Snapdragon X1E78100 laptop as v7.3-rc3 good,
v7.3-rc4 bad, culprit 143755bdabaa9.

Switching msm to sg_dma_address()/sg_dma_len() is not a trivial fix
either: those fields are only valid for sg_tables that msm has
dma-mapped itself, which native non-MSM_BO_WC objects' sg_tables
are not, so the conversion needs more work.  The msm maintainer has
therefore requested restoring the previous default for v7.3, to be
revisited once msm no longer consumes struct page and sg->length of
imported sg_tables.

Link: https://lore.kernel.org/linux-arm-msm/20260923074256.9357-1-liujianfeng1994@gmail.com/
Suggested-by: Rob Clark <robin.clark@oss.qualcomm.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Karl Mehltretter <kmehltretter@gmail.com>

Signed-off-by: Jianfeng Liu <liujianfeng1994@gmail.com>
---

 drivers/dma-buf/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
index e4f078a326a41..7efc0f0d07126 100644
--- a/drivers/dma-buf/Kconfig
+++ b/drivers/dma-buf/Kconfig
@@ -43,7 +43,7 @@ config UDMABUF
 config DMABUF_DEBUG
 	bool "DMA-BUF debug checks"
 	depends on DMA_SHARED_BUFFER
-	default y if DEBUG_KERNEL
+	default y if DEBUG
 	help
 	  This option enables additional checks for DMA-BUF importers and
 	  exporters. Specifically it validates that importers do not peek at the
---
base-commit: 93f51579e7df248780214094418f205253383cc5
branch: revert-dmabuf-debug-for-7.3

-- 
2.47.3