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

[Rob Clark]

On Fri, Sep 25, 2026 at 7:20 PM Jianfeng Liu <liujianfeng1994@xxxxxxxxx> wrote:
>
> 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.

I sent a series[1] to remove the remaining (direct) use of pages for
imported dma-bufs, and remove use of drm_prime_sg_to_page_array() (so
one less caller of the deprecated function).

This on its own won't solve the problems with CONFIG_DMABUF_DEBUG.
There is still the indirect dependency on pages when mapping sgt's.  I
won't have time to work on that until after XDC, so it won't be a v7.3
thing, and _probably_ won't be a v7.4 thing at this point.  But
hopefully I can come up with something for v7.5.  Until then, please
apply this revert.

BR,
-R

[1] https://patchwork.freedesktop.org/series/175045/

> Link: https://lore.kernel.org/linux-arm-msm/20260923074256.9357-1-liujianfeng1994@xxxxxxxxx/
> Suggested-by: Rob Clark <robin.clark@xxxxxxxxxxxxxxxx>
> Cc: Christian König <christian.koenig@xxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> Cc: Karl Mehltretter <kmehltretter@xxxxxxxxx>
>
> Signed-off-by: Jianfeng Liu <liujianfeng1994@xxxxxxxxx>
> ---
>
>  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
>