[PATCH v2 00/21] DRM: fix struct sg_table nents vs. orig_nents misuse

[Marek Szyprowski]

Dear All,

During the Exynos DRM GEM rework and fixing the issues in the 
drm_prime_sg_to_page_addr_arrays() function [1] I've noticed that most
drivers in DRM framework incorrectly use nents and orig_nents entries of
the struct sg_table.

In case of the most DMA-mapping implementations exchanging those two
entries or using nents for all loops on the scatterlist is harmless,
because they both have the same value. There exists however a DMA-mapping
implementations, for which such incorrect usage breaks things. The nents
returned by dma_map_sg() might be lower than the nents passed as its
parameter and this is perfectly fine. DMA framework or IOMMU is allowed
to join consecutive chunks while mapping if such operation is supported
by the underlying HW (bus, bridge, IOMMU, etc). Example of the case
where dma_map_sg() might return 1 'DMA' chunk for the 4 'physical' pages
is described here [2]

The DMA-mapping framework documentation [3] states that dma_map_sg()
returns the numer of the created entries in the DMA address space.
However the subsequent calls to dma_sync_sg_for_{device,cpu} and
dma_unmap_sg must be called with the original number of entries passed to
dma_map_sg. The common pattern in DRM drivers were to assign the
dma_map_sg() return value to sg_table->nents and use that value for
the subsequent calls to dma_sync_sg_* or dma_unmap_sg functions. Also
the code iterated over nents times to access the pages stored in the
processed scatterlist, while it should use orig_nents as the numer of
the page entries.

I've tried to identify all such incorrect usage of sg_table->nents and
this is a result of my research. It looks that the incorrect pattern has
been copied over the many drivers mainly in the DRM subsystem. Too bad in
most cases it even worked correctly if the system used simple,
linear DMA-mapping implementation, for which swapping nents and
orig_nents doesn't make any difference.

The biggest TODO is DRM/i915 driver and I don't feel brave enough to fix
it fully. The driver creatively uses sg_table->orig_nents to store the
size of the allocate scatterlist and ignores the number of the entries
returned by dma_map_sg function. In this patchset I only fixed the
sg_table objects exported by dmabuf related functions. I hope that I
didn't break anything there.

As a follow-up for this patchset I will prepare a common
dma_{map,sync,unmap}_sgtable() wrappers, which will use a proper sg_table
entries and call respective DMA-mapping functions. I hope this will help
to avoid issue like this in the future.

Patches are based on top of Linux next-20200504.

Best regards,
Marek Szyprowski


References:

[1] https://lkml.org/lkml/2020/3/27/555 
[2] https://lkml.org/lkml/2020/3/29/65
[3] Documentation/DMA-API-HOWTO.txt


Changelog:

v2:
- dropped most of the changes to drm/i915
- added fixes for rcar-du, xen, media and ion
- fixed a few issues pointed by kbuild test robot
- added wide cc: list for each patch

v1: https://lore.kernel.org/linux-iommu/c01c9766-9778-fd1f-f36e-2dc7bd376ba4@xxxxxxx/T/#m879a727e4e46b5479ef8603994b1a006fb2aa337
- initial version


Patch summary:

Marek Szyprowski (21):
  drm: core: fix sg_table nents vs. orig_nents misuse
  drm: amdgpu: fix sg_table nents vs. orig_nents misuse
  drm: armada: fix sg_table nents vs. orig_nents misuse
  drm: etnaviv: fix sg_table nents vs. orig_nents misuse
  drm: exynos: fix sg_table nents vs. orig_nents misuse
  drm: i915: fix sg_table nents vs. orig_nents misuse for dmabuf objects
  drm: lima: fix sg_table nents vs. orig_nents misuse
  drm: msm: fix sg_table nents vs. orig_nents misuse
  drm: panfrost: fix sg_table nents vs. orig_nents misuse
  drm: radeon: fix sg_table nents vs. orig_nents misuse
  drm: rockchip: fix sg_table nents vs. orig_nents misuse
  drm: tegra: fix sg_table nents vs. orig_nents misuse
  drm: virtio: fix sg_table nents vs. orig_nents misuse
  drm: vmwgfx: fix sg_table nents vs. orig_nents misuse
  drm: xen: fix sg_table nents vs. orig_nents misuse
  drm: host1x: fix sg_table nents vs. orig_nents misuse
  drm: rcar-du: fix sg_table nents vs. orig_nents misuse
  xen: gntdev: fix sg_table nents vs. orig_nents misuse
  dmabuf: fix sg_table nents vs. orig_nents misuse
  media: pci: fix common ALSA DMA-mapping related code
  staging: ion: fix sg_table nents vs. orig_nents misuse

 drivers/dma-buf/heaps/heap-helpers.c             |  7 ++++---
 drivers/dma-buf/udmabuf.c                        |  5 +++--
 drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c      |  7 ++++---
 drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c          |  8 ++++----
 drivers/gpu/drm/armada/armada_gem.c              | 14 ++++++++-----
 drivers/gpu/drm/drm_cache.c                      |  2 +-
 drivers/gpu/drm/drm_gem_shmem_helper.c           |  7 ++++---
 drivers/gpu/drm/drm_prime.c                      |  9 +++++----
 drivers/gpu/drm/etnaviv/etnaviv_gem.c            | 10 ++++++----
 drivers/gpu/drm/exynos/exynos_drm_g2d.c          |  7 ++++---
 drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c       |  9 +++++----
 drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c |  5 +++--
 drivers/gpu/drm/lima/lima_gem.c                  |  4 ++--
 drivers/gpu/drm/msm/msm_gem.c                    |  8 ++++----
 drivers/gpu/drm/msm/msm_iommu.c                  |  3 ++-
 drivers/gpu/drm/panfrost/panfrost_gem.c          |  3 ++-
 drivers/gpu/drm/panfrost/panfrost_mmu.c          |  4 +++-
 drivers/gpu/drm/radeon/radeon_ttm.c              | 11 ++++++-----
 drivers/gpu/drm/rockchip/rockchip_drm_gem.c      | 15 +++++++-------
 drivers/gpu/drm/tegra/gem.c                      | 25 ++++++++++++------------
 drivers/gpu/drm/tegra/plane.c                    | 13 ++++++------
 drivers/gpu/drm/virtio/virtgpu_object.c          | 11 ++++++-----
 drivers/gpu/drm/virtio/virtgpu_vq.c              |  8 ++++----
 drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c       |  6 +++---
 drivers/gpu/drm/xen/xen_drm_front_gem.c          |  2 +-
 drivers/gpu/host1x/job.c                         | 17 ++++++++--------
 drivers/media/pci/cx23885/cx23885-alsa.c         |  2 +-
 drivers/media/pci/cx25821/cx25821-alsa.c         |  2 +-
 drivers/media/pci/cx88/cx88-alsa.c               |  2 +-
 drivers/media/pci/saa7134/saa7134-alsa.c         |  2 +-
 drivers/media/platform/vsp1/vsp1_drm.c           |  7 ++++---
 drivers/staging/android/ion/ion.c                | 17 ++++++++--------
 drivers/staging/android/ion/ion_heap.c           |  6 +++---
 drivers/staging/android/ion/ion_system_heap.c    |  2 +-
 drivers/xen/gntdev-dmabuf.c                      | 10 ++++++----
 35 files changed, 149 insertions(+), 121 deletions(-)

-- 
1.9.1