[RFC 00/17] DRM: fix struct sg_table nents vs. orig_nents misuse

[Marek Szyprowski <m.szyprowski@samsung.com>]

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 in the
DRM subsystem and this is a result of my research. It looks that the
incorrect pattern has been copied over the many drivers. 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.

I wonder what to do to avoid such misuse in the future, as this case
clearly shows that the current sg_table structure is a bit hard to
understand. I have the following ideas and I would like to know your
opinion before I prepare more patches and check sg_table usage all over
the kernel:

1. introduce a dma_{map,sync,unmap}_sgtable() wrappers, which will use
   a proper sg_table entries and call respective DMA-mapping functions
   and adapt current code to it

2. rename nents and orig_nents to nr_pages, nr_dmas to clearly state
   which one refers to which part of the scatterlist; I'm open for
   other names for those entries

I will appreciate your comments.

Patches are based on top of Linux next-20200428. I've reduced the
recipients list mainly to mailing lists, the next version I will try to
send to the maintainers of the respective drivers.

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


Patch summary:

Marek Szyprowski (17):
  drm: core: fix sg_table nents vs. orig_nents usage
  drm: amdgpu: fix sg_table nents vs. orig_nents usage
  drm: armada: fix sg_table nents vs. orig_nents usage
  drm: etnaviv: fix sg_table nents vs. orig_nents usage
  drm: exynos: fix sg_table nents vs. orig_nents usage
  drm: i915: fix sg_table nents vs. orig_nents usage
  drm: lima: fix sg_table nents vs. orig_nents usage
  drm: msm: fix sg_table nents vs. orig_nents usage
  drm: panfrost: fix sg_table nents vs. orig_nents usage
  drm: radeon: fix sg_table nents vs. orig_nents usage
  drm: rockchip: fix sg_table nents vs. orig_nents usage
  drm: tegra: fix sg_table nents vs. orig_nents usage
  drm: virtio: fix sg_table nents vs. orig_nents usage
  drm: vmwgfx: fix sg_table nents vs. orig_nents usage
  drm: xen: fix sg_table nents vs. orig_nents usage
  drm: host1x: fix sg_table nents vs. orig_nents usage
  dmabuf: fix sg_table nents vs. orig_nents usage

 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       | 13 ++++++------
 drivers/gpu/drm/i915/gem/i915_gem_internal.c     |  4 ++--
 drivers/gpu/drm/i915/gem/i915_gem_region.c       |  4 ++--
 drivers/gpu/drm/i915/gem/i915_gem_shmem.c        |  5 +++--
 drivers/gpu/drm/i915/gem/selftests/huge_pages.c  | 10 +++++-----
 drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c |  5 +++--
 drivers/gpu/drm/i915/gt/intel_ggtt.c             | 12 ++++++------
 drivers/gpu/drm/i915/i915_gem_gtt.c              | 12 +++++++-----
 drivers/gpu/drm/i915/i915_scatterlist.c          |  4 ++--
 drivers/gpu/drm/i915/selftests/scatterlist.c     |  8 ++++----
 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              | 10 +++++-----
 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 ++++++++--------
 34 files changed, 154 insertions(+), 128 deletions(-)

-- 
1.9.1

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