[PATCH/RFC 0/7] dma-mapping: DMA_ATTR_NO_KERNEL_MAPPING related fixes

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

Hi All,

arch_dma_prep_coherent() is called by the generic DMA code on pages
which may be located in highmem: dma_direct_alloc() forces the remap
path for a highmem page returned from CMA and calls this hook on it
before creating the uncached mapping. However, the hook has never been
documented and some implementations assume that the page has a kernel
linear mapping. This issue affects MIPS, xtensa and PowerPC. It gets
fixed by patches #2-#4.

xtensa also only invalidates the cache, which discards the zeroes
written by the dma_direct_alloc(). Patch #1 switches it to a
writeback+invalidate.

Patch #5 documents that arch_dma_prep_coherent() must write back dirty
lines and must accept highmem pages. With that in place, patch #6 drops
the PageHighMem() check in dma_direct_alloc_no_mapping(), which so far
skipped cache maintenance for highmem CMA pages allocated with
DMA_ATTR_NO_KERNEL_MAPPING.

Patch #7 makes dma_direct_alloc_no_mapping() always return zeroed
memory, like all other dma_alloc_* paths. Passing __GFP_ZERO is not
enough there, because CMA ignores.

The analysis and the patchset was prepared with the help of an LLM. The
code compiles fine, but it has not been runtime tested on any of the
affected architectures yet. Testing on real hardware would be
appreciated. I've reviewed the resulting changes.

Best regards
Marek Szyprowski, PhD
Samsung R&D Institute Poland


Patch summary:

Marek Szyprowski (7):
  xtensa: write back dirty lines in arch_dma_prep_coherent()
  xtensa: handle highmem pages in arch_dma_prep_coherent()
  MIPS: mm: handle highmem pages in arch_dma_prep_coherent()
  powerpc/mm: handle highmem pages in arch_dma_prep_coherent()
  dma-mapping: document that arch_dma_prep_coherent() must handle
    highmem
  dma-direct: do not skip cache maintenance for highmem pages in
    no-mapping allocations
  dma-direct: zero memory allocated with DMA_ATTR_NO_KERNEL_MAPPING

 arch/mips/mm/dma-noncoherent.c    | 10 +++++-----
 arch/powerpc/mm/dma-noncoherent.c |  4 +---
 arch/xtensa/kernel/pci-dma.c      | 12 +++++++++++-
 include/linux/dma-map-ops.h       | 10 ++++++++++
 kernel/dma/direct.c               |  9 +++++++--
 5 files changed, 34 insertions(+), 11 deletions(-)

-- 
2.43.0