[PATCH 0/2] ARM: preserve DMA_FROM_DEVICE buffer contents

[Karl Mehltretter <kmehltretter@gmail.com>]

This series prevents a DMA_FROM_DEVICE map from discarding CPU-written
buffer contents on non-coherent 32-bit ARM. It is based on
v7.3-rc1-324-g986c24e0fe44.

ARM currently invalidates these buffers before the device writes them.
If the device writes only part of a buffer, discarded dirty cache lines
can expose older memory contents in the untouched bytes. A stock USB
webcam demonstrated this through usbfs. Short isochronous packets left
gaps, and usbfs returned non-zero data to userspace from bytes it had
cleared.

arm64 changed this handoff from invalidate to clean in 2022 with commit
c50f11c6196f ("arm64: mm: Don't invalidate FROM_DEVICE buffers at start
of DMA transfer"). Arnd Bergmann's 2023 ARM32 cache-maintenance series
left DMA_FROM_DEVICE preservation unresolved [1]. This series keeps the
existing ownership hooks.

Patch 1 covers v6, v7, v7-M and the common outer-cache path. These paths
already invalidate when ownership returns to the CPU. Patch 2 makes the
legacy write-back backends use their existing clean-and-invalidate
operation at device handoff. Their no-op ownership-return hooks remain
unchanged, avoiding the additional completion traversal raised during
review [2].

[1] https://lore.kernel.org/r/20230327121317.4081816-1-arnd@kernel.org
[2] https://lists.infradead.org/pipermail/linux-riscv/2023-March/029740.html

Testing:

  - An ARM11 MPCore (ARMv6) in a New Nintendo 2DS XL lost 402,080 of
    409,600 CPU-written bytes in a mapping-only test. With patch 1, none
    were lost. The UP kernel used out-of-tree platform support and the
    same map-time invalidate as mainline.

  - On an ARM926EJ-S (ARMv5TEJ) SAM9X75, eleven mapping-only runs lost
    339,936 to 351,744 of 409,600 bytes with the existing code. Five
    usbfs runs checked 245,708 unwritten gap bytes and exposed 29,403 of
    them as non-zero. With this series, five fresh 100-iteration
    mapping-only runs on the same board each lost 0 of 409,600 bytes.

  - A Cortex-A72 running AArch32/LPAE did not reproduce the loss in 22
    existing-kernel runs. This does not establish whether other v7 cores
    are affected.

The one-pass series passed focused W=1 object builds for ARM926,
Feroceon, XScale, ARM940 and v7. The remaining legacy backends have
source review and compile coverage from the previous two-pass revision.

Neither patch adds a cache traversal. Patch 2 changes the legacy
DMA_FROM_DEVICE operation from invalidate to clean-and-invalidate. This
can add per-line work and memory writes on dirty buffers. Throughput and
CPU cost have not been measured.

Karl Mehltretter (2):
  ARM: dma-mapping: preserve DMA_FROM_DEVICE buffer contents
  ARM: dma-mapping: flush FROM_DEVICE buffers on legacy backends

 arch/arm/mm/cache-fa.S          | 1 -
 arch/arm/mm/cache-v4wb.S        | 1 -
 arch/arm/mm/cache-v6.S          | 2 --
 arch/arm/mm/cache-v7.S          | 2 --
 arch/arm/mm/cache-v7m.S         | 2 --
 arch/arm/mm/dma-mapping-nommu.c | 5 +----
 arch/arm/mm/dma-mapping.c       | 8 +-------
 arch/arm/mm/proc-arm1020.S      | 1 -
 arch/arm/mm/proc-arm1020e.S     | 1 -
 arch/arm/mm/proc-arm1022.S      | 1 -
 arch/arm/mm/proc-arm1026.S      | 1 -
 arch/arm/mm/proc-arm920.S       | 1 -
 arch/arm/mm/proc-arm922.S       | 1 -
 arch/arm/mm/proc-arm925.S       | 1 -
 arch/arm/mm/proc-arm926.S       | 1 -
 arch/arm/mm/proc-arm940.S       | 1 -
 arch/arm/mm/proc-arm946.S       | 1 -
 arch/arm/mm/proc-feroceon.S     | 2 --
 arch/arm/mm/proc-mohawk.S       | 1 -
 arch/arm/mm/proc-xsc3.S         | 1 -
 arch/arm/mm/proc-xscale.S       | 1 -
 21 files changed, 2 insertions(+), 34 deletions(-)

-- 
2.53.0