[PATCH v6 0/2] dma: support DMA zone starting above 4GB

From: Baruch Siach
Date: Sun Aug 11 2024 - 02:33:03 EST


From: Baruch Siach <baruchs-c@xxxxxxxxxxxxx>

DMA zones code assumes that DMA lower limit is zero. When there is no RAM
below 4GB, arm64 platform code sets DMA/DMA32 zone limits to cover the entire
RAM[0].

My target platform has RAM starting at 32GB. Devices with 30-bit DMA mask are
mapped to 1GB at the bottom of RAM, between 32GB - 33GB. DMA zone over the
entire RAM breaks DMA allocation for these devices.

In response to a previous RFC hack[1] Catalin Marinas suggested to add a
separate offset value as base address for the DMA zone, and then refined the
suggestion to use start of RAM[3]. This series attempts to implement that
suggestion.

With this series applied, the DMA zone covers the right RAM range for my
platform.

v6:

* Drop the first patch; existing logic is just fine

* Modify powerpc code to avoid off by one issue

v5:

* Test the correct kernel

* Add missing patch that actually makes DMA zone work

* Extend the treatment of zone_dma_limit > DMA_BIT_MASK(32)

* Use max() to make the code somewhat more readable

* Change zone_dma_limit type to u64 to match DMA_BIT_MASK()

v4:

* Drop last patch. zone_dma_limit includes RAM base address.

* Adjust DMA zone selection in swiotlb as well.

* Don't change max_zone_phys() behaviour

* Update code to fallback to DMA zone when zone_dma_limit > DMA_BIT_MASK(32)

v3:

* Rebase on v6.11-rc1.

* Drop zone_dma_base. Use memblock_start_of_DRAM() instead.

* Drop DT patches. Low DMA range limit no longer needed.

* Add patch to improve dma_direct_optimal_gfp_mask() heuristics as Catalin
suggested.

RFC v2:

* Add patch from Catalin[2] changing zone_dma_bits to zone_dma_limit to
simplify subsequent patches

* Test on real hardware

RFC v1: https://lore.kernel.org/all/cover.1703683642.git.baruch@xxxxxxxxxx/

[0] See commit 791ab8b2e3db ("arm64: Ignore any DMA offsets in the
max_zone_phys() calculation")

[1] https://lore.kernel.org/all/9af8a19c3398e7dc09cfc1fbafed98d795d9f83e.1699464622.git.baruch@xxxxxxxxxx/

[2] https://lore.kernel.org/all/ZZ2HnHJV3gdzu1Aj@xxxxxxx/

[3] https://lore.kernel.org/all/ZnH-VU2iz9Q2KLbr@xxxxxxx/

Catalin Marinas (2):
dma: replace zone_dma_bits by zone_dma_limit
arm64: support DMA zone above 4GB

arch/arm64/mm/init.c | 32 ++++++++++----------------------
arch/powerpc/mm/mem.c | 5 ++++-
arch/s390/mm/init.c | 2 +-
include/linux/dma-direct.h | 2 +-
kernel/dma/direct.c | 6 +++---
kernel/dma/pool.c | 4 ++--
kernel/dma/swiotlb.c | 6 +++---
7 files changed, 24 insertions(+), 33 deletions(-)


base-commit: 8400291e289ee6b2bf9779ff1c83a291501f017b
--
2.43.0