linux-kernel - [PATCH v2 0/8] dma-mapping: arm64: support batched cache sync

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Message-ID: <20251226225254.46197-1-21cnbao@gmail.com>
Date: Sat, 27 Dec 2025 11:52:40 +1300
From: Barry Song <21cnbao@...il.com>
To: catalin.marinas@....com,
	m.szyprowski@...sung.com,
	robin.murphy@....com,
	will@...nel.org,
	iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org,
	xen-devel@...ts.xenproject.org,
	Barry Song <baohua@...nel.org>,
	Leon Romanovsky <leon@...nel.org>,
	Ada Couprie Diaz <ada.coupriediaz@....com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Marc Zyngier <maz@...nel.org>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Ryan Roberts <ryan.roberts@....com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Joerg Roedel <joro@...tes.org>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
	Tangquan Zheng <zhengtangquan@...o.com>,
	Huacai Zhou <zhouhuacai@...o.com>
Subject: [PATCH v2 0/8] dma-mapping: arm64: support batched cache sync

From: Barry Song <baohua@...nel.org>

Many embedded ARM64 SoCs still lack hardware cache coherency support, which
causes DMA mapping operations to appear as hotspots in on-CPU flame graphs.

For an SG list with *nents* entries, the current dma_map/unmap_sg() and DMA
sync APIs perform cache maintenance one entry at a time. After each entry,
the implementation synchronously waits for the corresponding region’s
D-cache operations to complete. On architectures like arm64, efficiency can
be improved by issuing all entries’ operations first and then performing a
single batched wait for completion.

Tangquan's results show that batched synchronization can reduce
dma_map_sg() time by 64.61% and dma_unmap_sg() time by 66.60% on an MTK
phone platform (MediaTek Dimensity 9500). The tests were performed by
pinning the task to CPU7 and fixing the CPU frequency at 2.6 GHz,
running dma_map_sg() and dma_unmap_sg() on 10 MB buffers (10 MB / 4 KB
sg entries per buffer) for 200 iterations and then averaging the
results.

I also ran this patch set on an RK3588 Rock5B+ board and
observed that millions of DMA sync operations were batched.

v2:
 * Refine a large amount of arm64 asm code based on feedback from
   Robin, thanks!
 * Drop batch_add APIs and always use arch_sync_dma_for_* + flush,
   even for a single buffer, based on Leon’s suggestion, thanks!
 * Refine a large amount of code based on feedback from Leon, thanks!
 * Also add batch support for iommu_dma_sync_sg_for_{cpu,device}
v1 link:
 https://lore.kernel.org/lkml/20251219053658.84978-1-21cnbao@gmail.com/

v1, diff with RFC:
 * Drop a large number of #ifdef/#else/#endif blocks based on feedback
   from Catalin and Marek, thanks!
 * Also add batched iova link/unlink support, marked as RFC since I lack
   the required hardware. This was suggested by Marek, thanks!
RFC link:
 https://lore.kernel.org/lkml/20251029023115.22809-1-21cnbao@gmail.com/

Barry Song (8):
  arm64: Provide dcache_by_myline_op_nosync helper
  arm64: Provide dcache_clean_poc_nosync helper
  arm64: Provide dcache_inval_poc_nosync helper
  dma-mapping: Separate DMA sync issuing and completion waiting
  dma-mapping: Support batch mode for dma_direct_sync_sg_for_*
  dma-mapping: Support batch mode for dma_direct_{map,unmap}_sg
  dma-iommu: Support DMA sync batch mode for IOVA link and unlink
  dma-iommu: Support DMA sync batch mode for iommu_dma_sync_sg_for_{cpu,
    device}

 arch/arm64/include/asm/assembler.h  | 24 +++++++++---
 arch/arm64/include/asm/cache.h      |  6 +++
 arch/arm64/include/asm/cacheflush.h |  2 +
 arch/arm64/kernel/relocate_kernel.S |  3 +-
 arch/arm64/mm/cache.S               | 57 +++++++++++++++++++++++------
 arch/arm64/mm/dma-mapping.c         |  4 +-
 drivers/iommu/dma-iommu.c           | 35 ++++++++++++++----
 drivers/xen/swiotlb-xen.c           | 24 ++++++++----
 include/linux/dma-map-ops.h         |  6 +++
 kernel/dma/direct.c                 | 23 +++++++++---
 kernel/dma/direct.h                 | 21 ++++++++---
 kernel/dma/mapping.c                |  6 +--
 kernel/dma/swiotlb.c                |  4 +-
 13 files changed, 165 insertions(+), 50 deletions(-)

Cc: Leon Romanovsky <leon@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will@...nel.org>
Cc: Ada Couprie Diaz <ada.coupriediaz@....com>
Cc: Ard Biesheuvel <ardb@...nel.org>
Cc: Marc Zyngier <maz@...nel.org>
Cc: Anshuman Khandual <anshuman.khandual@....com>
Cc: Ryan Roberts <ryan.roberts@....com>
Cc: Suren Baghdasaryan <surenb@...gle.com>
Cc: Robin Murphy <robin.murphy@....com>
Cc: Joerg Roedel <joro@...tes.org>
Cc: Juergen Gross <jgross@...e.com>
Cc: Stefano Stabellini <sstabellini@...nel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Cc: Tangquan Zheng <zhengtangquan@...o.com>
Cc: Huacai Zhou <zhouhuacai@...o.com>
--
2.43.0