On Sat, Dec 27, 2025 at 11:52:44AM +1300, Barry Song wrote:
> From: Barry Song <[email protected]>
>
> Currently, arch_sync_dma_for_cpu and arch_sync_dma_for_device
> always wait for the completion of each DMA buffer. That is,
> issuing the DMA sync and waiting for completion is done in a
> single API call.
>
> For scatter-gather lists with multiple entries, this means
> issuing and waiting is repeated for each entry, which can hurt
> performance. Architectures like ARM64 may be able to issue all
> DMA sync operations for all entries first and then wait for
> completion together.
>
> To address this, arch_sync_dma_for_* now issues DMA operations in
> batch, followed by a flush. On ARM64, the flush is implemented
> using a dsb instruction within arch_sync_dma_flush().
>
> For now, add arch_sync_dma_flush() after each
> arch_sync_dma_for_*() call. arch_sync_dma_flush() is defined as a
> no-op on all architectures except arm64, so this patch does not
> change existing behavior. Subsequent patches will introduce true
> batching for SG DMA buffers.
>
> Cc: Leon Romanovsky <[email protected]>
> Cc: Catalin Marinas <[email protected]>
> Cc: Will Deacon <[email protected]>
> Cc: Marek Szyprowski <[email protected]>
> Cc: Robin Murphy <[email protected]>
> Cc: Ada Couprie Diaz <[email protected]>
> Cc: Ard Biesheuvel <[email protected]>
> Cc: Marc Zyngier <[email protected]>
> Cc: Anshuman Khandual <[email protected]>
> Cc: Ryan Roberts <[email protected]>
> Cc: Suren Baghdasaryan <[email protected]>
> Cc: Joerg Roedel <[email protected]>
> Cc: Juergen Gross <[email protected]>
> Cc: Stefano Stabellini <[email protected]>
> Cc: Oleksandr Tyshchenko <[email protected]>
> Cc: Tangquan Zheng <[email protected]>
> Signed-off-by: Barry Song <[email protected]>
> ---
> arch/arm64/include/asm/cache.h | 6 ++++++
> arch/arm64/mm/dma-mapping.c | 4 ++--
> drivers/iommu/dma-iommu.c | 37 +++++++++++++++++++++++++---------
> drivers/xen/swiotlb-xen.c | 24 ++++++++++++++--------
> include/linux/dma-map-ops.h | 6 ++++++
> kernel/dma/direct.c | 8 ++++++--
> kernel/dma/direct.h | 9 +++++++--
> kernel/dma/swiotlb.c | 4 +++-
> 8 files changed, 73 insertions(+), 25 deletions(-)
<...>
> +#ifndef arch_sync_dma_flush
> +static inline void arch_sync_dma_flush(void)
> +{
> +}
> +#endif
Over the weekend I realized a useful advantage of the ARCH_HAVE_* config
options: they make it straightforward to inspect the entire DMA path simply
by looking at the .config.
Thanks,
Reviewed-by: Leon Romanovsky <[email protected]>
