[RFC PATCH 0/5] dma-mapping: arm64: support batched cache sync

[Barry Song]

On Wed, Oct 29, 2025 at 10:31 AM Barry Song <21cnbao at gmail.com> wrote:
>
> From: Barry Song <v-songbaohua at oppo.com>
>
> 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 initial 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.
>
> Barry Song (5):
>   arm64: Provide dcache_by_myline_op_nosync helper
>   arm64: Provide dcache_clean_poc_nosync helper
>   arm64: Provide dcache_inval_poc_nosync helper
>   arm64: Provide arch_sync_dma_ batched helpers
>   dma-mapping: Allow batched DMA sync operations if supported by the
>     arch
>

Hi Catalin, Will, Marek, Robin, and all,
Do you have any feedback on this before I send the formal
patchset (dropping the RFC tag)?

Thanks
Barry