linux-kernel - Re: [PATCH v2 4/8] dma-mapping: Separate DMA sync issuing and completion waiting

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Message-ID: <8acc1980-add8-4e0d-bba3-f7452fe53bce@suse.com>
Date: Mon, 5 Jan 2026 13:28:27 +0100
From: Jürgen Groß <jgross@...e.com>
To: Barry Song <21cnbao@...il.com>, 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>, Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 Tangquan Zheng <zhengtangquan@...o.com>
Subject: Re: [PATCH v2 4/8] dma-mapping: Separate DMA sync issuing and
 completion waiting

On 26.12.25 23:52, Barry Song wrote:
> From: Barry Song <baohua@...nel.org>
> 
> 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 <leon@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Robin Murphy <robin.murphy@....com>
> 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: 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>
> Signed-off-by: Barry Song <baohua@...nel.org>

Reviewed-by: Juergen Gross <jgross@...e.com> # drivers/xen/swiotlb-xen.c


Juergen

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