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

[Leon Romanovsky <leon@kernel.org>]

On Mon, Dec 29, 2025 at 10:38:26AM +1300, Barry Song wrote:
> On Mon, Dec 29, 2025 at 3:49 AM Leon Romanovsky <leon@kernel.org> wrote:
> >
> > On Sun, Dec 28, 2025 at 10:45:13AM +1300, Barry Song wrote:
> > > On Sun, Dec 28, 2025 at 9:07 AM Leon Romanovsky <leon@kernel.org> wrote:
> > > >
> > > > On Sat, Dec 27, 2025 at 11:52:44AM +1300, Barry Song wrote:
> > > > > From: Barry Song <baohua@kernel.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@kernel.org>
> > > > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > > > Cc: Will Deacon <will@kernel.org>
> > > > > Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> > > > > Cc: Robin Murphy <robin.murphy@arm.com>
> > > > > Cc: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> > > > > Cc: Ard Biesheuvel <ardb@kernel.org>
> > > > > Cc: Marc Zyngier <maz@kernel.org>
> > > > > Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> > > > > Cc: Ryan Roberts <ryan.roberts@arm.com>
> > > > > Cc: Suren Baghdasaryan <surenb@google.com>
> > > > > Cc: Joerg Roedel <joro@8bytes.org>
> > > > > Cc: Juergen Gross <jgross@suse.com>
> > > > > Cc: Stefano Stabellini <sstabellini@kernel.org>
> > > > > Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > > > Cc: Tangquan Zheng <zhengtangquan@oppo.com>
> > > > > Signed-off-by: Barry Song <baohua@kernel.org>
> > > > > ---
> > > > >  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.
> > >
> > > I am not quite sure how much this benefits users, as the same
> > > information could also be obtained by grepping for
> > > #define arch_sync_dma_flush in the source code.
> >
> > It differs slightly. Users no longer need to grep around or guess whether this
> > platform used the arch_sync_dma_flush path. A simple grep for ARCH_HAVE_ in
> > /proc/config.gz provides the answer.
> 
> In any case, it is only two or three lines of code, so I am fine with
> either approach. Perhaps Marek, Robin, and others have a point here?
> 
> >
> > >
> > > >
> > > > Thanks,
> > > > Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
> > >
> > > Thanks very much, Leon, for reviewing this over the weekend. One thing
> > > you might have missed is that I place arch_sync_dma_flush() after all
> > > arch_sync_dma_for_*() calls, for both single and sg cases. I also
> > > used a Python script to scan the code and verify that every
> > > arch_sync_dma_for_*() is followed by arch_sync_dma_flush(), to ensure
> > > that no call is left out.
> > >
> > > In the subsequent patches, for sg cases, the per-entry flush is
> > > replaced by a single flush of the entire sg. Each sg case has
> > > different characteristics: some are straightforward, while others
> > > can be tricky and involve additional contexts.
> >
> > I didn't overlook it, and I understand your rationale. However, this is
> > not how kernel patches should be structured. You should not introduce
> > code in patch X and then move it elsewhere in patch X + Y.
> 
> I am not quite convinced by this concern. This patch only
> separates DMA sync issuing from completion waiting, and it
> reflects that the development is done step by step.
> 
> >
> > Place the code in the correct location from the start. Your patches are
> > small enough to review as is.
> 
> My point is that this patch places the code in the correct locations
> from the start. It splits arch_sync_dma_for_*() into
> arch_sync_dma_for_*() plus arch_sync_dma_flush() everywhere, without
> introducing any functional changes from the outset.
> The subsequent patches clearly show which parts are truly batched.
> 
> In the meantime, I do not have a strong preference here. If you think
> it is better to move some of the straightforward batching code here,
> I can follow that approach. Perhaps I could move patch 5, patch 8,
> and the iommu_dma_iova_unlink_range_slow change from patch 7 here,
> while keeping
> 
>   [PATCH 6] dma-mapping: Support batch mode for
>   dma_direct_{map,unmap}_sg
> 
> and the IOVA link part from patch 7 as separate patches, since that
> part is not straightforward. The IOVA link changes affect both
> __dma_iova_link() and dma_iova_sync(), which are two separate
> functions and require a deeper understanding of the contexts to
> determine correctness. That part also lacks testing.

Don't worry about testing. NVME, RDMA and GPU are using this path
and someone will test it.

> 
> Would that be okay with you?

I don't know, need to see the code.

Thanks

> 
> Thanks
> Barry