Re: [PATCH v2 5/8] dma-mapping: Support batch mode for dma_direct_sync_sg_for_*

[Leon Romanovsky <leon@kernel.org>]

On Wed, Jan 07, 2026 at 08:47:36AM +1300, Barry Song wrote:
> On Wed, Jan 7, 2026 at 8:12 AM Robin Murphy <robin.murphy@arm.com> wrote:
> >
> > On 2026-01-06 6:41 pm, Barry Song wrote:
> > > On Mon, Dec 29, 2025 at 3:50 AM Leon Romanovsky <leon@kernel.org> wrote:
> > >>
> > >> On Sun, Dec 28, 2025 at 09:52:05AM +1300, Barry Song wrote:
> > >>> On Sun, Dec 28, 2025 at 9:09 AM Leon Romanovsky <leon@kernel.org> wrote:
> > >>>>
> > >>>> On Sat, Dec 27, 2025 at 11:52:45AM +1300, Barry Song wrote:
> > >>>>> From: Barry Song <baohua@kernel.org>
> > >>>>>
> > >>>>> Instead of performing a flush per SG entry, issue all cache
> > >>>>> operations first and then flush once. This ultimately benefits
> > >>>>> __dma_sync_sg_for_cpu() and __dma_sync_sg_for_device().
> > >>>>>
> > >>>>> 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: Tangquan Zheng <zhengtangquan@oppo.com>
> > >>>>> Signed-off-by: Barry Song <baohua@kernel.org>
> > >>>>> ---
> > >>>>>   kernel/dma/direct.c | 14 +++++++-------
> > >>>>>   1 file changed, 7 insertions(+), 7 deletions(-)
> > >>>>
> > >>>> <...>
> > >>>>
> > >>>>> -             if (!dev_is_dma_coherent(dev)) {
> > >>>>> +             if (!dev_is_dma_coherent(dev))
> > >>>>>                        arch_sync_dma_for_device(paddr, sg->length,
> > >>>>>                                        dir);
> > >>>>> -                     arch_sync_dma_flush();
> > >>>>> -             }
> > >>>>>        }
> > >>>>> +     if (!dev_is_dma_coherent(dev))
> > >>>>> +             arch_sync_dma_flush();
> > >>>>
> > >>>> This patch should be squashed into the previous one. You introduced
> > >>>> arch_sync_dma_flush() there, and now you are placing it elsewhere.
> > >>>
> > >>> Hi Leon,
> > >>>
> > >>> The previous patch replaces all arch_sync_dma_for_* calls with
> > >>> arch_sync_dma_for_* plus arch_sync_dma_flush(), without any
> > >>> functional change. The subsequent patches then implement the
> > >>> actual batching. I feel this is a better approach for reviewing
> > >>> each change independently. Otherwise, the previous patch would
> > >>> be too large.
> > >>
> > >> Don't worry about it. Your patches are small enough.
> > >
> > > My hardware does not require a bounce buffer, but I am concerned that
> > > this patch may be incorrect for systems that do require one.
> > >
> > > Now it is:
> > >
> > > void dma_direct_sync_sg_for_cpu(struct device *dev,
> > >                  struct scatterlist *sgl, int nents, enum dma_data_direction dir)
> > > {
> > >          struct scatterlist *sg;
> > >          int i;
> > >
> > >          for_each_sg(sgl, sg, nents, i) {
> > >                  phys_addr_t paddr = dma_to_phys(dev, sg_dma_address(sg));
> > >
> > >                  if (!dev_is_dma_coherent(dev))
> > >                          arch_sync_dma_for_cpu(paddr, sg->length, dir);
> > >
> > >                  swiotlb_sync_single_for_cpu(dev, paddr, sg->length, dir);
> > >
> > >                  if (dir == DMA_FROM_DEVICE)
> > >                          arch_dma_mark_clean(paddr, sg->length);
> > >          }
> > >
> > >          if (!dev_is_dma_coherent(dev)) {
> > >                  arch_sync_dma_flush();
> > >                  arch_sync_dma_for_cpu_all();
> > >          }
> > > }
> > >
> > > Should we call swiotlb_sync_single_for_cpu() and
> > > arch_dma_mark_clean() after the flush to ensure the CPU sees the
> > > latest data and that the memcpy is correct? I mean:
> >
> > Yes, this and the equivalents in the later patches are broken for all
> > the sync_for_cpu and unmap paths which may end up bouncing (beware some
> > of them get a bit fiddly) - any cache maintenance *must* be completed
> > before calling SWIOTLB. As for mark_clean, IIRC that was an IA-64 thing,
> > and appears to be entirely dead now.
> 
> Thanks, Robin. Personally, I would prefer an approach like the one below—
> that is, not optimizing the bounce buffer cases, as they are already slow
> due to hardware limitations with memcpy, and optimizing them would make
> the code quite messy.
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 550a1a13148d..a4840f7e8722 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -423,8 +423,11 @@ void dma_direct_sync_sg_for_cpu(struct device *dev,
>         for_each_sg(sgl, sg, nents, i) {
>                 phys_addr_t paddr = dma_to_phys(dev, sg_dma_address(sg));
> 
> -               if (!dev_is_dma_coherent(dev))
> +               if (!dev_is_dma_coherent(dev)) {
>                         arch_sync_dma_for_cpu(paddr, sg->length, dir);
> +                       if (unlikely(dev->dma_io_tlb_mem))
> +                               arch_sync_dma_flush();
> +               }
> 
>                 swiotlb_sync_single_for_cpu(dev, paddr, sg->length, dir);
> 
> I’d like to check with you, Leon, and Marek on your views about this.

I agree with your point that the non‑SWIOTLB path is the performant one and
should be preferred. My concern is that you are accessing the
dma_io_tlb_mem variable directly from direct.c, which looks like a layer
violation.

You likely need to introduce an is_swiotlb_something() helper for this.

BTW, please send a v3 instead of posting incremental follow‑ups.
It's hard to track the changes across multiple small additions.

Thanks.
> 
> Thanks
> Barry