On Wed, Jan 7, 2026 at 8:12 AM Robin Murphy [off-list ref] wrote:

On 2026-01-06 6:41 pm, Barry Song wrote:

quoted

On Mon, Dec 29, 2025 at 3:50 AM Leon Romanovsky [off-list ref] wrote:

quoted

On Sun, Dec 28, 2025 at 09:52:05AM +1300, Barry Song wrote:

quoted

On Sun, Dec 28, 2025 at 9:09 AM Leon Romanovsky [off-list ref] wrote:

quoted

On Sat, Dec 27, 2025 at 11:52:45AM +1300, Barry Song wrote:

quoted

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 <redacted>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Anshuman Khandual <redacted>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Tangquan Zheng <redacted>
Signed-off-by: Barry Song <baohua@kernel.org>
---
  kernel/dma/direct.c | 14 +++++++-------
  1 file changed, 7 insertions(+), 7 deletions(-)

<...>

quoted

-             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.

Thanks
Barry