Re: [PATCH 20/21] ARM: dma-mapping: split out arch_dma_mark_clean() helper

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Hi Arnd,

On Mon, Mar 27, 2023 at 2:16 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
From: Arnd Bergmann <arnd@xxxxxxxx>

The arm version of the arch_sync_dma_for_cpu() function annotates pages as
PG_dcache_clean after a DMA, but no other architecture does this here. On
ia64, the same thing is done in arch_sync_dma_for_cpu(), so it makes sense
to use the same hook in order to have identical arch_sync_dma_for_cpu()
semantics as all other architectures.

Splitting this out has multiple effects:

 - for dma-direct, this now gets called after arch_sync_dma_for_cpu()
   for DMA_FROM_DEVICE mappings, but not for DMA_BIDIRECTIONAL. While
   it would not be harmful to keep doing it for bidirectional mappings,
   those are apparently not used in any callers that care about the flag.

 - Since arm has its own dma-iommu abstraction, this now also needs to
   call the same function, so the calls are added there to mirror the
   dma-direct version.

 - Like dma-direct, the dma-iommu version now marks the dcache clean
   for both coherent and noncoherent devices after a DMA, but it only

[ HELP NEEDED: can anyone confirm that it is a correct assumption
  on arm that a cache-coherent device writing to a page always results
  in it being in a PG_dcache_clean state like on ia64, or can a device
  write directly into the dcache?]

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Thanks for your patch, which is now commit 322dbe898f82fd8a
("ARM: dma-mapping: split out arch_dma_mark_clean() helper") in

If CONFIG_ARM_DMA_USE_IOMMU=y, the build fails.

--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c

@@ -1294,6 +1298,17 @@ static int arm_iommu_map_sg(struct device *dev, struct scatterlist *sg,
        return -EINVAL;

+static void arm_iommu_sync_dma_for_cpu(phys_addr_t phys, size_t len,
+                                      enum dma_data_direction dir,
+                                      bool dma_coherent)
+       if (!dma_coherent)
+               arch_sync_dma_for_cpu(phys, s->length, dir);


+       if (dir == DMA_FROM_DEVICE)
+               arch_dma_mark_clean(phys, s->length);


  * arm_iommu_unmap_sg - unmap a set of SG buffers mapped by dma_map_sg
  * @dev: valid struct device pointer

@@ -1425,9 +1438,9 @@ static void arm_iommu_unmap_page(struct device *dev, dma_addr_t handle,
        if (!iova)

-       if (!dev->dma_coherent && !(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
+       if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))

Missing opening curly brace.

                phys = iommu_iova_to_phys(mapping->domain, handle);
-               arch_sync_dma_for_cpu(phys, size, dir);
+               arm_iommu_sync_dma_for_cpu(phys, size, dir, dev->dma_coherent);

        iommu_unmap(mapping->domain, iova, len);

With the above fixed, it builds and boots fine (on R-Car M2-W).



Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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