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SubjectRe: [PATCH 3/3] videobuf2: replace a layering violation with dma_map_resource
Hi Christoph,

On 2019-01-17 18:21, Christoph Hellwig wrote:
> On Mon, Jan 14, 2019 at 01:42:26PM +0100, Marek Szyprowski wrote:
>> On 2019-01-11 19:17, Christoph Hellwig wrote:
>>> vb2_dc_get_userptr pokes into arm direct mapping details to get the
>>> resemblance of a dma address for a a physical address that does is
>>> not backed by a page struct. Not only is this not portable to other
>>> architectures with dma direct mapping offsets, but also not to uses
>>> of IOMMUs of any kind. Switch to the proper dma_map_resource /
>>> dma_unmap_resource interface instead.
>>> Signed-off-by: Christoph Hellwig <>
>> There are checks for IOMMU presence in other places in vb2-dma-contig,
>> so it was used only when no IOMMU is available, but I agree that the
>> hacky code should be replaced by a generic code if possible.
>> Tested-by: Marek Szyprowski <>
>> V4L2 pipeline works fine on older Exynos-based boards with CMA and IOMMU
>> disabled.
> Do you know if these rely on the offsets? E.g. would they still work
> with the patch below applied on top. That would keep the map_resource
> semantics as-is as solve the issue pointed out by Robin for now.

AFAIK that code was only used for sharing buffers between hardware
modules that are a part of the same SoC, usually implemented as platform
devices. AFAIR this never worked for devices on different buses. So far
I wasn't aware on ANY which would require an offset for the DMA access.

The first version of videobuf2-dc code even incorrectly used paddr
instead of dma_addr as a buffer 'address' returned to the client
drivers, because in case of those SoC this was exactly the same (see
commits 472af2b05bdefcaee7e754e22cbf131110017ad6 and

> If not I can only think of a flag to bypass the offseting for now, but
> that would be pretty ugly. Or go for the long-term solution of
> discovering the relationship between the two devices, as done by the
> PCIe P2P code..
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 8e0359b04957..25bd19974223 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -359,7 +359,7 @@ EXPORT_SYMBOL(dma_direct_map_sg);
> dma_addr_t dma_direct_map_resource(struct device *dev, phys_addr_t paddr,
> size_t size, enum dma_data_direction dir, unsigned long attrs)
> {
> - dma_addr_t dma_addr = phys_to_dma(dev, paddr);
> + dma_addr_t dma_addr = paddr;
> if (unlikely(!dma_direct_possible(dev, dma_addr, size))) {
> report_addr(dev, dma_addr, size);
Best regards
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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