On Mon, Jan 14, 2019 at 01:42:26PM +0100, Marek Szyprowski wrote:

Hi Christoph,

On 2019-01-11 19:17, Christoph Hellwig wrote:

quoted

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 <hch@lst.de>

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 <m.szyprowski@samsung.com>

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.

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);

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