Subject: Re: [PATCH 3/3] videobuf2: replace a layering violation with dma_map_resource

From: Christoph Hellwig <[email-masked]>
Date: Thu, 17 Jan 2019 18:21:52 +0100

On Mon, Jan 14, 2019 at 01:42:26PM +0100, Marek Szyprowski wrote:
> Hi Christoph,
> 
> 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 <[email-masked]>
> 
> 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 <[email-masked]>
> 
> 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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