Messages in this thread | | | Date | Thu, 9 May 2024 14:28:33 +0100 | Subject | Re: [PATCH] dma-direct: Set SG_DMA_SWIOTLB flag for dma-direct | From | Robin Murphy <> |
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On 04/05/2024 9:53 am, Petr Tesařík wrote: > On Fri, 3 May 2024 18:37:12 +0000 > "T.J. Mercier" <tjmercier@google.com> wrote: > >> As of commit 861370f49ce4 ("iommu/dma: force bouncing if the size is >> not cacheline-aligned") sg_dma_mark_swiotlb is called when >> dma_map_sgtable takes the IOMMU path and uses SWIOTLB for some portion >> of a scatterlist. It is never set for the direct path, so drivers >> cannot always rely on sg_dma_is_swiotlb to return correctly after >> calling dma_map_sgtable. Fix this by calling sg_dma_mark_swiotlb in the >> direct path like it is in the IOMMU path. >> >> Signed-off-by: T.J. Mercier <tjmercier@google.com> >> --- >> kernel/dma/direct.c | 4 +++- >> 1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c >> index 4d543b1e9d57..52f0dcb25ca2 100644 >> --- a/kernel/dma/direct.c >> +++ b/kernel/dma/direct.c >> @@ -12,7 +12,7 @@ >> #include <linux/pfn.h> >> #include <linux/vmalloc.h> >> #include <linux/set_memory.h> >> -#include <linux/slab.h> >> +#include <linux/swiotlb.h> >> #include "direct.h" >> >> /* >> @@ -497,6 +497,8 @@ int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents, >> goto out_unmap; >> } >> sg_dma_len(sg) = sg->length; >> + if (is_swiotlb_buffer(dev, dma_to_phys(dev, sg->dma_address))) >> + sg_dma_mark_swiotlb(sg); >> } >> >> return nents; > > I'm not sure this does the right thing. IIUC when the scatterlist flags > include SG_DMA_SWIOTLB, iommu_dma_sync_sg_for_*() will call > iommu_dma_sync_single_for_*(), which in turn translates the DMA address > to a physical address using iommu_iova_to_phys(). It seems to me that > this function may not work correctly if there is no IOMMU, but it also > seems to me that the scatterlist may contain such non-IOMMU addresses.
In principle dma-direct *could* make use of the SG_DMA_SWIOTLB flag for an ever-so-slightly cheaper check than is_swiotlb_buffer() in sync_sg and unmap_sg, the same way as iommu-dma does. However the benefit would be a lot less significant than for iommu-dma, where it's really about the overhead of needing to perform iommu_iova_to_phys() translations for every segment every time in order to *get* the right thing to check is_swiotlb_buffer() on - that's what would be unreasonably prohibitive otherwise.
Thanks, Robin
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