Re: [PATCH] dma-direct: Set SG_DMA_SWIOTLB flag for dma-direct

[Petr Tesařík]

On Fri,  3 May 2024 18:37:12 +0000
"T.J. Mercier" <tjmercier@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  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.

I'm no expert, so correct DMA-to-physical translation might in fact be
somehow implicitly guaranteed. If that's the case, could you explain it
in the commit message, please?

Petr T