Re: [PATCH v2 1/4] dma-iommu: fix sync_sg with swiotlb

[Will Deacon <will@kernel.org>]

On Fri, Jul 09, 2021 at 12:34:59PM +0900, David Stevens wrote:
> From: David Stevens <stevensd@chromium.org>
> 
> The is_swiotlb_buffer function takes the physical address of the swiotlb
> buffer, not the physical address of the original buffer. The sglist
> contains the physical addresses of the original buffer, so for the
> sync_sg functions to work properly when a bounce buffer might have been
> used, we need to use iommu_iova_to_phys to look up the physical address.
> This is what sync_single does, so call that function on each sglist
> segment.
> 
> The previous code mostly worked because swiotlb does the transfer on map
> and unmap. However, any callers which use DMA_ATTR_SKIP_CPU_SYNC with
> sglists or which call sync_sg would not have had anything copied to the
> bounce buffer.
> 
> Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
> Signed-off-by: David Stevens <stevensd@chromium.org>
> ---
>  drivers/iommu/dma-iommu.c | 26 +++++++++++++-------------
>  1 file changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 7bcdd1205535..eac65302439e 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -811,14 +811,14 @@ static void iommu_dma_sync_sg_for_cpu(struct device *dev,
>  	if (dev_is_dma_coherent(dev) && !dev_is_untrusted(dev))
>  		return;
>  
> -	for_each_sg(sgl, sg, nelems, i) {
> -		if (!dev_is_dma_coherent(dev))
> -			arch_sync_dma_for_cpu(sg_phys(sg), sg->length, dir);
> -
> -		if (is_swiotlb_buffer(sg_phys(sg)))
> +	if (dev_is_untrusted(dev))
> +		for_each_sg(sgl, sg, nelems, i)
> +			iommu_dma_sync_single_for_cpu(dev, sg_dma_address(sg),
> +						      sg->length, dir);
> +	else
> +		for_each_sg(sgl, sg, nelems, i)
>  			swiotlb_sync_single_for_cpu(dev, sg_phys(sg),
>  						    sg->length, dir);

Doesn't this skip arch_sync_dma_for_cpu() for non-coherent trusted devices?

Why not skip the extra dev_is_untrusted(dev) call here and just call
iommu_dma_sync_single_for_cpu() for each entry regardless?

Will