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Date:   Sat, 5 Nov 2016 15:39:30 -0400
From:   Konrad Rzeszutek Wilk <konrad@...nok.org>
To:     Alexander Duyck <alexander.h.duyck@...el.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [mm PATCH v2 03/26] swiotlb: Add support for
 DMA_ATTR_SKIP_CPU_SYNC

.. snip..
> @@ -561,6 +565,7 @@ void swiotlb_tbl_unmap_single(struct device *hwdev, phys_addr_t tlb_addr,
>  	 * First, sync the memory before unmapping the entry
>  	 */
>  	if (orig_addr != INVALID_PHYS_ADDR &&
> +	    !(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
>  	    ((dir == DMA_FROM_DEVICE) || (dir == DMA_BIDIRECTIONAL)))
>  		swiotlb_bounce(orig_addr, tlb_addr, size, DMA_FROM_DEVICE);
>  
> @@ -654,7 +659,8 @@ void swiotlb_tbl_sync_single(struct device *hwdev, phys_addr_t tlb_addr,
>  		 * GFP_DMA memory; fall back on map_single(), which
>  		 * will grab memory from the lowest available address range.
>  		 */
> -		phys_addr_t paddr = map_single(hwdev, 0, size, DMA_FROM_DEVICE);
> +		phys_addr_t paddr = map_single(hwdev, 0, size,
> +					       DMA_FROM_DEVICE, 0);
>  		if (paddr == SWIOTLB_MAP_ERROR)
>  			goto err_warn;
>  
> @@ -669,7 +675,8 @@ void swiotlb_tbl_sync_single(struct device *hwdev, phys_addr_t tlb_addr,
>  
>  			/* DMA_TO_DEVICE to avoid memcpy in unmap_single */
>  			swiotlb_tbl_unmap_single(hwdev, paddr,
> -						 size, DMA_TO_DEVICE);
> +						 size, DMA_TO_DEVICE,
> +						 DMA_ATTR_SKIP_CPU_SYNC);

This I believe is redundant. That is swiotlb_tbl_unmap_single only
does an bounce if the dir is DMA_FROM_DEVICE or DMA_BIDIRECTIONAL.

I added /* optional. */
>  			goto err_warn;
>  		}
>  	}
> @@ -699,7 +706,7 @@ void swiotlb_tbl_sync_single(struct device *hwdev, phys_addr_t tlb_addr,
>  		free_pages((unsigned long)vaddr, get_order(size));
>  	else
>  		/* DMA_TO_DEVICE to avoid memcpy in swiotlb_tbl_unmap_single */
> -		swiotlb_tbl_unmap_single(hwdev, paddr, size, DMA_TO_DEVICE);
> +		swiotlb_tbl_unmap_single(hwdev, paddr, size, DMA_TO_DEVICE, 0);

.. but here you choose to put 0? I changed that to
DMA_ATTR_SKIP_CPU_SYNC and expanded the comment above.

Time to test the patches.

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