linux-kernel - Re: [PATCH 08/10] swiotlb-xen: introduce phys_to_dma/dma_to

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Date:   Thu, 21 May 2020 09:07:53 +0100
From:   Julien Grall <julien@....org>
To:     Stefano Stabellini <sstabellini@...nel.org>, jgross@...e.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>
Subject: Re: [PATCH 08/10] swiotlb-xen: introduce phys_to_dma/dma_to_phys
 translations

Hi,

On 21/05/2020 00:45, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@...inx.com>
> 
> Call dma_to_phys in is_xen_swiotlb_buffer.
> Call phys_to_dma in xen_phys_to_bus.
> Call dma_to_phys in xen_bus_to_phys.
> 
> Everything is taken care of by these changes except for
> xen_swiotlb_alloc_coherent and xen_swiotlb_free_coherent, which need a
> few explicit phys_to_dma/dma_to_phys calls.

The commit message explains what the code is doing but doesn't explain 
why this is needed.

> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
> ---
>   drivers/xen/swiotlb-xen.c | 20 ++++++++++++--------
>   1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index c50448fd9b75..d011c4c7aa72 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -64,14 +64,16 @@ static inline dma_addr_t xen_phys_to_bus(struct device *dev, phys_addr_t paddr)
>   
>   	dma |= paddr & ~XEN_PAGE_MASK;
>   
> -	return dma;
> +	return phys_to_dma(dev, dma);
>   }
>   
> -static inline phys_addr_t xen_bus_to_phys(struct device *dev, dma_addr_t baddr)
> +static inline phys_addr_t xen_bus_to_phys(struct device *dev,
> +					  dma_addr_t dma_addr)
>   {
> +	phys_addr_t baddr = dma_to_phys(dev, dma_addr);
>   	unsigned long xen_pfn = bfn_to_pfn(XEN_PFN_DOWN(baddr));
> -	dma_addr_t dma = (dma_addr_t)xen_pfn << XEN_PAGE_SHIFT;
> -	phys_addr_t paddr = dma;
> +	phys_addr_t paddr = (xen_pfn << XEN_PAGE_SHIFT) |
> +			    (baddr & ~XEN_PAGE_MASK);
>   
>   	paddr |= baddr & ~XEN_PAGE_MASK;
>   
> @@ -99,7 +101,7 @@ static inline int range_straddles_page_boundary(phys_addr_t p, size_t size)
>   
>   static int is_xen_swiotlb_buffer(struct device *dev, dma_addr_t dma_addr)
>   {
> -	unsigned long bfn = XEN_PFN_DOWN(dma_addr);
> +	unsigned long bfn = XEN_PFN_DOWN(dma_to_phys(dev, dma_addr));
>   	unsigned long xen_pfn = bfn_to_local_pfn(bfn);
>   	phys_addr_t paddr = XEN_PFN_PHYS(xen_pfn);
>   
> @@ -304,11 +306,11 @@ xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
>   	if (hwdev && hwdev->coherent_dma_mask)
>   		dma_mask = hwdev->coherent_dma_mask;
>   
> -	/* At this point dma_handle is the physical address, next we are
> +	/* At this point dma_handle is the dma address, next we are
>   	 * going to set it to the machine address.
>   	 * Do not use virt_to_phys(ret) because on ARM it doesn't correspond
>   	 * to *dma_handle. */
> -	phys = *dma_handle;
> +	phys = dma_to_phys(hwdev, *dma_handle);
>   	dev_addr = xen_phys_to_bus(hwdev, phys);
>   	if (((dev_addr + size - 1 <= dma_mask)) &&
>   	    !range_straddles_page_boundary(phys, size))
> @@ -319,6 +321,7 @@ xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
>   			xen_free_coherent_pages(hwdev, size, ret, (dma_addr_t)phys, attrs);
>   			return NULL;
>   		}
> +		*dma_handle = phys_to_dma(hwdev, *dma_handle);
>   		SetPageXenRemapped(virt_to_page(ret));
>   	}
>   	memset(ret, 0, size);
> @@ -351,7 +354,8 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr,
>   	    TestClearPageXenRemapped(pg))
>   		xen_destroy_contiguous_region(phys, order);
>   
> -	xen_free_coherent_pages(hwdev, size, vaddr, (dma_addr_t)phys, attrs);
> +	xen_free_coherent_pages(hwdev, size, vaddr, phys_to_dma(hwdev, phys),
> +				attrs);
>   }
>   
>   /*
> 

Cheers,

-- 
Julien Grall