linux-kernel - Re: [PATCH 02/10] swiotlb-xen: remove start_dma

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Message-ID: <6241b8f6-5c51-0486-55ae-d571b117a184@xen.org>
Date:   Thu, 21 May 2020 09:05:07 +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 02/10] swiotlb-xen: remove start_dma_addr

Hi,

On 21/05/2020 00:45, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@...inx.com>
> 
> It is not strictly needed. Call virt_to_phys on xen_io_tlb_start
> instead. It will be useful not to have a start_dma_addr around with the
> next patches.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
> ---
>   drivers/xen/swiotlb-xen.c | 5 +----
>   1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index a42129cba36e..b5e0492b07b9 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -52,8 +52,6 @@ static unsigned long xen_io_tlb_nslabs;
>    * Quick lookup value of the bus address of the IOTLB.
>    */
>   
> -static u64 start_dma_addr;
> -
>   /*
>    * Both of these functions should avoid XEN_PFN_PHYS because phys_addr_t
>    * can be 32bit when dma_addr_t is 64bit leading to a loss in
> @@ -241,7 +239,6 @@ int __ref xen_swiotlb_init(int verbose, bool early)
>   		m_ret = XEN_SWIOTLB_EFIXUP;
>   		goto error;
>   	}
> -	start_dma_addr = xen_virt_to_bus(xen_io_tlb_start);
>   	if (early) {
>   		if (swiotlb_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs,
>   			 verbose))
> @@ -389,7 +386,7 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
>   	 */
>   	trace_swiotlb_bounced(dev, dev_addr, size, swiotlb_force);
>   
> -	map = swiotlb_tbl_map_single(dev, start_dma_addr, phys,
> +	map = swiotlb_tbl_map_single(dev, virt_to_phys(xen_io_tlb_start), phys,

xen_virt_to_bus() is implemented as xen_phys_to_bus(virt_to_phys()). Can 
you explain how the two are equivalent?

Cheers,

-- 
Julien Grall