linux-kernel - Re: [PATCH 09/10] xen/arm: introduce phys/dma translations in xen_dma_sync_for

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Message-ID: <83c1120f-fe63-dc54-7b82-15a91c748de8@xen.org>
Date:   Thu, 21 May 2020 09:18:35 +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 09/10] xen/arm: introduce phys/dma translations in
 xen_dma_sync_for_*

Hi,

On 21/05/2020 00:45, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@...inx.com>
> 
> Add phys_to_dma/dma_to_phys calls to
> xen_dma_sync_for_cpu, xen_dma_sync_for_device, and
> xen_arch_need_swiotlb.
> 
> In xen_arch_need_swiotlb, take the opportunity to switch to the simpler
> pfn_valid check we use everywhere else.
> 
> dma_cache_maint is fixed by the next patch.

Like patch #8, this explains what the code is doing not why this is 
necessary.

> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
> ---
>   arch/arm/xen/mm.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm/xen/mm.c b/arch/arm/xen/mm.c
> index f2414ea40a79..7639251bcc79 100644
> --- a/arch/arm/xen/mm.c
> +++ b/arch/arm/xen/mm.c
> @@ -1,5 +1,6 @@
>   // SPDX-License-Identifier: GPL-2.0-only
>   #include <linux/cpu.h>
> +#include <linux/dma-direct.h>
>   #include <linux/dma-noncoherent.h>
>   #include <linux/gfp.h>
>   #include <linux/highmem.h>
> @@ -75,7 +76,7 @@ void xen_dma_sync_for_cpu(struct device *dev, dma_addr_t handle,
>   			  phys_addr_t paddr, size_t size,
>   			  enum dma_data_direction dir)
>   {
> -	if (pfn_valid(PFN_DOWN(handle)))
> +	if (pfn_valid(PFN_DOWN(dma_to_phys(dev, handle))))
>   		arch_sync_dma_for_cpu(paddr, size, dir);
>   	else if (dir != DMA_TO_DEVICE)
>   		dma_cache_maint(handle, size, GNTTAB_CACHE_INVAL);
> @@ -85,7 +86,7 @@ void xen_dma_sync_for_device(struct device *dev, dma_addr_t handle,
>   			     phys_addr_t paddr, size_t size,
>   			     enum dma_data_direction dir)
>   {
> -	if (pfn_valid(PFN_DOWN(handle)))
> +	if (pfn_valid(PFN_DOWN(dma_to_phys(dev, handle))))
>   		arch_sync_dma_for_device(paddr, size, dir);
>   	else if (dir == DMA_FROM_DEVICE)
>   		dma_cache_maint(handle, size, GNTTAB_CACHE_INVAL);
> @@ -97,8 +98,7 @@ bool xen_arch_need_swiotlb(struct device *dev,
>   			   phys_addr_t phys,
>   			   dma_addr_t dev_addr)
>   {
> -	unsigned int xen_pfn = XEN_PFN_DOWN(phys);
> -	unsigned int bfn = XEN_PFN_DOWN(dev_addr);
> +	unsigned int bfn = XEN_PFN_DOWN(dma_to_phys(dev, dev_addr));
>   
>   	/*
>   	 * The swiotlb buffer should be used if
> @@ -115,7 +115,7 @@ bool xen_arch_need_swiotlb(struct device *dev,
>   	 * require a bounce buffer because the device doesn't support coherent
>   	 * memory and we are not able to flush the cache.
>   	 */
> -	return (!hypercall_cflush && (xen_pfn != bfn) &&
> +	return (!hypercall_cflush && !pfn_valid(bfn) &&

I believe this change is incorrect. The bfn is a frame based on Xen page 
granularity (always 4K) while pfn_valid() is expecting a frame based on 
the Kernel page granularity.

>   		!dev_is_dma_coherent(dev));
>   }
>   
> 

Cheers,

-- 
Julien Grall