linux-kernel - Re: [PATCH 03/11] xen/arm: simplify dma_cache

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Date:   Thu, 29 Aug 2019 15:49:04 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Christoph Hellwig <hch@....de>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        xen-devel@...ts.xenproject.org, iommu@...ts.linux-foundation.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 03/11] xen/arm: simplify dma_cache_maint

On Tue, 27 Aug 2019, Christoph Hellwig wrote:
> And this was still buggy I think, it really needs some real Xen/Arm
> testing which I can't do.  Hopefully better version below:
> 
> --
> >From 5ad4b6e291dbb49f65480c9b769414931cbd485a Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@....de>
> Date: Wed, 24 Jul 2019 15:26:08 +0200
> Subject: xen/arm: simplify dma_cache_maint
> 
> Calculate the required operation in the caller, and pass it directly
> instead of recalculating it for each page, and use simple arithmetics
> to get from the physical address to Xen page size aligned chunks.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  arch/arm/xen/mm.c | 61 ++++++++++++++++-------------------------------
>  1 file changed, 21 insertions(+), 40 deletions(-)
> 
> diff --git a/arch/arm/xen/mm.c b/arch/arm/xen/mm.c
> index 90574d89d0d4..2fde161733b0 100644
> --- a/arch/arm/xen/mm.c
> +++ b/arch/arm/xen/mm.c
> @@ -35,64 +35,45 @@ unsigned long xen_get_swiotlb_free_pages(unsigned int order)
>  	return __get_free_pages(flags, order);
>  }
>  
> -enum dma_cache_op {
> -       DMA_UNMAP,
> -       DMA_MAP,
> -};
>  static bool hypercall_cflush = false;
>  
> -/* functions called by SWIOTLB */
> -
> -static void dma_cache_maint(dma_addr_t handle, unsigned long offset,
> -	size_t size, enum dma_data_direction dir, enum dma_cache_op op)
> +/* buffers in highmem or foreign pages cannot cross page boundaries */
> +static void dma_cache_maint(dma_addr_t handle, size_t size, u32 op)
>  {
>  	struct gnttab_cache_flush cflush;
> -	unsigned long xen_pfn;
> -	size_t left = size;
>  
> -	xen_pfn = (handle >> XEN_PAGE_SHIFT) + offset / XEN_PAGE_SIZE;
> -	offset %= XEN_PAGE_SIZE;
> +	cflush.a.dev_bus_addr = handle & XEN_PAGE_MASK;
> +	cflush.offset = xen_offset_in_page(handle);
> +	cflush.op = op;
>  
>  	do {
> -		size_t len = left;
> -	
> -		/* buffers in highmem or foreign pages cannot cross page
> -		 * boundaries */
> -		if (len + offset > XEN_PAGE_SIZE)
> -			len = XEN_PAGE_SIZE - offset;
> -
> -		cflush.op = 0;
> -		cflush.a.dev_bus_addr = xen_pfn << XEN_PAGE_SHIFT;
> -		cflush.offset = offset;
> -		cflush.length = len;
> -
> -		if (op == DMA_UNMAP && dir != DMA_TO_DEVICE)
> -			cflush.op = GNTTAB_CACHE_INVAL;
> -		if (op == DMA_MAP) {
> -			if (dir == DMA_FROM_DEVICE)
> -				cflush.op = GNTTAB_CACHE_INVAL;
> -			else
> -				cflush.op = GNTTAB_CACHE_CLEAN;
> -		}
> -		if (cflush.op)
> -			HYPERVISOR_grant_table_op(GNTTABOP_cache_flush, &cflush, 1);
> +		if (size + cflush.offset > XEN_PAGE_SIZE)
> +			cflush.length = XEN_PAGE_SIZE - cflush.offset;
> +		else
> +			cflush.length = size;
> +
> +		HYPERVISOR_grant_table_op(GNTTABOP_cache_flush, &cflush, 1);
>  
> -		offset = 0;
> -		xen_pfn++;
> -		left -= len;
> -	} while (left);
> +		cflush.offset = 0;
> +		cflush.a.dev_bus_addr += cflush.length;
> +		size -= cflush.length;

Yes that's better

Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>


> +	} while (size);
>  }
>  
>  static void __xen_dma_page_dev_to_cpu(struct device *hwdev, dma_addr_t handle,
>  		size_t size, enum dma_data_direction dir)
>  {
> -	dma_cache_maint(handle & PAGE_MASK, handle & ~PAGE_MASK, size, dir, DMA_UNMAP);
> +	if (dir != DMA_TO_DEVICE)
> +		dma_cache_maint(handle, size, GNTTAB_CACHE_INVAL);
>  }
>  
>  static void __xen_dma_page_cpu_to_dev(struct device *hwdev, dma_addr_t handle,
>  		size_t size, enum dma_data_direction dir)
>  {
> -	dma_cache_maint(handle & PAGE_MASK, handle & ~PAGE_MASK, size, dir, DMA_MAP);
> +	if (dir == DMA_FROM_DEVICE)
> +		dma_cache_maint(handle, size, GNTTAB_CACHE_INVAL);
> +	else
> +		dma_cache_maint(handle, size, GNTTAB_CACHE_CLEAN);
>  }
>  
>  void __xen_dma_map_page(struct device *hwdev, struct page *page,
> -- 
> 2.20.1
>