linux-kernel - Re: [PATCH 1/1] arm64/dma-mapping: remove an unnecessary conversion

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Date:	Tue, 15 Mar 2016 15:37:58 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Zhen Lei <thunder.leizhen@...wei.com>
Cc:	Will Deacon <will.deacon@....com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Zefan Li <lizefan@...wei.com>, Xinwei Hu <huxinwei@...wei.com>,
	Tianhong Ding <dingtianhong@...wei.com>,
	Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH 1/1] arm64/dma-mapping: remove an unnecessary conversion

On Tue, Mar 15, 2016 at 10:12:11AM +0800, Zhen Lei wrote:
> 1. In swiotlb_alloc_coherent, the branch of __get_free_pages. Directly
>    return vaddr on success, and pass vaddr to free_pages on failure.
> 2. So, we can directly transparent pass vaddr from __dma_free to
>    swiotlb_free_coherent, keep consistent with swiotlb_alloc_coherent.
> 
> This patch have no functional change,

I don't think so.

> but can obtain a bit performance improvement.

Have you actually measured it?

> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
> index a6e757c..b2f2834 100644
> --- a/arch/arm64/mm/dma-mapping.c
> +++ b/arch/arm64/mm/dma-mapping.c
> @@ -187,8 +187,6 @@ static void __dma_free(struct device *dev, size_t size,
>  		       void *vaddr, dma_addr_t dma_handle,
>  		       struct dma_attrs *attrs)
>  {
> -	void *swiotlb_addr = phys_to_virt(dma_to_phys(dev, dma_handle));
> -
>  	size = PAGE_ALIGN(size);
> 
>  	if (!is_device_dma_coherent(dev)) {
> @@ -196,7 +194,7 @@ static void __dma_free(struct device *dev, size_t size,
>  			return;
>  		vunmap(vaddr);
>  	}
> -	__dma_free_coherent(dev, size, swiotlb_addr, dma_handle, attrs);
> +	__dma_free_coherent(dev, size, vaddr, dma_handle, attrs);
>  }

What happens when !is_device_dma_coherent(dev)? (hint: read two lines
above __dma_free_coherent).

-- 
Catalin