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

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Date:	Thu, 17 Mar 2016 19:06:27 +0800
From:	"Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:	Catalin Marinas <catalin.marinas@....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>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/1] arm64/dma-mapping: remove an unnecessary conversion



On 2016/3/16 9:56, Leizhen (ThunderTown) wrote:
> 
> 
> On 2016/3/15 23:37, Catalin Marinas wrote:
>> 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?
> I have not run any performance testing, but reduced a line of code. So I said "a bit".
> 
>>
>>> 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).
Do you afraid "vaddr" maybe modified by these statement?
First, it could not be __free_from_pool. Otherwise, the function vunmap(which after it) can not work well.
Then, it count not be vunmap too, the parameter is defined as "const void *".

In the call chain: __dma_free_coherent-->__dma_free_coherent-->swiotlb_free_coherent, only swiotlb_free_coherent finally use "vaddr".

>>
> The whole function of __dma_free as below: (nobody use swiotlb_addr except __dma_free_coherent)
> 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)) {
>                 if (__free_from_pool(vaddr, size))
>                         return;
>                 vunmap(vaddr);
>         }
>         __dma_free_coherent(dev, size, swiotlb_addr, dma_handle, attrs);
> }
>