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

From: Leizhen (ThunderTown)
Date: Tue Mar 15 2016 - 21:57:04 EST




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).
>
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);
}