Messages in this thread | | | Subject | Re: [PATCH 1/1] arm64/dma-mapping: remove an unnecessary conversion | From | "Leizhen (ThunderTown)" <> | Date | Thu, 17 Mar 2016 19:06:27 +0800 |
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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); > } >
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