[PATCH v2 0/1] arm64/dma-mapping: reduce an unnecessary conversion for coherent DMA

[PATCH v2 0/1] arm64/dma-mapping: reduce an unnecessary conversion for coherent DMA

[Zhen Lei]

Changelog:
v1 -> v2:
1. Give up removing the conversion, because of thoughtless, instead moved it
into branch if (!is_device_dma_coherent(dev)). Thanks for Catalin's detailed
explanation, I directly take some relies as comment in the code.

Zhen Lei (1):
  arm64/dma-mapping: reduce an unnecessary conversion for coherent DMA

 arch/arm64/mm/dma-mapping.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

-- 
2.5.0

[PATCH v2 1/1] arm64/dma-mapping: reduce an unnecessary conversion for coherent DMA

[Zhen Lei]

1.For coherent DMA
  In swiotlb_alloc_coherent, it directly return vaddr on success, and
pass vaddr to free_pages on failure. So, we can directly transparent pass
vaddr from __dma_free to swiotlb_free_coherent.

2.Keep no change for non-coherent DMA.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 arch/arm64/mm/dma-mapping.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
index a6e757c..ceb2018 100644
--- a/arch/arm64/mm/dma-mapping.c
+++ b/arch/arm64/mm/dma-mapping.c
@@ -187,16 +187,22 @@ 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);
+
+		/*
+		 * For non-coherent DMA, the vaddr is not part of the linear
+		 * mapping as it has been remapped by __dma_alloc() via
+		 * dma_common_contiguous_remap(), hence for swiotlb freeing we
+		 * need the actual linear map address.
+		 */
+		vaddr = phys_to_virt(dma_to_phys(dev, dma_handle));
 	}
-	__dma_free_coherent(dev, size, swiotlb_addr, dma_handle, attrs);
+	__dma_free_coherent(dev, size, vaddr, dma_handle, attrs);
 }

 static dma_addr_t __swiotlb_map_page(struct device *dev, struct page *page,
--
2.5.0

Re: [PATCH v2 1/1] arm64/dma-mapping: reduce an unnecessary conversion for coherent DMA

[Catalin Marinas]

On Fri, Mar 18, 2016 at 10:26:07AM +0800, Zhen Lei wrote:
> 1.For coherent DMA
>   In swiotlb_alloc_coherent, it directly return vaddr on success, and
> pass vaddr to free_pages on failure. So, we can directly transparent pass
> vaddr from __dma_free to swiotlb_free_coherent.
> 
> 2.Keep no change for non-coherent DMA.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>  arch/arm64/mm/dma-mapping.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)

You sent a patch without any real reason like "improved performance",
just making the code slightly harder to read. Currently it is pretty
clear to me that phys_to_virt is meant for computing the swiotlb
address and we ignore the actual vaddr in this context.

So, I'm not taking this patch (if you want, you can send one just adding
a comment why vaddr cannot always be passed to the swiotlb API).

-- 
Catalin