[PATCH V2] cma: use macro PFN_DOWN when converting size to pages

[PATCH V2] cma: use macro PFN_DOWN when converting size to pages

[Xishi Qiu]

Use "PFN_DOWN(r->size)" instead of "r->size >> PAGE_SHIFT".

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
---
 drivers/base/dma-contiguous.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/drivers/base/dma-contiguous.c b/drivers/base/dma-contiguous.c
index 0ca5442..b3d711d 100644
--- a/drivers/base/dma-contiguous.c
+++ b/drivers/base/dma-contiguous.c
@@ -206,8 +206,7 @@ static int __init cma_init_reserved_areas(void)
 
 	for (; i; --i, ++r) {
 		struct cma *cma;
-		cma = cma_create_area(PFN_DOWN(r->start),
-				      r->size >> PAGE_SHIFT);
+		cma = cma_create_area(PFN_DOWN(r->start), PFN_DOWN(r->size));
 		if (!IS_ERR(cma))
 			dev_set_cma_area(r->dev, cma);
 	}
-- 
1.7.1

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Re: [PATCH V2] cma: use macro PFN_DOWN when converting size to pages

[Johannes Weiner]

On Mon, Aug 05, 2013 at 05:09:17PM +0800, Xishi Qiu wrote:
> Use "PFN_DOWN(r->size)" instead of "r->size >> PAGE_SHIFT".
> 
> Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
> ---
>  drivers/base/dma-contiguous.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/dma-contiguous.c b/drivers/base/dma-contiguous.c
> index 0ca5442..b3d711d 100644
> --- a/drivers/base/dma-contiguous.c
> +++ b/drivers/base/dma-contiguous.c
> @@ -206,8 +206,7 @@ static int __init cma_init_reserved_areas(void)
>  
>  	for (; i; --i, ++r) {
>  		struct cma *cma;
> -		cma = cma_create_area(PFN_DOWN(r->start),
> -				      r->size >> PAGE_SHIFT);
> +		cma = cma_create_area(PFN_DOWN(r->start), PFN_DOWN(r->size));

PFN_DOWN(r->start) makes sense because you are dividing and rounding a
byte-granular address to a PFN.

r->size >> PAGE_SHIFT translates number of bytes into number of pages.

It ends up being the same arithmetic operation to do both things, but
the units are different; the result of the second expression is not a
PFN.  I think this change actually worsens readability of the code.

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