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

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Date:	Mon, 5 Aug 2013 15:58:05 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Xishi Qiu <qiuxishi@...wei.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] cma: use macro PFN_DOWN when converting size to pages

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@...wei.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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