linux-kernel - Re: [PATCH 1/2] scatterlist: use sg

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Date:	Wed, 10 Jun 2015 09:00:31 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Jens Axboe <axboe@...nel.dk>, Michal Simek <monstr@...str.eu>,
	Russell King <linux@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Julia Lawall <Julia.Lawall@...6.fr>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Christoph Hellwig <hch@....de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] scatterlist: use sg_phys()

On Wed, Jun 10, 2015 at 2:32 AM, Joerg Roedel <joro@...tes.org> wrote:
> On Tue, Jun 09, 2015 at 12:27:10PM -0400, Dan Williams wrote:
>> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
>> index 7e7583ddd607..9f6ff6671f01 100644
>> --- a/arch/arm/mm/dma-mapping.c
>> +++ b/arch/arm/mm/dma-mapping.c
>> @@ -1502,7 +1502,7 @@ static int __map_sg_chunk(struct device *dev, struct scatterlist *sg,
>>               return -ENOMEM;
>>
>>       for (count = 0, s = sg; count < (size >> PAGE_SHIFT); s = sg_next(s)) {
>> -             phys_addr_t phys = page_to_phys(sg_page(s));
>> +             phys_addr_t phys = sg_phys(s) - s->offset;
>
> So sg_phys() turns out to be 'page_to_phys(sg_page(s)) + s->offset',
> which makes the above statement to:
>
>         page_to_phys(sg_page(s)) + s->offset - s->offset;
>
> The compiler will probably optimize that away, but it still doesn't look
> like an improvement.

The goal is to eventually stop leaking struct page deep into the i/o
stack.  Anything that relies on being able to retrieve a struct page
out of an sg entry needs to be converted.  I think we need a new
helper for this case "sg_phys_aligned()?".
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