linux-kernel - Re: [RFC PATCH 1/7] swiotlb: Instead of tracking the end of the swiotlb region just calculate it

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Date:	Thu, 4 Oct 2012 12:31:02 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Konrad Rzeszutek Wilk <konrad@...nel.org>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, rob@...dley.net,
	akpm@...ux-foundation.org, joerg.roedel@....com,
	bhelgaas@...gle.com, shuahkhan@...il.com,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	x86@...nel.org
Subject: Re: [RFC PATCH 1/7] swiotlb: Instead of tracking the end of the
 swiotlb region just calculate it

On Thu, Oct 04, 2012 at 08:54:09AM -0700, Alexander Duyck wrote:
> On 10/04/2012 06:01 AM, Konrad Rzeszutek Wilk wrote:
> > On Wed, Oct 03, 2012 at 05:38:47PM -0700, Alexander Duyck wrote:
> >> In the case of swiotlb we already have the start of the region and the number
> >> of slabs that give us the region size.  Instead of having to call
> >> virt_to_phys on two pointers we can just take advantage of the fact that the
> >> region is linear and just compute the end based on the start plus the size.
> > Why not take advantage of 'the fact that the region is linear' and just
> > pre-compute the end in swiotlb_init_with_tbl?
> >
> > That way the logic in is_swiotlb_buffer is even simpler?
> >
> 
> Using a pre-computed end point based on a virtual address is more
> expensive in the x86_64 case.  The calls to __phys_addr require a
> separate function call.  By just using the physical address of the start
> and adding the offset I can avoid the second call and the compiler will
> take advantage of the smaller function size.  The result is that
> is_swiotlb_buffer will be inlined.

Right, that is not what I was thinking. My thought was that since you
are already computing the start of the DMA address, you can also
compute the end - and save both values in a global variable.

Then use said global variables to check for your DMA instead of
doing the extra compution of shifting to find the end of the DMA address.

> 
> Thanks,
> 
> Alex
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