linux-kernel - Re: [PATCH 1/2] x86 swiotlb: Verify we can perform the remapping requested.

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Date:	Thu, 27 Oct 2011 09:01:53 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	ebiederm@...ssion.com
Cc:	konrad.wilk@...cle.com, fujita.tomonori@....ntt.co.jp,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86 swiotlb: Verify we can perform the remapping
 requested.

On Mon, 24 Oct 2011 08:07:31 -0700
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> writes:
> 
> > On Mon, Oct 17, 2011 at 02:19:18PM -0700, Eric W. Biederman wrote:
> >> 
> >> Recently I had a driver try with a peculiar 2G dma memory limit.
> >> It failed in weird and strange ways because my bounce buffers were
> >> being allocated above 2G where the driver could not reach, and
> >> no error was reported when the mappings were setup.
> >
> > OK, so the overflow buffer was used instead.. which presumarily
> > also was allocated above the 2G? That seems to point that
> > alloc_bootmem_low_pages is not doing its job?
> 
> I just looked alloc_bootmem_low_pages allocates memory below
> ARCH_ADDRESS_LIMIT which for everything except s390 is 4G.
> 
> I know I was mostly using the amd gart driver.  So I may be mistaken
> that the swiotlb driver had the same issue.  However my only solution
> at the time was to boot with mem=2G.  So I believe the swiotlb did
> have this issue.
> 
> Mostly the patch was.  Hmm.  That looks stupid not wiring up the
> swiotlb address space limit check when someone has already written it.

http://www.gossamer-threads.com/lists/linux/kernel/1333685?do=post_view_threaded#1333685

swiotlb isn't designed to handle hardware having odd dma mask (e.g. 2G
or something). Such device needs the own bouncing mechanism.
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