linux-kernel - Re: [PATCH] swiotlb: fix dma_alloc_coherent allocation failures with swiotlb

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Date:	Wed, 3 Sep 2008 21:54:15 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	joerg.roedel@....com, tony.luck@...el.com,
	linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
	mingo@...hat.com, hpa@...or.com, tglx@...utronix.de,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH] swiotlb: fix dma_alloc_coherent allocation failures with swiotlb

On Thu, Sep 04, 2008 at 03:05:21AM +0900, FUJITA Tomonori wrote:
> On Wed, 3 Sep 2008 17:03:44 +0200
> Joerg Roedel <joerg.roedel@....com> wrote:
> 
> > The SWIOTLB version of dma_alloc_coherent allocates all memory with
> > GFP_DMA unconditionally. This leads sometimes unnecessary to allocation
> > failures. This patch makes the allocation strategy to use the DMA32 zone
> > first if this is possible. The changes are boot tested on AMD64 and
> > compile tested for i386 and IA64.
> > 
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > Cc: tony.luck@...el.com
> > Signed-off-by: Joerg Roedel <joerg.roedel@....com>
> > ---
> >  lib/swiotlb.c |   23 +++++++++++++++++------
> >  1 files changed, 17 insertions(+), 6 deletions(-)
> 
> This doesn't look correct since IA64 doesn't need this flag hack.

Thats why the patch checks the DMA mask against ISA_DMA_THRESHOLD. This
is defined as the border of the ZONE_DMA (16MB on x86 and 4G on IA64).
So is correct for x86 and ia64.

> Another problem about this patch is that swiotlb doesn't use GFP_DMA
> for the fallback_dev.

Thats not true, it does. ISA_DMA_THRESHOLD is defined to be the same as
DMA_24BIT_MASK, which is the DMA mask of the fallback_dev.

Joerg

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