Re: [PATCH] swiotlb: fix dma_alloc_coherent allocation failures with swiotlb

[Joerg Roedel <joro@8bytes.org>]

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@amd.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@jp.fujitsu.com>
> > Cc: tony.luck@intel.com
> > Signed-off-by: Joerg Roedel <joerg.roedel@amd.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