linux-kernel - Re: [PATCH] x86: avoid unnecessary low zone allocation in AMD IOMMU's alloc

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Message-ID: <20080910135347.GI6329@amd.com>
Date:	Wed, 10 Sep 2008 15:53:47 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH] x86: avoid unnecessary low zone allocation in AMD IOMMU's alloc_coherent

On Wed, Sep 10, 2008 at 10:37:45PM +0900, FUJITA Tomonori wrote:
> On Wed, 10 Sep 2008 15:10:32 +0200
> Joerg Roedel <joerg.roedel@....com> wrote:
> > Can't we simply make the gfp hacks depend on
> > dma_ops->is_phys and avoid further gfp hacks in the hardware iommu
> > implementations?
> 
> I thought about it but adding a new dma_ops->we_don't_want_gfp_flag
> hook doesn't make the code simpler much. Currently, we have the gfp
> setting hack in just one place. It's not bad. Adding such new hook
> means adding more lines than we can remove.

The is_phys flas is already in place and its meaning is "the dma_ops
return bus addresses equal to physical addresses". This is exactly the
case when we need the gfp hacks. So I don't see a problem in just
skipping the gfp rewrite if is_phys is zero. I don't see a point in
adding gfp flags in dma_alloc_coherent and remove them again
dma_ops->alloc_coherent code. Specially in this case where we already
know in dma_alloc_coherent if we really need the flag rewrite.

> Yeah, I was against your patch to adding the gfp setting hack to
> swiotlb but it's because gfp is kinda architecture specific stuff and
> swiotlb should not. It's the bad design IMO. It's ok for me that
> architecture specific IOMMUs can do the architecture specific stuff
> (and it's about just clearing the gfp flag).

The generic swiotlb code already contained a gfp hack for IA64 and I
added another one for x86 which is ok in my opinion. But the #ifdef was
ugly, I agree with that now :-)
Your solution of removing the flag hacks from the generic code completly
was the other possible way.

Joerg

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