Date: Sat, 18 Jul 2009 12:41:01 +0200
From: Ingo Molnar <mingo@elte.hu>
To: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Ian.Campbell@eu.citrix.com, jeremy@goop.org, linux-kernel@vger.kernel.org,
       linux-ia64@vger.kernel.org, linuxppc-dev@ozlabs.org,
       benh@kernel.crashing.org, tony.luck@intel.com, x86@kernel.org,
       beckyb@kernel.crashing.org, joerg.roedel@amd.com
Subject: Re: [00/15] swiotlb cleanup
Message-ID: <20090718104101.GA27239@elte.hu>
References: <1247234512.4002.417.camel@zakaz.uk.xensource.com> <20090710141248.GE26264@elte.hu> <20090713131157V.fujita.tomonori@lab.ntt.co.jp> <20090713181558B.fujita.tomonori@lab.ntt.co.jp>
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* FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:

> On Mon, 13 Jul 2009 13:20:22 +0900
> FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:
> 
> > On Fri, 10 Jul 2009 16:12:48 +0200
> > Ingo Molnar <mingo@elte.hu> wrote:
> > 
> > > > functionality and reimplemented the surrounding infrastructure in 
> > > > terms of that (and incorporating our additional requirements). I 
> > > > prototyped this (it is currently unworking, in fact it seems to 
> > > > have developed rather a taste for filesystems :-() but the 
> > > > diffstat of my WIP patch is:
> > > >
> > > >          arch/x86/kernel/pci-swiotlb.c |    6 
> > > >          arch/x86/xen/pci-swiotlb.c    |    2 
> > > >          drivers/pci/xen-iommu.c       |  385 ++++++++++++++++++++++++++++++++++++++++--
> > > >          include/linux/swiotlb.h       |   12 +
> > > >          lib/swiotlb.c                 |   10 -
> > > >          5 files changed, 385 insertions(+), 30 deletions(-)
> > > >
> > > > where a fair number of the lines in xen-iommu.c are copies of 
> > > > functions from swiotlb.c with minor modifications. As I say it 
> > > > doesn't work yet but I think it's roughly indicative of what such 
> > > > an approach would look like. I don't like it much but am happy to 
> > > > run with it if it looks to be the most acceptable approach. [...]
> > > 
> > > +400 lines of code to avoid much fewer lines of generic code impact 
> > > on the lib/swiotlb.c side sounds like a bad technical choice to me. 
> > 
> > The amount of code is not the point. The way to impact on the
> > lib/swiotlb.c is totally wrong from the perspective of the kernel
> > design; it uses architecture code in the very original (xen) way.
> 
> btw, '+400 lines of code to avoid much fewer lines of generic code
> impact on the lib/swiotlb.c' doesn't sound true to me.
> 
> Here is a patch in the way that Xen people want to do:
> 
> http://patchwork.kernel.org/patch/26343/
> 
> ---
>  arch/x86/Kconfig                 |    4 +
>  arch/x86/include/asm/io.h        |    2 +
>  arch/x86/include/asm/pci_x86.h   |    1 +
>  arch/x86/include/asm/xen/iommu.h |   12 ++
>  arch/x86/kernel/pci-dma.c        |    3 +
>  arch/x86/pci/Makefile            |    1 +
>  arch/x86/pci/init.c              |    6 +
>  arch/x86/pci/xen.c               |   51 +++++++
>  drivers/pci/Makefile             |    2 +
>  drivers/pci/xen-iommu.c          |  271 ++++++++++++++++++++++++++++++++++++++
> 
> Even with the way that Xen people want to do, 
> drivers/pci/xen-iommu.c is about 300 lines. And my patchset 
> removes the nice amount of lines for dom0 support. I don't see 
> much difference wrt lines.

ok, that kind of impact looks reasonable. If we are wrong and the 
Xen model becomes duplicated anywhere else it can still be 
generalized into core swiotlb code.

	Ingo
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