Date: Fri, 10 Apr 2009 04:43:55 +0900
To: jeremy@goop.org
Cc: fujita.tomonori@lab.ntt.co.jp, galak@kernel.crashing.org,
       hch@infradead.org, linux-kernel@vger.kernel.org, mingo@elte.hu,
       ian.campbell@citrix.com, beckyb@kernel.crashing.org
Subject: Re: [PATCH 4/7] swiotlb: Allow arch override of
 address_needs_mapping
From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
In-Reply-To: <49DE4A37.3080702@goop.org>
References: <49DD3C9C.7060101@goop.org>
	<20090410033357S.fujita.tomonori@lab.ntt.co.jp>
	<49DE4A37.3080702@goop.org>
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On Thu, 09 Apr 2009 12:19:19 -0700
Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> FUJITA Tomonori wrote:
> >> Well, Becky's patches also added the hwdev argument to them, so 
> >> presumably the powerpc implementation needs that (different 
> >> devices/buses have differing views of physical memory, I guess).
> >>     
> >
> > Until I see the ppc specific swiotlb patchset, I'm not sure but I
> > think that we can remove phys_to_bus in swiotlb.
> >   
> 
> Kumar's comment was: "For our SoC chips we don't need any mapping 
> between phys & bus.  However something like PCI does have a mapping (a 
> simple offset)."

I meant that swiotlb doesn't need to use phys_to_bus stuff. But I as
said, I'm not sure until I see the ppc swiotlb code.


> Kumar, could a single system have different phys<->bus mappings on a 
> single system, or could it differ from device to device (or bus to bus)?
> 
> > Even if we need phys_to_bus, we can remove the rest of __weak tricks
> > for only dom0. And we can make phys_to_bus arch-specific. Then we
> > don't need any __weak tricks in swiotlb (and x86's swiotlb). dom0
> > support adds many hacks to swiotlb.
> >   
> 
> Well, we'd still need a way to do hook the swiotlb_alloc(_boot) 
> allocation.  At the moment its effectively arch-specific because x86 
> only uses swiotlb_alloc_boot(), and ia64 only uses swiotlb_alloc().  One 
> option would be to simply make that function arch-defined, which would 
> remove the need for any kind of override mechanism in lib/swiotlb; that 
> would match the handling of phys_to_bus.  And its more appealing if we 
> manage to drop swiotlb_alloc_boot, so there's only a single function for 
> the arches to worry about.
> 
> > Yeah, ISA DMA comment is misleading. swiotlb can't handle it. And it
> > doesn't need to handle it because the block layer can thanks to
> > the bouncing (the network layer does the similar, I think).
> >
> > As you said, we could remove the latter though I'm not sure.
> >   
> 
> It would take a bit of rearranging the x86 swiotlb/iommu init sequence, 
> but I don't think it would be too complex.  I'll look into it.

Unfortunately, not that simple. IA64_64 and x86 share the iommu init
sequence. So you need to look at both. I put some cleanups on it in
30-rc1. But I need to do more cleanups.


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