linux-kernel - Re: [PATCH 4/7] swiotlb: Allow arch override of address_needs

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Date:	Thu, 09 Apr 2009 12:19:19 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	galak@...nel.crashing.org, hch@...radead.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	ian.campbell@...rix.com, beckyb@...nel.crashing.org
Subject: Re: [PATCH 4/7] swiotlb: Allow arch override of address_needs_mapping

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)."

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.

    J
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