Re: [Xen-devel] Re: ATI radeon fails with "iommu=soft swiotlb=force" (seen on RV730/RV740 and RS780/RS800)

[Alex Deucher <alexdeucher@xxxxxxxxx>]

On Mon, Oct 5, 2009 at 10:01 AM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Mon, Oct 05, 2009 at 11:32:31AM +0100, Jan Beulich wrote:
>> >>> Jeremy Fitzhardinge <jeremy@xxxxxxxx> 02.10.09 20:42 >>>
>> >On 10/02/09 10:23, Boris Derzhavets wrote:
>> >> Jeremy,
>> >> Please,  be aware of bugzilla.xensource.com [1519]  the most recent
>> >> entries :-
>> >>
>> >> http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=1519
>> >>
>> >
>> >Ah, OK.  I pushed a variant of Konrad's patches.  Could you try them out?
>>
>> Are you really convinced fixing this in DRM is the right thing to do? To
>> me, the use of vmalloc_32() in drivers/ieee1394/ seems to make similar
>> assumptions (pci_map_sg() not modifying the buffer addresses), and
>> who knows where else such assumptions exist. After all, vmalloc_32()
>> is *defined* to have the property assumed by both of the users, and
>> other than for most kmalloc() cases using GFP_DMA{,32} we're talking
>> about double buffering generally large amounts of data here even in
>> the cases where the DMA API is being used properly.
>
> I was chatting with Jeremy about this, and I realized that in some
> sense the radeon driver assumes that physical != bus addresses. Which is
> OK on x86, but if this card was ever moved to a Sun box it would fail.
>

FWIW, the radeon drm has been working fine on both sparc and ppc for years.

Alex

> The patch here thwarts this by allocating memory from the IOMMU, so that
> even if bus != physical address from pci_map_page, that is OK.
>
> Another way to make this work right is to modify the drivers
> that use vmalloc_32, with pci_map_[sg|page], is for them to check if the 
> physical
> != bus address, and if so, remap the virtual address (page_to_vmalloc) to
> point to the new bus addresses (and free the "old" page). Obviously this
> requires some book-keeping, but it does fix those drivers if they were to 
> move to
> non-x86 architecture. Or on machines where the IOMMU hands out non-physical
> addresses.
>
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