Re: [Xen-devel] USB virt status --- Help please!!!

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 14 Nov 2005, at 10:12, harry wrote:

> > When running on Xen, just because Linux has allocated adjacent pages
> > from its 'physical' memory map, doesn't mean they really are 
> > physically
> > contiguous. For I/O we have to go to extra effort to really allocate
> > truly contiguous multi-page extents.
> >
> > So your code is correct. If you can preallocate buffers then you could
> > do that with dma_alloc_coherent and that would guarantee contigous
> > buffers....
> I'm given the buffers by the assorted USB drivers.  I could stage the
> data using memcpy into my own buffer allocated using 
> dma_alloc_coherent.
> Or should I go through all the USB drivers and change the buffer
> allocation?
Your first approach would be equivalent to just using swiotlb. But  it 
is easy to implement and at least avoids 'use swiotlb=force' crashes. 
One possibility is to switch to having swiotlb default to 'on' and then 
need to force it off.
The second approach would never get past the kernel maintainers. This 
is a very xen-specific problem and the maintainers won't allow us to 
solve it by hacking at generic driver code.
 -- Keir


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