EXA for radeon experimental patch

Eric Anholt eta at lclark.edu
Thu Sep 1 11:59:53 PDT 2005

On Thu, 2005-09-01 at 14:57 +0200, Lars Knoll wrote:
> On Thursday 01 September 2005 13:15, Benjamin Herrenschmidt wrote:
> > > > They do, though they require some kernel support to get to the physical
> > > > address of pages and need some proper scatter/gather support on the
> > > > card side.
> > >
> > > The all you need is a drm module for your card. Even if the card doesn't
> > > have scatter/gather support, drm allows you to allocate a piece of
> > > consistent physical ram, and mmap it in the server. The handle you get is
> > > the physical address, so you should be able to use that to implement PCI
> > > dma transfers.
> >
> > Yup, but it's very likely that allocating physically contiguous memory
> > will fail. The kernel isn't that good as keeping physical memory non
> > fragmented, and thus, physical allocations above PAGE_SIZE are quite
> > likely fail after boot.
> Why this? The kernel has support for paging, so it could easily free up some 
> continuous pages just by swapping them out if they are used.

It would be nice if it were true.  But getting physically contiguous
pages any time after boot (of any significant size) is hard.  I'm
guessing due to wired pages being sprinkled around?

Eric Anholt                                     eta at lclark.edu
http://people.freebsd.org/~anholt/              anholt at FreeBSD.org
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