On Mon, 13 Jun 2011 10:55:59 -0700
"Michael K. Edwards" [off-list ref] wrote:

As far as I can tell, there is not yet any way to get real
cache-bypassing write-combining from userland in a mainline kernel,
for x86/x86_64 or ARM. 

Well only if things are really broken.  sysfs exposes _wc resource
files to allow userland drivers to map a given PCI BAR using write
combining, if the underlying platform supports it.

Similarly, userland mapping of GEM objects through the GTT are supposed
to be write combined, though I need to verify this (we've had trouble
with it in the past).

-- 
Jesse Barnes, Intel Open Source Technology Center