Having looked into this further, I find that adding the kernel boot
parameter "enable_mtrr_cleanup" does exactly what I'd done in replacing
the 4GB entry, and for the same purpose (opening up a range where a new
write-combining entry can go). Plus, it's a general algorithm, not
something specific to this one motherboard. The sanitizer can be
selected by default when compiling the kernel
(CONFIG_MTRR_SANITIZER_ENABLE_DEFAULT=1), but Ubuntu doesn't do so.
Looking at this thread:
http://lkml.org/lkml/2008/4/28/52
it seems that the kernel developers are moving towards using PAT, rather
than MTRRs, for the write-combining regions; but the Ubuntu kernel
doesn't have PAT enabled.
So people who are getting the "write error: invalid argument" message
might try adding that boot parameter.
--
[i915GM] MTRR entry gets removed when restarting xorg - causes corruption on
ttys
https://bugs.launchpad.net/bugs/314928
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