I was glancing through the FreeBSD kernel source recently, and noticed
that they use MTRR write-combining over the VGA region (as lomg as
there is a VGA compatible device detected). Since this is about as far
as their MTRR support goes, they have a nice simple way of doing this:
They just hard-code the appropriate values for the fixed range
registers.
Presumably this works, so it would be nice if Linux did the same. I
doubt it's worth doing for VGA text-mode alone (though I'm not a heavy
text-mode user), but it would be good for fbcon and the XFree86
SVGA-based servers.
I can see an easy and a not so easy way to do this:
Not so easy way: Expose the fixed registers through mtrr_add() and
/proc/mtrr. There are no great problems with this, but we would need
to keep a list of "virtual" variable ranges for the 0-1MB area and
translate to the corresponding fixed range settings.
Easy way: Just disable the fixed ranges, so the variable ranges cover
0-1MB. Clearly this isn't appropriate if the BIOS does something with
the fixed ranges other than setting them all to write-back, but I
suspect that no BIOS tries to be that clever. This would mean we use
up a variable range for the VGA region, but I doubt anyone is yet down
to their last one.
I'll have a go at the second option sometime soon.
<rant>
I see that the PAT feature of the Xeon puts MTRR attributes in the page
table entries, which is a far neater solution than this variable/fixed
range stuff. As I understand it, the PPro caches MTRR attributes in
the TLB anyway, so why did Intel come up with this fiddly ranges
scheme in the first place? Ditto with regard to PAE/PSE36.
</rant>
-- Dave Wragg- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.altern.org/andrebalsa/doc/lkml-faq.html