video drivers, agp, mtrr, and write-combining

[Alice Corbin <ali@axian.com>]

I have a silly question.

I've noticed that some, though no all, video drivers add their video memory
to MTRR as 'write-combining' if both MTRR and AGP are configured in.

Is there a reason that all video drivers don't do this?  Is it all would 
benefit from write-combining memory, but that some simply haven't been 
updated to take advantage of it?  Or could write-combining actually be 
detrimental to some video drivers?  (And if the former, would it make
more sense to do the mtrr_add in a central location, say agpgart?)

Ali
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