While experimenting with framebuffer access performances, we noticed a very significant improvement in write access to it when not setting the "guarded" bit on the MMU mappings. This bit basically says that reads and writes won't have side effects (it allows speculation).
Unless the data is already in cache.
It appears that it also disables write combining.
When the page is also cache-inhibited, it indeed does.
Btw, did you ever get to fix the problem with mapping the last page of physical address space via /dev/mem ?
Segher
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