On Fri, 2015-06-26 at 21:31 +0200, Luis R. Rodriguez wrote:

quoted

Yeah either, we need to fix our relaxed implementation (patch
welcome :-)

Yikes, so although powerpc has useful heuristics to automatically
enable WC the default write ops have been nullifying its effects?

The heuristic is for userspace mapping which don't use the kernel
accessors. To cut a long story short, this was all done back in the day
to speed up X.org which doesn't use fancy accessors with barriers to
access the framebuffer (neither does the kernel fbdev layer).

Shouldn't this have been blatantly obvious in performance benchmarks
and expectations? 

If not a change should give considerable performance
gains... If the WC was nullified on powerpc then the experience and
value of the heuristics have less value and even if they are going
to be only considered for userspace mmap() it should be taken with
a bit grain of salt it seems.

It wasn't nullified for the main user at the time, the fb. And I
mentioned an IB adapter or two for which the code had been hand tuned.

Cheers,
Ben.