On Fri, Jun 26, 2015 at 08:51:58AM +1000, Benjamin Herrenschmidt wrote:

On Thu, 2015-06-25 at 21:40 +0000, Casey Leedom wrote:

quoted

Hhmmm, so what do PowerPC Drivers do when they want to take
advantage of Write Combining?  Do their own Endian Swizzling
with the __raw_*() APIs?

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?
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.

  Luis