On Tue, 2008-05-27 at 08:50 -0700, Roland Dreier wrote:

quoted

Though it's my understanding that at least ia64 does require the

 > explicit barriers anyway, so we are still in a dodgy situation here
 > where it's not clear what drivers should do and we end up with
 > possibly excessive barriers on powerpc where I end up with both
 > the wmb/rmb/mb that were added for ia64 -and- the ones I have in
 > readl/writel to make them look synchronous... Not nice.

ia64 is a disaster with a slightly different ordering problem -- the
mmiowb() issue.  I know Ben knows far too much about this, but for big
SGI boxes, you sometimes need mmiowb() to avoid problems with driver
code that does totally sane stuff like

This is a different issue. We deal with it on powerpc by having writel
set a per-cpu flag and spin_unlock() test it, and do the barrier if
needed there.

However, drivers such as e1000 -also- have a wmb() between filling the
ring buffer and kicking the DMA with MMIO, with a comment about this
being needed for ia64 relaxed ordering.

Ben.