MMIO and gcc re-ordering issue

[Benjamin Herrenschmidt]

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