Re: [Linux-ia64] pio barriers

[Jesse Barnes <jbarnes@sgi.com>]

On Mon, Dec 10, 2001 at 03:58:19PM -0800, David Mosberger wrote:
> The comment is wrong, or at least misleading (I wrote it, so hopefully
> nobody is offended... ;-).  mf.a is needed for inX/outX emulation, not
> really for ordering.  Uncached accesses are not re-ordered by the CPU
> and mf will do just fine as far as ordering of cached accesses are
> concerned.

On our platform, uncached accesses can arrive out of orderif they come
from different cpus, so simple spinlock protection isn't quite enough
to guarantee that pios don't arrive out of order.  We'll need a pio
barrier macro and (ideally) an spin_unlock variant that includes it.

> Can you get away with forcing the proper ordering with a dummy-read?
> If so, I suspect that would be preferable as that is the only platform
> independent way to do this (as far as I know).

Do you mean that a driver should do a dummy read following writes that
have to arrive prior to others?  I guess that would be just as
invasive as introducing a pio barrier or another spinlock...

Jesse