Re: readl/writel and memory barriers

[Jesse Barnes <jbarnes@sgi.com>]

On Tue, Feb 19, 2002 at 11:33:22AM -0800, David Mosberger wrote:
> It certainly does for on ia64-compliant system.  Check section 9.3
> "Multi-threaded Code" in the "Itanium Software Conventions and Runtime
> Architecture manual".

I don't have that doc handy, but I'll trust your judgement...

> Now, with NUMA platforms, where the chipsets/switch may re-order
> accesses, the performance hit will be much bigger, so the old scheme
> may not be sufficient.

Right.  I still have to do some performance measurements, but I
suspect that as the system size goes up, we'll see the I/O ordering
penalty increase.  It'll probably get noticable at around 64p.

> I'm no NUMA expert, but my guess is that nobody will want to go
> through all the existing drivers to change them to use mmiob().  For
> new drivers, it might be OK.

The source level impact should actually be pretty small.  An mmiob()
prior to the spin_unlock in a critical section that does I/O usually
suffices.  Maybe it would be a good idea to have io_spin_lock and
io_spin_unlock for this purpose?

Thanks,
Jesse