From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Wed, 29 Nov 2006 09:57:24 +1100

quoted

This looks mostly fine.

I was thinking about the lockless stuff, and I wonder if there
is a clever way you can get it back down to one PIO on the
GREG_STAT register.

I think you'd need to have the ->poll() clear gp->status, then
do a smp_wb(), right before it re-enables interrupts.

Then in the interrupt handler, you need to find a way to safely
OR-in any unset bits in gp->status in a race-free manner.

Having it atomic might work at a slightly smaller cost than a lock,
though atomics don't have strong ordering requirements so you'd still
have to be a bit careful.

At least in theory the atomic + any necessary memory barriers
would be cheaper than the extra PIO read we need otherwise.