Re: [patch 1/2] powerpc: rmb fix

quoted

The powerpc kernel needs to have full sync insns in every I/O
accessor in order to enforce all the ordering rules Linux demands.
It's a bloody shame, but the alternative would be to make the
barriers lots more expensive.  A third alternative would be to

Well lots more expensive compared to what you have now. But what
you have now is like having those expensive barriers between
*every* io access.

Yeah.  But I/O reads are very expensive anyway, and the barriers
are used for more than just I/O ordering.

I/O writes are a different thing; ideally, they would use only
eieio, if anything at all.

Maybe the tradeoff isn't optimal.  The I/O primitives didn't have
all those "sync"s in there before, they got added because some bad
interaction with spinlocks was discovered, if my memory isn't failing
me.

quoted

have barrier ops that do not order everything, but just A vs. B
for various choices of A and B (coherent accesses, MMIO accesses,
etc.)

The non-smp_ variant is supposed to order everything, AFAIK. Maybe
you could get more fancy and have PIO vs MMIO etc etc. but it looks
like this whole area is in a pretty sticky state anyway so let's
not think about that.

*Thinking* about it is fun.  Trying to get the code merged would be
a different thing ;-)


Segher