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

From: Segher Boessenkool <[email-masked]>
Date: Wed, 22 Aug 2007 05:29:50 +0200

>>> If this isn't causing any problems maybe there
>>> is some loigic we are overlooking?
>>
>> The I/O accessor functions enforce the necessary ordering
>> already I believe.
>
> Ah, it looks like you might be right, IO should appear to go in-order, 
> in
> which case the rmb() would simply need to order cacheable loads. 
> Interesting
> way to do things... are drivers simply not up to scratch enough to 
> allow
> out of order IO?

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
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.)

> Anyway, this raises another question -- if IO accessors have the right
> ordering, why is wmb() not an lwsync as well? There appears to be many
> more wmb() calls than rmb()...

Input MMIO accessors are {sync, load, stall pipeline until load came 
back}.
That's a full ordering on both sides.

Output MMIO on the other hand is done with {sync, store}.  Now since
wmb() has to order MMIO writes vs. main memory writes, we need a full
sync here.  On some (most, all?) CPUs an eieio is actually enough btw.
The barrier insn could be put at the end of all MMIO write ops too,
but I believe that would be more expensive (in execution time; in code
size it definitely would be, of course).

Segher