MMIO and gcc re-ordering issue

[Benjamin Herrenschmidt]

On Wed, 2008-06-11 at 13:29 +1000, Nick Piggin wrote:
> 
> Exactly, yes. I guess everybody has had good intentions here, but
> as noticed, what is lacking is coordination and documentation.
> 
> You mention strong ordering WRT spin_unlock, which suggests that
> you would prefer to take option #2 (the current powerpc one): io/io
> is ordered and io is contained inside spinlocks, but io/cacheable
> in general is not ordered.

IO/cacheable -is- ordered on powepc in what we believe is the direction
that matter: IO reads are fully ordered vs. anything and IO writes are
ordered vs. previous cacheable stores. The only "relaxed" situation is
IO writes followed by cacheable stores, which I believe shouldn't be
a problem. (except for spinlocks for which we use the flag trick)

> I *would* prefer that io/cacheable actually is strongly ordered with
> the default accessors. Because if you have that, then the driver
> writer never has to care about memory ordering, provided they use
> correct locking for SMP issues. Same as x86. With option 2, there
> are still windows where you could possibly have issues.
> 
> For any high performance drivers that are well maintained (ie. the
> ones where slowdown might be noticed), everyone should have a pretty
> good handle on memory ordering requirements, so it shouldn't take
> long to go through and convert them to relaxed accessors.

Ben.