RFC on writel and writel_relaxed
will.deacon at arm.com
Thu Mar 29 20:20:47 AEDT 2018
On Thu, Mar 29, 2018 at 08:31:32AM +1100, Benjamin Herrenschmidt wrote:
> On Thu, 2018-03-29 at 02:23 +1000, Nicholas Piggin wrote:
> > This is a variation on the mandatory write barrier that causes writes to weakly
> > ordered I/O regions to be partially ordered. Its effects may go beyond the
> > CPU->Hardware interface and actually affect the hardware at some level.
> > How can a driver writer possibly get that right?
> > IIRC it was added for some big ia64 system that was really expensive
> > to implement the proper wmb() semantics on. So wmb() semantics were
> > quietly downgraded, then the subsequently broken drivers they cared
> > about were fixed by adding the stronger mmiowb().
> > What should have happened was wmb and writel remained correct, sane, and
> > expensive, and they add an mmio_wmb() to order MMIO stores made by the
> > writel_relaxed accessors, then use that to speed up the few drivers they
> > care about.
> > Now that ia64 doesn't matter too much, can we deprecate mmiowb and just
> > make wmb ordering talk about stores to the device, not to some
> > intermediate stage of the interconnect where it can be subsequently
> > reordered wrt the device? Drivers can be converted back to using wmb
> > or writel gradually.
> I was under the impression that mmiowb was specifically about ordering
> writel's with a subsequent spin_unlock, without it, MMIOs from
> different CPUs (within the same lock) would still arrive OO.
> If that's indeed the case, I would suggest ia64 switches to a similar
> per-cpu flag trick powerpc uses.
... or we could remove ia64.
/me runs for cover
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