RFC on writel and writel_relaxed

[Benjamin Herrenschmidt]

On Tue, 2018-03-27 at 11:44 +0200, Arnd Bergmann wrote:
> > The interesting thing is that we do seem to have a whole LOT of these
> > spurrious wmb before writel all over the tree, I suspect because of
> > that incorrect recommendation in memory-barriers.txt.
> > 
> > We should fix that.
> 
> Maybe the problem is just that it's so counter-intuitive that we don't
> need that barrier in Linux, when the hardware does need one on some
> architectures.
> 
> How about we define a barrier type instruction specifically for this
> purpose, something like wmb_before_mmio() and have all architectures
> define that to an empty macro?

This is exactly what wmb() is about and exactly what Linux rejected
back in the day (and in hindsight I agree with him).

> That way, having correct code using wmb_before_mmio() will not
> trigger an incorrect review comment that leads to extra wmb(). ;-)

Ah, you mean have an empty macro that will always be empty on all
architectures just to fool people ? :-)

Not sure that will fly ... I think we just need to be documenting that
stuff better and not have incorrect examples. Also a sweep to remove
some useless ones like the one in e1000e would help.

Cheers,
Ben.