On 2018-03-27 07:23, Benjamin Herrenschmidt wrote:
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.
I have been converting wmb+writel to wmb+writel_relaxed. (About 30
patches)
I will have to just remove the wmb and keep writel, then repost.
Some of these got applied. It will cause some churn for the maintainers.
Cheers,
Ben.
