On Tue, Mar 27, 2018 at 08:22:55AM -0400, okaya@xxxxxxxxxxxxxx wrote: > 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. Okay, but before you do that, can we get a statement how this works for WC? Some of these writels are to WC memory, do they need the wmb()?!? Jason -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
