On Wed, 28 Mar 2018 11:55:09 -0400 (EDT) David Miller <davem@xxxxxxxxxxxxx> wrote: > From: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> > Date: Thu, 29 Mar 2018 02:13:16 +1100 > > > Let's fix all archs, it's way easier than fixing all drivers. Half of > > the archs are unused or dead anyway. > > Agreed. While we're making decrees here, can we do something about mmiowb? The semantics are basically indecipherable. 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. Thanks, Nick -- 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
