On Wed, Mar 28, 2018 at 09:01:27PM +1100, Benjamin Herrenschmidt wrote: > On Wed, 2018-03-28 at 11:55 +0200, Arnd Bergmann wrote: > > > powerpc and ARM can't quite make them synchronous I think, but at least > > > they should have the same semantics as writel. > > > > One thing that ARM does IIRC is that it only guarantees to order writel() within > > one device, and the memory mapped PCI I/O space window almost certainly > > counts as a separate device to the CPU. > > That sounds bogus. To elaborate, if you do the following on arm: writel(DEVICE_FOO); writel(DEVICE_BAR); we generally cannot guarantee in which order those accesses will hit the devices even if we add every barrier under the sun. You'd need something in between, specific to DEVICE_FOO (probably a read-back) to really push the first write out. This doesn't sound like it would be that uncommon to me. On the other hand: writel(DEVICE_FOO); writel(DEVICE_FOO); is obviously ordered and also things like: writel(DEVICE_FOO_IN_PCI_MEM_SPACE); writel(DEVICE_BAR_IN_SAME_PCI_MEM_SPACE); are ordered up to the PCI host bridge, because that's really the "device" here. Will -- 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
