On Mon, May 19, 2014 at 02:19:58PM +0100, Arnd Bergmann wrote: > On Friday 16 May 2014 10:53:33 Will Deacon wrote: > > On Thu, May 15, 2014 at 04:55:52PM +0100, Arnd Bergmann wrote: > > > On Thursday 15 May 2014 16:34:30 Will Deacon wrote: > > > > > If this is used to synchronize with a DMA, there is no guarantee that the > > > > > transaction from PCI will be visible in memory by then. > > > > > > > > Can you elaborate on this scenario please? When would we use an I/O space > > > > write to synchronise with a DMA transfer from a PCI endpoint? You're > > > > definitely referring to I/O space as opposed to Configuration Space, right? > > > > > > Correct. Assume a PCI device uses PIO and DMA. It sends a DMA to main memory > > > and lets the CPU know about the data using a level (IntA as opposed to MSI) > > > interrupt. The CPU performs an outl() operation to an I/O port to let the > > > hardware know it has received the IRQ and the response of the outl() is > > > guaranteed to flush the DMA transaction: by the time the outl() completes > > > we know that the data in memory is valid because it is strongly ordered > > > relative to the DMA. > > > > Hmm, when you say `guaranteed to flush the DMA transaction', is that a PCI > > requirement? If so, whether or not that DMA data is then visible to the CPU > > is really specific to the host-controller implementation. It could easily be > > buffered somewhere between the host controller and memory, for example. > > It's something that drivers are supposed to rely on, and the PCI host > already has to make the same guarantee about ordering of MSI, MMIO-read > and DMA transactions, all of which we definitely rely on in the kernel. Supposed to rely on for x86, sure. I don't think we can even give you this guarantee for arm64, where nE is nothing more than a *hint* to the memory subsystem. For the MSI case, I thought we had to go and poke the SCU for the Marvell SoC? > > > outl() actually does a dsb() internally, but unfortunately that is > > > before the store, not after, so I assume that a driver relying on the > > > behavior above would still be racy. > > > > Yup, we'd need an additional dsb. I think we're confusing what the PCI > > specification says about ordering and what the inb/outb instructions provide > > on x86. It may well be that we want to emulate the x86 behaviour on ARM, but > > that's not going to come cheap and I don't think it's a decision we should > > take lightly. > > outb() is expected to be an extremely heavyweight operation, that's why nobody > uses it. Why would you care about whether it's one or two microsecond latency > on a MIDI port or an ISDN adapter? Fair enough -- I just don't think we should dress up an erratum workaround as a bug fix, especially when it's adding a new user of strongly-ordered memory to the kernel which we can't honour for arm64. > We can decide that we don't care about correctness here, but the performance > argument doesn't seem overly important. In this case, I think that's true. Will -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
