On Mon, Mar 26, 2018 at 09:44:15PM +0200, Arnd Bergmann wrote: > On Mon, Mar 26, 2018 at 6:54 PM, Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > On Mon, Mar 26, 2018 at 11:08:45AM +0000, David Laight wrote: > >> > > This is a super performance critical operation for most drivers and > >> > > directly impacts network performance. > >> > >> Perhaps there ought to be writel_nobarrier() (etc) that never contain > >> any barriers at all. > >> This might mean that they are always just the memory operation, > >> but it would make it more obvious what the driver was doing. > > > > I think that is what writel_relaxed is supposed to be. > > > > The only restriction it has is that the writes to a single device > > using UC memory must be kept in program order.. > > Not sure about whether we have ever defined what happens to > writel_relaxed() on WC memory though: On ARM, we disallow > the compiler to combine writes, but the CPU still might. If the driver uses WC memory then I think it should not expect anything in terms of how writes map to TLPs other than nothing combines across mmiowb() and mmiowb() is fully globally ordered when enclosed in a spinlock. The entire point of using WC memory is usually to get combining :) If the driver doesn't want that then it should map UC.. > It's also not entirely clear to me what we want writel() inside a > spinlock to mean: should the spinlock guarantee that two writel() > calls on different CPUs that are protected by spinlocks are > serialized by those locks, or not? Yes for writel, I think that is already defined by the barriers document The same document says that _relaxed() does not give that guarentee. The lwn articule on this went into some depth on the interaction with spinlocks. As far as I can see, containment in a spinlock seems to be the only different between writel and writel_relaxed.. 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
