On Tue, 2018-03-27 at 20:26 -1000, Linus Torvalds wrote: > On Tue, Mar 27, 2018 at 6:33 PM, Benjamin Herrenschmidt > <b...@kernel.crashing.org> wrote: > > > > This is why, I want (with your agreement) to define clearly and once > > and for all, that the Linux semantics of writel are that it is ordered > > with previous writes to coherent memory (*) > > Honestly, I think those are the sane semantics. In fact, make it > "ordered with previous writes" full stop, since it's not only ordered > wrt previous writes to memory, but also previous writel's.
Of course. It was somewhat a given that it's ordered vs. any previous MMIO actually, but it doesn't hurt to spell it out once more. > > Also, can I assume the above ordering with writel() equally applies to > > readl() or not ? > > > > IE: > > dma_buf->foo = 1; > > readl(STUPID_DEVICE_DMA_KICK_ON_READ); > > If that KICK_ON_READ is UC, then that's definitely the case. And > honestly, status registers like that really should always be UC. > > But if somebody sets the area WC (which is crazy), then I think it > might be at least debatable. x86 semantics does allow reads to be done > before previous writes (or, put another way, writes to be buffered - > the buffers are ordered so writes don't get re-ordered, but reads can > happen during the buffering). Right, for now I worry about UC semantics. Once we have nailed that, we can look at WC, which is a lot more tricky as archs differs more widely, but one thing at a time. > But UC accesses are always done entirely ordered, and honestly, any > status register that starts a DMA would not make sense any other way. > > Of course, you'd have to be pretty odd to want to start a DMA with a > read anyway - partly exactly because it's bad for performance since > reads will be synchronous and not buffered like a write). I have bad memories of old adaptec controllers ... That said, I think the above might not be right on ARM if we want to make it the rule, Will, what do you reckon ? Cheers, Ben.