RFC on writel and writel_relaxed

[Benjamin Herrenschmidt]

On Tue, 2018-03-27 at 20:26 -1000, Linus Torvalds wrote:
> On Tue, Mar 27, 2018 at 6:33 PM, Benjamin Herrenschmidt
> <benh at 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.