RFC on writel and writel_relaxed

[Linus Torvalds]

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.

> 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).

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).

                   Linus