Re: RFC on writel and writel_relaxed

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Tue, Mar 27, 2018, 20:43 Benjamin Herrenschmidt <benh@kernel.crashing.org>
wrote:

> >
> > 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 ...
>

*Old* adaptec controllers were likely to use the in/out instructions for
status and command data.

Those are actually even more ordered than UC reads and writes: the in/out
instructions are not just fully ordered, but are fully *synchronous* on
x86.

So not just doing accesses in order, but actually waiting for everything to
drain before they start executing, but they also wait for the operation
itself to complete (ie "out" will not just queue the write, it will then
wait for the queue to empty and the write data to hit the line).

That's why in/out were *so* slow, and why nobody uses them any more (well,
the address size limitations and the lack of any remapping of the address
obviously also are a reason).

    Linus

>

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