RFC on writel and writel_relaxed

[Benjamin Herrenschmidt]

On Wed, 2018-03-28 at 06:53 +0000, Linus Torvalds wrote:
> 
> 
> On Tue, Mar 27, 2018, 20:43 Benjamin Herrenschmidt <benh at kernel.crash
> ing.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).

All true indeed, though a lot of other archs never quite made them
fully synchronous, which was another can of worms ... oh well.

As for Adaptec, you might be right, I do remember having cases of old
stuff triggering DMA on reads, it might have been "Mac" variants of
Adaptec using MMIO or something...

Cheers,
Ben.