On Wed, 2018-03-28 at 06:53 +0000, Linus Torvalds wrote:

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

quoted

quoted

Of course, you'd have to be pretty odd to want to start a DMA

with a

quoted

read anyway - partly exactly because it's bad for performance

since

quoted

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.