Re: Memory mapped IO vs Port IO

Andi Kleen
2003-09-11T17:33:30Z

On Thu, 11 Sep 2003 18:12:05 +0100
Jamie Lokier <jamie@shareable.org> wrote:

> Andi Kleen wrote:
> > Even a memory write is tens to hundres of cycles.
>
> Not from the CPU's perspective. It is done in parallel with other
> instructions.

Only when there are more instructions to execute. But device
driver code often does a following read e.g. to check if it can submit
another request to the hardware.

My claim is basically:

Change everybody who currently does

#ifdef CONFIG_MMIO
writel(... )
readl(...)
#else
outl( ... )
inl ( ...)
#endif

to
if (dev->mmio) {
writel();
real();
} else {
outl();
inl();
}

and you will have a hard time to benchmark the difference on any non ancient system
in actual driver operation.

-Andi