Re: Memory mapped IO vs Port IO

[Andi Kleen <ak@suse.de>]

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