Date: Thu, 11 Sep 2003 19:25:50 +0200
From: Andi Kleen <ak@suse.de>
To: Jamie Lokier <jamie@shareable.org>
Cc: willy@debian.org, linux-kernel@vger.kernel.org
Subject: Re: Memory mapped IO vs Port IO
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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

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