Date: Thu, 11 Sep 2003 18:39:09 -0700
From: jw schultz <jw@pegasys.ws>
To: linux-kernel@vger.kernel.org
Subject: Re: Memory mapped IO vs Port IO
Message-ID: <20030912013909.GG15833@pegasys.ws>
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	linux-kernel@vger.kernel.org
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On Thu, Sep 11, 2003 at 07:25:50PM +0200, Andi Kleen wrote:
> 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.

Shouldn't that be

 	dev->dev_ops->writel(...); 
 	dev->dev_ops->readl(...);

with no conditionals?

-- 
________________________________________________________________
	J.W. Schultz            Pegasystems Technologies
	email address:		jw@pegasys.ws

		Remember Cernan and Schmitt
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