Re: Memory mapped IO vs Port IO

[Andi Kleen]

On Thu, 11 Sep 2003 17:25:04 +0100
Matthew Wilcox <willy@xxxxxxxxxx> wrote:

> On Thu, Sep 11, 2003 at 06:17:02PM +0200, Andi Kleen wrote:
> > The overhead of checking for PIO vs mmio at runtime in the drivers
> > should be completely in the noise on any non ancient CPU (both MMIO
> > and PIO typically take hundreds or thousands of CPU cycles for the bus
> > access, having an dynamic function call or an if before that is makes
> > no difference at all)
> 
> That's not true for MMIO writes which are posted.  They should take
> no longer than a memory write.  For MMIO reads and PIO reads & writes,
> you are, of course, correct.

Even a memory write is tens to hundres of cycles.

-Andi

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