Re: Memory mapped IO vs Port IO

[Anthony Dominic Truong]

On Thu, 2003-09-11 at 10:25, Andi Kleen wrote:
> On Thu, 11 Sep 2003 18:12:05 +0100
> Jamie Lokier <jamie@xxxxxxxxxxxxx> 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
> 
Hello,
Wouldn't it be better if we set the IN and OUT function pointers to the
right functions during driver init. based on the setting of dev->mmio.
And throughout the driver, we just call the IN and OUT functions by
their pointers.  Then we don't have to do if (dev->mmio) every time.
It's similar to the concept of virtual member function in C++.

ADT

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