PCI drivers - memory mapped vs. I/O ports

PCI drivers - memory mapped vs. I/O ports

[Ed Vance]

If a PCI device can be programmed equally well via I/O port space or memory
space, what are the reasons to chose one space over the other when writing
the driver?

---------------------------------------------------------------- 
Ed Vance              edv@macrolink.com
Macrolink, Inc.       1500 N. Kellogg Dr  Anaheim, CA  92807
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Re: PCI drivers - memory mapped vs. I/O ports

[Alan Cox]

> If a PCI device can be programmed equally well via I/O port space or memory
> space, what are the reasons to chose one space over the other when writing
> the driver?

mmio is posted on a PC i/o ports are not. That means you have to be more
careful but also means that

	write
	write
	write
	read

is a lot faster

Re: PCI drivers - memory mapped vs. I/O ports

[Richard B. Johnson]

On Mon, 18 Mar 2002, Ed Vance wrote:

> If a PCI device can be programmed equally well via I/O port space or memory
> space, what are the reasons to chose one space over the other when writing
> the driver?
> 

o       There is more 'memory' I/O space. Therefore you are more likely
        to have enough space for your device when other boards are
        installed. IOTW, you may find that there is no I/O space allocated
        because there isn't any available.

o       Memory address space I/O is faster on Intel Machines. You can
        write or read whole buffers of I/O space in memory address space.
        the best you can do in port space is one long-word at a time.

o       PCI posted writes work when accessing memory-address I/O space.
        Port I/O space cannot do this. The FIFO is written immediately
        before any other access is allowed. This improves the access
        speed in certain circumstances.

Basically, if you have a choice, it's hands-down to use memory-mapped
I/O space.

Cheers,
Dick Johnson

Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).

                 Windows-2000/Professional isn't.

Re: PCI drivers - memory mapped vs. I/O ports

[Jeff Garzik]

Richard B. Johnson wrote:

>On Mon, 18 Mar 2002, Ed Vance wrote:
>
>>If a PCI device can be programmed equally well via I/O port space or memory
>>space, what are the reasons to chose one space over the other when writing
>>the driver?
>>

>Basically, if you have a choice, it's hands-down to use memory-mapped
>I/O space.
>

Yep, I couldn't agree more.

    Jeff