Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64 with MCP51 laptops

[Rene Herman <rene.herman@keyaccess.nl>]

On 11-12-07 20:16, Pavel Machek wrote:

> > Pavel Machek already posted one. His udelay(8) wants to be less -- 1 or "to 
> > be safe" perhaps 2.
> >
> > http://lkml.org/lkml/2007/12/9/131
>
> 2 at least; that's how long outb(0x80) takes on one of my
> machines. Actually, ISA can go down to 4MHz, so maybe we should be
> using 4 usec.... but I guess I'm paranoid here.

4 isn't sensible. There have been machines capable both of running Linux and 
their ISA bus at less than 8 MHz (if only for example by picking a 5 divisor 
on a system that was capable of hosting a 40 Mhz 386/486 but using a slower 
CPU) but not by much. And machines doing that and running Linux, even more 
so "today": 0.

My posted test program (although there seems to be something wrong with it 
since it's influenced by compiler optimisation) is showing more than 1 but 
note that on the vast majority of machines, 0 would in fact do. 1 will on 
all, 2 will as well.

Rene.

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