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

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Message-ID: <475EEC10.8020300@keyaccess.nl>
Date:	Tue, 11 Dec 2007 20:59:12 +0100
From:	Rene Herman <rene.herman@...access.nl>
To:	Pavel Machek <pavel@....cz>
CC:	Andi Kleen <andi@...stfloor.org>,
	"linux-os (Dick Johnson)" <linux-os@...logic.com>,
	David Newall <david@...idnewall.com>,
	Paul Rolland <rol@...917.net>,
	"H. Peter Anvin" <hpa@...or.com>, Krzysztof Halasa <khc@...waw.pl>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"David P. Reed" <dpreed@...d.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, rol@...be.net
Subject: Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64
 with MCP51 laptops

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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