linux-kernel - Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.

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Date:	Tue, 01 Jan 2008 13:16:49 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"David P. Reed" <dpreed@...d.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Rene Herman <rene.herman@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Rolland <rol@...917.net>,
	Pavel Machek <pavel@....cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	rol@...be.net
Subject: Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.

David P. Reed wrote:
> Alan, thank you for the pointers.  I have been doing variations on this 
> testing theme for a while - I get intrigued by a good debugging 
> challenge, and after all it's my machine...
> 
> Two relevant new data points, and then some more suggestions:
> 
> 1. It appears to be a real port.  SMI traps are not happening in the 
> normal outb to 80.  Hundreds of them execute perfectly with the expected 
> instruction counts.  If I can trace the particular event that creates 
> the hard freeze (getting really creative, here) and stop before the 
> freeze disables the entire computer, I will.  That may be an SMI, or 
> perhaps any other kind of interrupt or exception.  Maybe someone knows 
> how to safely trace through an impending SMI while doing printk's or 
> something?
> 
> 2. It appears to be the standard POST diagnostic port.  On a whim, I 
> disassembled my DSDT code, and studied it more closely.   It turns out 
> that there are a bunch of "Store(..., DBUG)" instructions scattered 
> throughout, and when you look at what DBUG is defined as, it is defined 
> as an IO Port at IO address DBGP, which is a 1-byte value = 0x80.  So 
> the ACPI BIOS thinks it has something to do with debugging.   There's a 
> little strangeness here, however, because the value sent to the port 
> occasionally has something to do with arguments to the ACPI operations 
> relating to sleep and wakeup ...  could just be that those arguments are 
> distinctive.
> 

Dumb question: if you change your iodelay function so it always writes 
zero to port 0x80, does it start working?

	-hpa
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