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

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Message-Id: <E1JBxnV-00046d-CS@be1.7eggert.dyndns.org>
Date:	Mon, 07 Jan 2008 20:38:09 +0100
From:	Bodo Eggert <7eggert@....de>
To:	Christer Weinigel <christer@...nigel.se>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"David P. Reed" <dpreed@...d.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Rene Herman <rene.herman@...il.com>,
	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.

Christer Weinigel <christer@...nigel.se> wrote:

> How do you find out the speed of the ISA bus?  AFAIK there is no
> standardized way to do that.  On the Geode SC2200 the ISA bus speed is
> usually the PCI clock divided by 4 giving 33MHz/4=8.3MHz or
> 30/4=7.5MHz, but with no external ISA devices it's possible to
> overclock the ISA bus to /3 to run it at 11MHz or so.  But without
> poking at some CPU and southbridge specific registers to find out the
> PCI bus speed and the ISA bus divisor you can't really tell.

If you overclock, you are on your own. IIRC I've used 13,3 MHz for some time
and used a lower PIO mode to compensate.

> So if you do udelay based on a 6MHz clock (I think you can safely
> assume that any 386 based system runs the ISA bus at least that fast)
> you'll waste at least 30% and maybe even 100% more time for the delay
> after every _p call.

Defaulting to 8 MHz and offering an option to set another clock speed
(like idebus=) should be OK.

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