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

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Date:	Tue, 1 Jan 2008 21:07:34 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"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.

> > #1 udelay has to be for the worst case bus clock (6MHz) while the 
> > #device may be at 10Mhz or even 12MHz ISA. So it slows it down stuff
> > unneccessarily- and stuff that really really is slow enough as is.
> 
> udelay is supposed to be reliable. If someone runs a new kernel and has 
> no TSC (which might happen even on modern hardware or with notsc) _and_ 
> finds that udelay is not calibrated well enough then that's a kernel bug 
> we want to fix.

You miss the point entirely. The delay is in bus clocks not CPU clocks,
not tsc clocks not PIT clocks, and it is permitted to vary by a factor of
two. So you'll worst case halve the speed of network packet up/download
even if your udelay is accurate.

> > #2 Most of the ancient wind up relics with ISA bus don't have a tsc so
> > their udelay value is kind of iffy.
> 
> iffy in what way? Again, we might be hiding real udelay bugs.

As you say - its only a few instructions so small udelays tend to be
inaccurate - overlong.

> yes, there are always risks in changing something, but using udelay is a 
> common-sense consolidation of code.

Not for ISA bus hardware. For chipset logic, for PCI yes - for ISA stuff
no. It's all about ISA clocks not wall clocks.

Alan
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