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

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Date:	Fri, 18 Jan 2008 00:06:12 +1030
From:	David Newall <davidn@...idnewall.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	"David P. Reed" <dpreed@...d.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Rene Herman <rene.herman@...access.nl>,
	Zachary Amsden <zach@...are.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christer Weinigel <christer@...nigel.se>,
	Ondrej Zary <linux@...nbow-software.org>,
	Bodo Eggert <7eggert@....de>, 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 <rol@...be.net>
Subject: Re: Re: [PATCH] x86: provide a DMI based port 0x80
 I/O delay override.

Alan Cox wrote:
>> This is a timing issue, isn't it?  How are we synchronising, other than
>> by delaying for a (bus-dependant) period?  The characteristics of each
>> bus are known so a number can be assigned for "one bus cycle", without
>> having to use the bus.
>>     
>
> The characteristics of the bus are not known. It could be anything
> between 6 and about 16MHz.

In the early days of clone PCs, as you know but perhaps many on this
list might not, the bus speed could be changed, but this was
user-selectable.  For such a machine, delay values can be pre-calculated
for each bus speed, and a kernel parameter set accordingly.  Or are you
saying that the characteristics of the bus on a given machine vary for
reasons other than user selection?

The fact that busses run at different speeds on different machines is
not a problem because the delay value can be determined for each given
machine.

The question is, for a given machine, can we determine a delay value
instead of using a junk I/O?
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