From owner-freebsd-stable  Thu May 25 13:38: 2 2000
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From: "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net>
Message-Id: <200005252037.NAA70216@gndrsh.dnsmgr.net>
Subject: Re: 4-stable won't boot
In-Reply-To: <200005251840.LAA29149@freeway.dcfinc.com> from "Chad R. Larson" at "May 25, 2000 11:40:48 am"
To: chad@DCFinc.com
Date: Thu, 25 May 2000 13:37:23 -0700 (PDT)
Cc: vallo@matti.ee, msmith@FreeBSD.ORG, steve@sse0691.bri.hp.com,
	stable@FreeBSD.ORG
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> > > Did you take all of them out at once?  The AUTO_EOI options are BAD and 
> > > EVIL and we should _not_ be letting people use them.
> > 
> > Then please document it in the LINT, I remember that on the first time I
> > did custom kernel those EOI options seemed quite useful to include in
> > kernel config. After a while following the lists they didn't seem so...
> 
> As with most things, absolutes are rare.
> 
> My kernel config files on my 2.2.x systems have contained
> 
>     options		"AUTO_EOI_1"
>     options		"AUTO_EOI_2"
> 
> for ever, without problem.
> 
> What makes that e-vile?

Early versions of the 8259 (pre A version) and clones that copied
that broken design.  See Lint for some details:

# AUTO_EOI_1 enables the `automatic EOI' feature for the master 8259A
# interrupt controller.  This saves about 0.7-1.25 usec for each interrupt.
# This option breaks suspend/resume on some portables.
#
# AUTO_EOI_2 enables the `automatic EOI' feature for the slave 8259A
# interrupt controller.  This saves about 0.7-1.25 usec for each interrupt.
# Automatic EOI is documented not to work for for the slave with the
# original i8259A, but it works for some clones and some integrated  
# versions.

I've seen lots of modern hardware that works fine with AUTO_EOI_1, but
I don't dare turn ont AUTO_EOI_2 unless you like to crash and burn.

It is probably possible, but non-trivial, to write code that can detect
if the 8259 does AUTO EOI correctly and disable this if it doesn't.  But
like I said, non-trivial.

-- 
Rod Grimes - KD7CAX @ CN85sl - (RWG25)               rgrimes@gndrsh.dnsmgr.net


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