From owner-freebsd-hackers  Mon Feb 18 12:51:29 2002
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To: Matthew Dillon <dillon@apollo.backplane.com>
Cc: Mike Smith <msmith@hub.freebsd.org>,
	John Polstra <jdp@polstra.com>, hackers@FreeBSD.ORG
Subject: Re: ACPI timecounter tests original fast version vs masked version 
In-Reply-To: Your message of "Mon, 18 Feb 2002 12:40:59 PST."
             <200202182040.g1IKex936264@apollo.backplane.com> 
Date: Mon, 18 Feb 2002 21:48:18 +0100
Message-ID: <45878.1014065298@critter.freebsd.dk>
From: Poul-Henning Kamp <phk@critter.freebsd.dk>
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In message <200202182040.g1IKex936264@apollo.backplane.com>, Matthew Dillon wri
tes:

>    In anycase, I think this is reasonable especially if we explicitly
>    use the fast code for those chipsets known to be good.  I would like
>    to commit it.  It can handle *ANY* sort of ripple or fast-carry breakage,
>    really any type of breakage since it looks for two identical samples
>    after masking rather then using an inequality.

My only worry here is laptops.

If my laptop is connected to the wall-power when it boots it comes
up at full CPU speed during the probe and mask determination.

Then I unplug it and the CPU speed drops by a large factor and
suddenly the mask is too tight for the CPU to get two samples.

I think we need an approach where we loosen the mask at runtime if
some significant number of iterations fail to get a count may be
needed:

#define N	25

	static unsigned mask = 0xfffffffe;

	for (;;) {
		for (i = 0; i < N; i++) {
			c = READ_TIMER & mask;
			d = READ_TIMER & mask;
			if (c == d)
				return(d);
		}
		printf("Adjusting mask to %08x\n", mask);
		mask <<= 1;
	}


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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