Date: Thu, 18 May 2000 23:20:01 -0700 (PDT) From: Bruce Evans <bde@zeta.org.au> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/18598 Message-ID: <200005190620.XAA35403@freefall.freebsd.org>
next in thread | raw e-mail | index | archive | help
The following reply was made to PR kern/18598; it has been noted by GNATS.
From: Bruce Evans <bde@zeta.org.au>
To: phk@FreeBSD.ORG
Cc: y.grossel@hexanet.fr, freebsd-gnats-submit@FreeBSD.ORG
Subject: Re: kern/18598
Date: Fri, 19 May 2000 16:12:36 +1000 (EST)
On Thu, 18 May 2000 phk@FreeBSD.ORG wrote:
> Synopsis: Incessant messages "microuptime() went backwards"
>
> State-Changed-From-To: open->closed
> State-Changed-By: phk
> State-Changed-When: Thu May 18 15:19:07 PDT 2000
> State-Changed-Why:
> We resolved this in email to be the APM bios changing the frequency
> of the i8254 under our feet. Nothing much we can do in that case.
Um, we can easily make it jump forwards instead of backwards. From
i386/isa/clock.c:
> static u_int
> i8254_get_timecount(struct timecounter *tc)
> {
> u_int count;
^^^^^
> ...
> low = inb(TIMER_CNTR0);
> high = inb(TIMER_CNTR0);
> count = timer0_max_count - ((high << 8) | low);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This sometimes underflows to a large unsigned value if something changes
the actual maximum count to a value smaller than timer0_max_count
underneath us. We really want `count' to be a small negative value instead
of a large positive one.
> if (count < i8254_lastcount ||
^^^^^^^^^^^^^^^^^^^^^^^
Since both variables in the comparison are unsigned, this doesn't detect
the underflow.
Possible workarounds:
1) Used signed ints more and/or
2) replace the large unsigned counts by 0.
3) Whenever a large unsigned count is detected, adjust timer0_max_count
and associated variables to match it, or adjust timer0_max_count to
match the actual hardware max count (which was presumably adjusted
by APM), or adjust the hardware max count to match the original
timer0_max_count.
Bruce
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-bugs" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200005190620.XAA35403>