Date: Sun, 17 Feb 2002 15:18:01 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Michael Smith <msmith@FreeBSD.ORG> Cc: Poul-Henning Kamp <phk@critter.freebsd.dk>, Bruce Evans <bde@zeta.org.au>, freebsd-current@FreeBSD.ORG Subject: ACPI timer is screwed... (was Re: 'microuptime() went backwards ...' using ACPI timer. Shouldn't that be impossible? ) Message-ID: <200202172318.g1HNI1I02804@apollo.backplane.com> References: <200202172225.g1HMPKA01422@mass.dis.org>
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:I would like to see "the PIIX problem" caught on camera, personally.
:We're aware of one errata for it already, and we work around it. If
:there's another problem, or ideally if someone has some relatively quick
:code to test it, that would be much better.
Holy shit. We are screwed. It's a free-running counter with NO
synchronization whatsoever. None. Zip. Zero.
ACPI TIMER LSB MISREAD 128ababd 128abaa2 128abaa4
ACPI TIMER LSB MISREAD 128ea0cd 128ea0c9 128ea0cb
ACPI TIMER LSB MISREAD 129427fd 129425ff 12942801
So, for example: ABD AA2 AA4
ABD 101010111101
^ probably should be a 0
AA2 101010100010
AA2 101010100100
Another example: 0CD 0C9 0CB
0CD 000011001101
^ probably should be a 0
0C9 000011001001
0CB 000011001011
It looks like the hardware is using an adder with fast carry
(basically a forward looking carry calculation), which can result in a
later bit being set before the earlier bits are updated. However, if
this is the case then it is almost certain that you can catch the ripple
as well, or even catch the counter while the bit states are changing.
The only way to get a guarenteed accurate sample under these circumstances
is something like this, where you calculate a mask that results in
reasonable accurancy without causing the cpu to go into an infinite
loop and then read the timer until you get two samples that are the same:
u1 = TIMER_READ;
u2 = TIMER_READ;
while ((u1 ^ u2) & 0xFFFFFFF0) { <<<<<<<< mask must be chosen
u1 = u2;
u2 = TIMER_READ;
}
return(u2 & 0xFFFFFFF0) <<<<<<<< same mask here
Here are some more from my debug output:
ACPI TIMER LSB MISREAD 2cb0f97d 2cb0f961 2cb0f963
ACPI TIMER LSB MISREAD 2fad1ced 2fad1ce9 2fad1ceb
ACPI TIMER LSB MISREAD 33ed26ff 33ed2681 33ed2683
ACPI TIMER LSB MISREAD 34184d1d 34184d19 34184d1c
ACPI TIMER LSB MISREAD 344516ff 344516e1 344516e3
ACPI TIMER LSB MISREAD 392b2c6d 392b2c69 392b2c6b
ACPI TIMER LSB MISREAD 39c9073d 39c90739 39c9073b
ACPI TIMER LSB MISREAD 39f3161d 39f3161a 39f3161c
ACPI TIMER LSB MISREAD 3ad04bbf 3ad04bb1 3ad04bb3
ACPI TIMER LSB MISREAD 39f3161d 39f3161a 39f3161c
-Matt
Matthew Dillon
<dillon@backplane.com>
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