4.8 "Alternate system clock has died" error

John Baldwin jhb at freebsd.org
Tue Nov 22 13:43:33 GMT 2005

On Tuesday 22 November 2005 04:35 am, Uwe Doering wrote:
> Charles Sprickman wrote:
> > On Tue, 22 Nov 2005, Uwe Doering wrote:
> >> Charles Sprickman wrote:
> >>> [...]
> >>> So it certainly looks easy enough for me to change the first two
> >>> sections of the diff referenced above, but I'm having issues finding
> >>> that last one in cpu_initclocks().  It looks like that section really
> >>> has changed quite a bit. (see v.1.206)
> >>
> >> Just look for all instances of
> >>
> >>  writertc(RTC_STATUSB, rtc_statusb);
> >>
> >> and put
> >>
> >>  rtcin(RTC_INTR);
> >> directly behind them (into the next line).  There should be three of
> >> them, in 4.8 as well as RELENG_4 and CURRENT.
> >
> > I found the first two (line 721 and 979) and I see third at line 1064.
> >
> > One other question, looking at the initial patch
> > (http://www.freebsd.org/cgi/query-pr.cgi?pr=17800), I see that they
> > followed a slightly different line:
> >
> >     /* Initialize RTC. */
> >      writertc(RTC_STATUSA, rtc_statusa);
> >      writertc(RTC_STATUSB, RTCSB_24HR);  <<<---
> > +    rtcin(RTC_INTR); /* clear any pending interrupt */
> >
> > Should I worry about that at all?
> No.  User supplied patches in PRs aren't necessarily 100% correct.  In
> this case the PR submitter clears pending interrupts while interrupt
> generation is disabled.  However, the committer of 1.214 (John Baldwin,
> in fact) thought that this is wrong because you have to clear pending
> interrupts after interrupt generation has been re-enabled, or else you'd
> get a race condition.  And I agree with that.

Yeah, put the rtcin() after both writes have been completed.

> >>> Is there any interest in moving this back to 4-STABLE?
> >>
> >> Interest there is, I suppose.  Plenty of people still run 4.x.  The
> >> question is rather whether there is any committer willing to do the
> >> backport.  As far as I can tell, most of them are more focused on
> >> newer branches.  Perhaps we need special backporting committers for
> >> legacy branches.  Just a thought. ;-)
> >
> > Yeah, I work on a total of about 32 boxes, all still at either 4.8 or
> > 4.11.  Committers have to know C, right? :)
> Not only that.  In case of kernel issues they also have to have quite a
> lot of knowledge and experience in kernel hacking.  If you botch a
> kernel patch and don't notice in time you'll likely cause quite a lot of
> grief for the users.

Well, if your box boots ok with it then send me a diff in an e-mail and I'll 
backport it.  Note, however, that there isn't going to be a 4.12 release.

> > Does the RTC supply all timing in a system, or just "time of day"?
> Just the latter (once at boot time) and also the RTC interrupt
> ('stathz').  Perhaps 'profhz' too, if you enable profiling.  Can't tell
> offhand.

Yes, it does provide profhz as well.  When you start profiling a process, we 
reprogram the RTC to interrupt at profhz and divided down to still call 
statclock at stathz (see the fiddling with pscnt and psdiv).

> > How does this line relate to things? kern.clockrate: { hz = 100, tick =
> > 10000, tickadj = 5, profhz = 1024, stathz = 128 }
> > If the RTC doesn't supply the base timing for things like all the I/O
> > busses, processor, what does?
> There's another clock device, which 'hz' is derived of, for example.
> Perhaps one can call it the CPU clock.  It drives the CPU, memory, PCI
> bus etc.  Also, while the system is running it drives the kernel's
> time-of-day clock.  However, the CPU clock's frequency isn't overly
> accurate.  That's why the kernel's time-of-day clock usually deviates
> pretty much from the wall clock unless you apply some form of continuous
> adjustment, for instance NTP.

Actually, the timer for 'hz' is ISA timer #0.  There are three ISA timers, 0, 
1, and 2.  I have no idea if 1 has a common use though I don't think FreeBSD 
uses it.  timer 2 drives the pc speaker, and timer 0 can generate a periodic 
interrupt.  The three timers are in the i8254 "chip" (part of the southbridge 
LPC stuff in modern systems).  On 6.0 and later we do actually use a per-CPU 
timer known as the lapic (short for Local APIC) timer to drive 'hz', 
'stathz', and 'profhz' on systems using an APIC.

John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org

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