Re: DECstation 5000/200 timekeeping

"Maciej W. Rozycki" wrote:

> On Tue, 26 Oct 2021, Michael wrote:
>
> > > This really looks to me as though the clock interrupt is low enough
> > > priority to get locked out by SCSI, serial, and/or Ethernet interrupts;
> > > it reminds me of running NetBSD/mac68k, years ago.  Is that accurate?
> > > If so, is that an attribute of the hardware, or is it something that
> > > can be fixed in software?  I'm wondering if it can be fixed or if I'll
> > > just have to give up on decent timekeeping on this hardware.
> > 
> > Hmm, that's an r4400 or somesuch, isn't it?
> > Looking at the code, one difference between powerpc's clock.c and mips'
> > mips3_clockintr.c is that the powerpc code calls hardclock() for every
> > missed tick, while the mips code calls it once per interrupt.
> > Timekeeping on both should otherwise depend on the CPU's cycle counter
> > / decrementer, which both should take a lot more than a minute to
> > overflow.
>
>  Umm, the 5000/200 is R3000-based and has no high-precision timer hardware 
> of any kind available.  The only clocking source available is the DS1287 
> real-time clock, so the clock resolution is based on the RTC's interrupt 
> frequency.  If an RTC interrupt is lost due to its handling being delayed 
> by more than the interrupt's interval, then there's no way to recover.  It 
> also means the clock resolution is very coarse.

Mouse - what timecounter info is printing during boot?  This should give
some details on that the kernel is using for a clock source.  Also, does
"vmstat -i" show anything about missed clock interrupts?  We handle that
for some clock sources.

Both of my 5000/200's fail to power up :(

Cheers,
Simon.