Re: Strange macros set HZ value for timer channel zero

[george anzinger <george@mvista.com>]

george anzinger wrote:
> 
> "Richard B. Johnson" wrote:
> >
> > To whomever maintains the timer code, greetings.
> >
> > When using Linux on the AMD SC520 chip, the system time will
> > not be correct because the PIT clock is 1.1882 MHz instead of
> > the usual 1.19318 MHz. Therefore, I put a conditional value
> > in ../linux/include/asm/timex.h .
> >
> > #ifndef _ASMi386_TIMEX_H
> > #define _ASMi386_TIMEX_H
> >
> > #include <linux/config.h>
> > #include <asm/msr.h>
> > #ifdef SC520
> > #define CLOCK_TICK_RATE 1188200 /* Underlying HZ */
> > #else
> > #define CLOCK_TICK_RATE 1193180 /* Underlying HZ */
> > #endif
> >
> > Something is wrong! I now gain 3 hours in a 12 hour period. There
> > are some calculations performed somewhere that result in the
> > wrong divisor for the timer (PIT). I don't understand any of the
> > SHIFT stuff, nor FINE_TUNE stuff. It all seems bogus although
> > it might be the "new math" that's biting me.
> >
> > The correct value for 100 Hz should be 1188200/100 = 11882 = 0x2e6a
> > for the divisor. If I hard-code the value as a divisor in
> > /usr/src/linux/arch/i386/kernel/i8259.c,  it works. If I use the
> > #defines and macros in the headers, it doesn't.
> >
> Seems hard to believe.  Try this:
> 
> cd ../arch/i386/kernel
> gcc -D__KERNEL__ -I/usr/src/linux-2.4.7-hr/include -Wall
> -Wstrict-prototypes -Wno-trigraphs -O2 -fomit-frame-pointer
> -fno-strict-aliasing -fno-common -pipe  -march=i586    -c -o i8259.o
> i8259.c -E
> 
> (note the -E, the rest of the line is just cut from the make output, I
> usually make from a shell in emacs so all this output is available...
> If you are cutting this line from this message, make sure you change the
> -I path to match your kernel location.)
> 
> now exit i8259.c and search for your new number, i.e. 118820.  (Or if
      ^^^^ gosh, that should be "edit", don't know who grabbed my
fingers there...
> that fails search for "init_IRQ" and look at the outb_p in there.)  For
> the old 1192180 I find:
> 
>         outb_p(0x34,0x43);
>         outb_p(((1193180  + 100 /2) / 100 )  & 0xff , 0x40);
>         outb(((1193180  + 100 /2) / 100 )  >> 8 , 0x40);
> 
> Seems like the new number should do just what you need here.  The
> calibration in time.c is a little wonky, but that will not give the
> errors you are seeing, nor would your change of LATCH (I assume this is
> what you changed in i8259.c) affect that wonky stuff.
> 
> Oh, by the way, you will want to purge i8259.o as make will assume it is
> up to date and try to merge it as a *.o file, which will fail rather
> badly.
> 
> George
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