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SubjectRe: [PATCH] Re: time() glitch on 2.4.18: solved
On Tue, 5 Nov 2002, Jim Paris wrote:

> > BTW, why not trying to resync, with something like :
> >
> > if (count >= LATCH)
> > count = (count >> 8) | inb(0x40) << 8;
>
> That's best if we really are out of sync, but it's hard to tell. It
> could be that the 8253's latch value got clobbered somehow, in which
> case we definitely want to fix that or our timer interrupts will come
> out at the wrong rate. We also still need to double-check that
> count < LATCH after your code.
>
> Unless we assume that an unpaired read is more common than having the
> latch value clobbered in some other way, then I think just resetting
> the counter should be okay. Since none of this _should_ ever happen
> (but did on my system, grr), it's probably not worth making it too
> complicated just to try to figure out what went wrong.
>
> Updated patch with the printk and corrected conditional is below.
>
> -jim
>
> diff -urN linux-2.4.18/arch/i386/kernel/time.c linux-2.4.18-jim/arch/i386/kernel/time.c
> --- linux-2.4.18/arch/i386/kernel/time.c Fri Mar 15 18:28:53 2002
> +++ linux-2.4.18-jim/arch/i386/kernel/time.c Tue Nov 5 12:39:38 2002
> @@ -501,6 +501,18 @@
>
> count = inb_p(0x40); /* read the latched count */
> count |= inb(0x40) << 8;
> +
> + /* Any unpaired read will cause the above to swap MSB/LSB
> + forever. Try to detect this and reset the counter. */
> + if (count >= LATCH) {
> + printk(KERN_WARNING
> + "i8253 count too high! resetting..\n");
> + outb_p(0x34, 0x43);
> + outb_p(LATCH & 0xff, 0x40);
> + outb(LATCH >> 8, 0x40);
> + count = LATCH - 1;
> + }
> +
> spin_unlock(&i8253_lock);
>
> count = ((LATCH-1) - count) * TICK_SIZE;
> -

No! You will break many machines. You cannot use out_p() when
writing the latch it __must__ be out(). the "_p" puts a write
to another port between the two writes. That will screw up
the internal state-machine of most PITs including AMD-SC520.

Again, any time you must make two consecutive writes to the
same device port to set it, you must not use the "_p" version.
The above latch setting must be:

outb(LATCH & 0xff, 0x40);
outb(LATCH >> 8, 0x40);

Of course, the second write could have a "_p" option, but
you won't need it.



Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
Bush : The Fourth Reich of America


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