Re: apic and 8254 wraparound ...

[Herbert Poetzl]

On Sun, Dec 26, 2004 at 07:08:06PM +0100, Pavel Machek wrote:
> Hi!
> 
> > > When you read one 8bit value from an 8254 timer the values latch for
> > > read so that when you read the other half of the 16bit value you get the
> > > value from the moment of the first read. On 
> > > neptune that didn't work right so you got halves of two differing
> > > samples. That means the error would be worst case a bit under 300 (257
> > > for the wrap + a few for timing)
> > 
> > okay, I still wasn't able to find the documentation 
> > at the intel site, but I could extrapolate the issue
> > from your explanation (thanks by the way)
> > 
> > get_8254_timer_count() reads lo byte first, then the 
> > high byte, so assuming that the latch doesn't work
> > as expected on intel 430 NX and LX chipsets, can 
> > result in the following type of error:
> > 
> > counter >= 2^8 * N, 	LO is read (for example 0)
> > counter is decremented
> > counter <  2^8 * N  	HI is read (N - 1)
> > 
> > so the read value will be exactly 2^8 lower than
> > expected (assumed that the counter doesn't do more
> > than 256 counts between the two inb_p()s)
> > 
> > second the wrap-around will always happen _after_
> > the counter reached zero, so we can further assume
> > that the prev_count, has to be lower than 2^8, when
> > we observe a wraparound (otherwise we don't care)
> > 
> > let's further assume the counter does not decrement
> > more than 2^7 between two consecutive gets, then we
> > can change the wraparound check to something like
> > this:
> > 
> >         curr_count = get_8254_timer_count();
> > 
> > 	do {
> >         	prev_count = curr_count;
> > 	redo:
> >         	curr_count = get_8254_timer_count();
> > 
> > 		/* workaround for broken Mercury/Neptune */
> > 		if (prev_count - current_count >= 256)
> > 	    		goto redo;
> > 
> > 		/* ignore values far off from zero */
> >     		if (prev_count > 128)
> > 	    		continue;
> > 
> > 	} while (prev_count >= curr_count)
> > 
> > 
> > basically the check for (prev_count > 128) can be
> > removed but it feels a little more comfortable ...
> > 
> > would such change be acceptable for mainline?
> 
> Not sure... Reading time is quite performance critical; doing it twice
> would be bad. It should be acceptable if it was only done on
> Mercury/Neptune systems.

this important detail got lost over the thread

static void __init wait_8254_wraparound(void)

so I guess it should not be _too_ critical ;)

best,
Herbert

> -- 
> People were complaining that M$ turns users into beta-testers...
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