add 1 in __const_udelay() [Was: Re: Too much error in __const_udelay() ?]

The "mull" instruction in __const_udelay() cuts off the lower 32 bits -- so, it is "rounding down". This is both an issue for small ndelay()s for _all_ values for loops_per_jiffy and for certain {n,u}delay()s for many loops_per_jiffy values.

Assuming

LPJ = 1501115

udelay(87)

results in

130597 loops to be spent.

However, 1000 * 130597 / 1501115 is 86.999997 us, so we're actually _rounding down_. 1000 * 130598 / 1501115 is 87.000662841, which would be the technically correct thing to do. Of course, for the TSC case this won't matter as the maths take some time, so the actual delay is

1000 * __udelay(x) / lpj + __OVERHEAD(x)

Anybody worried about both the additional overhead and the fact that the overhead takes some time to run should add a check

        if (unlikely(xloops < OVERHEAD))
                return;
        xloops -= OVERHEAD;

to the delay() routines in arch/i386/kernel/timers/*.c and determine what the OVERHEAD is.

diff -ruN linux-original/arch/i386/lib/delay.c linux/arch/i386/lib/delay.c
@@ -35,7 +35,7 @@
 	__asm__("mull %0"
 		:"=d" (xloops), "=&a" (d0)
 		:"1" (xloops),"0" (current_cpu_data.loops_per_jiffy * (HZ/4)));
-        __delay(xloops);
+        __delay(++xloops);
 }
 
 void __udelay(unsigned long usecs)