[PATCH 3/3] fix for small xloops [Was: Re: Too much error in __const_udelay() ?]

[Dominik Brodowski]

The const_udelay calculation relies on the "overflow" of the lower 32 bits
of the mull operation. What's in the lower 32 bits is "cut off", so that a
"rounding down" phenomenon exists. For large arguments to {n,u}delay, this does
not matter, as udelay and ndelay round _up_ themselves. However, for small
delays (for cyclone timer: up to 20ns; for pmtmr-based delay timer it's even
up to 1500ns or 1us) it _is_ a critical error. Empirical testing has shown that
it happens only (for usual values of loops_per_jiffies) if xloops is lower or
equal to six. Let's be safe, and double that value, and add one xloop if
xloop is smaller than 13.

Signed-off-by: Dominik Brodowski <linux@xxxxxxxx>

diff -ruN linux-original/arch/i386/lib/delay.c linux/arch/i386/lib/delay.c
--- linux-original/arch/i386/lib/delay.c	2004-06-07 23:02:02.472656160 +0200
+++ linux/arch/i386/lib/delay.c	2004-06-07 22:55:40.063791144 +0200
@@ -34,6 +34,8 @@
 	__asm__("mull %0"
 		:"=d" (xloops), "=&a" (d0)
 		:"1" (xloops),"0" (current_cpu_data.loops_per_jiffy * HZ));
+	if (unlikely(xloops < 13))
+		xloops++;
         __delay(xloops);
 }
 
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