m68k: Implement ndelay() based on the existing udelay() logic

Add a ndelay macro modeled after the Coldfire udelay(). The ISP1160
driver needs a 150ns delay, so we need to have ndelay().
Signed-off-by: NMichael Schmitz <schmitz@debian.org>
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/delay.h

@@ -92,5 +92,28 @@ static inline void __udelay(unsigned long usecs)
 #define udelay(n) (__builtin_constant_p(n) ? \
 	((n) > 20000 ? __bad_udelay() : __const_udelay(n)) : __udelay(n))
 
+/*
+ * nanosecond delay:
+ *
+ * ((((HZSCALE) >> 11) * (loops_per_jiffy >> 11)) >> 6) is the number of loops
+ * per microsecond
+ *
+ * 1000 / ((((HZSCALE) >> 11) * (loops_per_jiffy >> 11)) >> 6) is the number of
+ * nanoseconds per loop
+ *
+ * So n / ( 1000 / ((((HZSCALE) >> 11) * (loops_per_jiffy >> 11)) >> 6) ) would
+ * be the number of loops for n nanoseconds
+ */
+
+/*
+ * The simpler m68k and ColdFire processors do not have a 32*32->64
+ * multiply instruction. So we need to handle them a little differently.
+ * We use a bit of shifting and a single 32*32->32 multiply to get close.
+ * This is a macro so that the const version can factor out the first
+ * multiply and shift.
+ */
+#define	HZSCALE		(268435456 / (1000000 / HZ))
+
+#define ndelay(n) __delay(DIV_ROUND_UP((n) * ((((HZSCALE) >> 11) * (loops_per_jiffy >> 11)) >> 6), 1000));
 
 #endif /* defined(_M68K_DELAY_H) */