Merge remote-tracking branch 'asoc/fix/intel' into asoc-linus

[sfrench/cifs-2.6.git] / arch / arm / include / asm / delay.h

diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
index b1ce037e4380dcea92f6774c0f7dc1ad9c9569f3..e986b7f717c4159f537e0d34b0abc1ae2d01b746 100644 (file)
--- a/arch/arm/include/asm/delay.h
+++ b/arch/arm/include/asm/delay.h
@@ -9,6 +9,33 @@
 #include <asm/memory.h>
 #include <asm/param.h> /* HZ */
 
+/*
+ * Loop (or tick) based delay:
+ *
+ * loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec
+ *
+ * where:
+ *
+ * jiffies_per_sec = HZ
+ * us_per_sec = 1000000
+ *
+ * Therefore the constant part is HZ / 1000000 which is a small
+ * fractional number. To make this usable with integer math, we
+ * scale up this constant by 2^31, perform the actual multiplication,
+ * and scale the result back down by 2^31 with a simple shift:
+ *
+ * loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31
+ *
+ * where:
+ *
+ * UDELAY_MULT = 2^31 * HZ / 1000000
+ *             = (2^31 / 1000000) * HZ
+ *             = 2147.483648 * HZ
+ *             = 2147 * HZ + 483648 * HZ / 1000000
+ *
+ * 31 is the biggest scale shift value that won't overflow 32 bits for
+ * delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000.
+ */
 #define MAX_UDELAY_MS  2
 #define UDELAY_MULT    UL(2147 * HZ + 483648 * HZ / 1000000)
 #define UDELAY_SHIFT   31