/*
* We want to do realistic conversions of time so we need to use the same
- * values the update wall clock code uses as the jiffie size. This value
- * is: TICK_NSEC (both of which are defined in timex.h). This
- * is a constant and is in nanoseconds. We will used scaled math and
- * with a scales defined here as SEC_JIFFIE_SC, USEC_JIFFIE_SC and
+ * values the update wall clock code uses as the jiffies size. This value
+ * is: TICK_NSEC (which is defined in timex.h). This
+ * is a constant and is in nanoseconds. We will used scaled math
+ * with a set of scales defined here as SEC_JIFFIE_SC, USEC_JIFFIE_SC and
* NSEC_JIFFIE_SC. Note that these defines contain nothing but
* constants and so are computed at compile time. SHIFT_HZ (computed in
* timex.h) adjusts the scaling for different HZ values.
+
+ * Scaled math??? What is that?
+ *
+ * Scaled math is a way to do integer math on values that would,
+ * otherwise, either overflow, underflow, or cause undesired div
+ * instructions to appear in the execution path. In short, we "scale"
+ * up the operands so they take more bits (more precision, less
+ * underflow), do the desired operation and then "scale" the result back
+ * by the same amount. If we do the scaling by shifting we avoid the
+ * costly mpy and the dastardly div instructions.
+
+ * Suppose, for example, we want to convert from seconds to jiffies
+ * where jiffies is defined in nanoseconds as NSEC_PER_JIFFIE. The
+ * simple math is: jiff = (sec * NSEC_PER_SEC) / NSEC_PER_JIFFIE; We
+ * observe that (NSEC_PER_SEC / NSEC_PER_JIFFIE) is a constant which we
+ * might calculate at compile time, however, the result will only have
+ * about 3-4 bits of precision (less for smaller values of HZ).
+ *
+ * So, we scale as follows:
+ * jiff = (sec) * (NSEC_PER_SEC / NSEC_PER_JIFFIE);
+ * jiff = ((sec) * ((NSEC_PER_SEC * SCALE)/ NSEC_PER_JIFFIE)) / SCALE;
+ * Then we make SCALE a power of two so:
+ * jiff = ((sec) * ((NSEC_PER_SEC << SCALE)/ NSEC_PER_JIFFIE)) >> SCALE;
+ * Now we define:
+ * #define SEC_CONV = ((NSEC_PER_SEC << SCALE)/ NSEC_PER_JIFFIE))
+ * jiff = (sec * SEC_CONV) >> SCALE;
+ *
+ * Often the math we use will expand beyond 32-bits so we tell C how to
+ * do this and pass the 64-bit result of the mpy through the ">> SCALE"
+ * which should take the result back to 32-bits. We want this expansion
+ * to capture as much precision as possible. At the same time we don't
+ * want to overflow so we pick the SCALE to avoid this. In this file,
+ * that means using a different scale for each range of HZ values (as
+ * defined in timex.h).
+ *
+ * For those who want to know, gcc will give a 64-bit result from a "*"
+ * operator if the result is a long long AND at least one of the
+ * operands is cast to long long (usually just prior to the "*" so as
+ * not to confuse it into thinking it really has a 64-bit operand,
+ * which, buy the way, it can do, but it take more code and at least 2
+ * mpys).
+
+ * We also need to be aware that one second in nanoseconds is only a
+ * couple of bits away from overflowing a 32-bit word, so we MUST use
+ * 64-bits to get the full range time in nanoseconds.
+
+ */
+
+/*
+ * Here are the scales we will use. One for seconds, nanoseconds and
+ * microseconds.
+ *
+ * Within the limits of cpp we do a rough cut at the SEC_JIFFIE_SC and
+ * check if the sign bit is set. If not, we bump the shift count by 1.
+ * (Gets an extra bit of precision where we can use it.)
+ * We know it is set for HZ = 1024 and HZ = 100 not for 1000.
+ * Haven't tested others.
+
+ * Limits of cpp (for #if expressions) only long (no long long), but
+ * then we only need the most signicant bit.
+ */
+
+#define SEC_JIFFIE_SC (31 - SHIFT_HZ)
+#if !((((NSEC_PER_SEC << 2) / TICK_NSEC) << (SEC_JIFFIE_SC - 2)) & 0x80000000)
+#undef SEC_JIFFIE_SC
+#define SEC_JIFFIE_SC (32 - SHIFT_HZ)
+#endif
+#define NSEC_JIFFIE_SC (SEC_JIFFIE_SC + 29)
+#define USEC_JIFFIE_SC (SEC_JIFFIE_SC + 19)
+#define SEC_CONVERSION ((unsigned long)((((u64)NSEC_PER_SEC << SEC_JIFFIE_SC))\
+ / (u64)TICK_NSEC))
+
+#define NSEC_CONVERSION ((unsigned long)((((u64)1 << NSEC_JIFFIE_SC))\
+ / (u64)TICK_NSEC))
+#define USEC_CONVERSION \
+ ((unsigned long)((((u64)NSEC_PER_USEC << USEC_JIFFIE_SC)) \
+ / (u64)TICK_NSEC))
+/*
+ * USEC_ROUND is used in the timeval to jiffie conversion. See there
+ * for more details. It is the scaled resolution rounding value. Note
+ * that it is a 64-bit value. Since, when it is applied, we are already
+ * in jiffies (albit scaled), it is nothing but the bits we will shift
+ * off.
+ */
+#define USEC_ROUND (u64)(((u64)1 << USEC_JIFFIE_SC) - 1)
+/*
+ * The maximum jiffie value is (MAX_INT >> 1). Here we translate that
+ * into seconds. The 64-bit case will overflow if we are not careful,
+ * so use the messy SH_DIV macro to do it. Still all constants.
*/
-#define SEC_JIFFIE_SC (30 - SHIFT_HZ)
-#define NSEC_JIFFIE_SC (SEC_JIFFIE_SC + 30)
-#define USEC_JIFFIE_SC (SEC_JIFFIE_SC + 20)
-#define SEC_CONVERSION ((unsigned long)(((u64)NSEC_PER_SEC << SEC_JIFFIE_SC) /\
- (u64)TICK_NSEC))
-#define NSEC_CONVERSION ((unsigned long)(((u64)1 << NSEC_JIFFIE_SC) /\
- (u64)TICK_NSEC))
-#define USEC_CONVERSION ((unsigned long)(((u64)NSEC_PER_USEC << USEC_JIFFIE_SC)/\
- (u64)TICK_NSEC))
#if BITS_PER_LONG < 64
# define MAX_SEC_IN_JIFFIES \
(long)((u64)((u64)MAX_JIFFY_OFFSET * TICK_NSEC) / NSEC_PER_SEC)
(SH_DIV((MAX_JIFFY_OFFSET >> SEC_JIFFIE_SC) * TICK_NSEC, NSEC_PER_SEC, 1) - 1)
#endif
-
+/*
+ * The TICK_NSEC - 1 rounds up the value to the next resolution. Note
+ * that a remainder subtract here would not do the right thing as the
+ * resolution values don't fall on second boundries. I.e. the line:
+ * nsec -= nsec % TICK_NSEC; is NOT a correct resolution rounding.
+ *
+ * Rather, we just shift the bits off the right.
+ *
+ * The >> (NSEC_JIFFIE_SC - SEC_JIFFIE_SC) converts the scaled nsec
+ * value to a scaled second value.
+ */
static __inline__ unsigned long
timespec_to_jiffies(struct timespec *value)
{
value->tv_sec = div_long_long_rem(nsec, NSEC_PER_SEC, &value->tv_nsec);
}
-/* Same for "timeval" */
+/* Same for "timeval"
+ *
+ * Well, almost. The problem here is that the real system resolution is
+ * in nanoseconds and the value being converted is in micro seconds.
+ * Also for some machines (those that use HZ = 1024, in-particular),
+ * there is a LARGE error in the tick size in microseconds.
+
+ * The solution we use is to do the rounding AFTER we convert the
+ * microsecond part. Thus the USEC_ROUND, the bits to be shifted off.
+ * Instruction wise, this should cost only an additional add with carry
+ * instruction above the way it was done above.
+ */
static __inline__ unsigned long
timeval_to_jiffies(struct timeval *value)
{
unsigned long sec = value->tv_sec;
- long usec = value->tv_usec
- + ((TICK_NSEC + 1000UL/2) / 1000UL) - 1;
+ long usec = value->tv_usec;
if (sec >= MAX_SEC_IN_JIFFIES){
sec = MAX_SEC_IN_JIFFIES;
usec = 0;
}
return (((u64)sec * SEC_CONVERSION) +
- (((u64)usec * USEC_CONVERSION) >>
+ (((u64)usec * USEC_CONVERSION + USEC_ROUND) >>
(USEC_JIFFIE_SC - SEC_JIFFIE_SC))) >> SEC_JIFFIE_SC;
}
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