[Minios-devel] [UNIKRAFT PATCHv2 2/7] plat/common: Calculate shift factors for coversion between ns and tick

[Jia He <justin.he@xxxxxxx>]

From: Wei Chen <wei.chen@xxxxxxx>

We had shift factor for coverting counter ticks to ns, but it's not
enough. Sometime, we need to covert ns to ticks. For example, we'll
transfer sleep(ns) to counter ticks. If we don't have the shift factor,
the conversion accuracy will be lower.

Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx>
Signed-off-by: Jia He <justin.he@xxxxxxx>
---
 plat/common/arm/time.c | 109 +++++++++++++++++++++++++++++++----------
 1 file changed, 84 insertions(+), 25 deletions(-)

diff --git a/plat/common/arm/time.c b/plat/common/arm/time.c
index 64a9cb6..88e04bd 100644
--- a/plat/common/arm/time.c
+++ b/plat/common/arm/time.c
@@ -43,21 +43,81 @@
 static uint64_t boot_ticks;
 static uint32_t counter_freq;
 
-/*
- * Shift factor for counter scaling multiplier; referred to as S in the
- * following comments.
- */
-static uint8_t counter_shift;
 
-/* Multiplier for converting counter ticks to nsecs. (0.S) fixed point. */
+/* Shift factor for converting ticks to ns */
+static uint8_t counter_shift_to_ns;
+
+/* Shift factor for converting ns to ticks */
+static uint8_t counter_shift_to_tick;
+
+/* Multiplier for converting counter ticks to nsecs */
 static uint32_t ns_per_tick;
 
+/* Multiplier for converting nsecs to counter ticks */
+static uint32_t tick_per_ns;
+
+/*
+ * The maximum time range in seconds which can be converted by multiplier
+ * and shift factors. This will guarantee the converted value not to exceed
+ * 64-bit unsigned integer. Increase the time range will reduce the accuracy
+ * of conversion, because we will get smaller multiplier and shift factors.
+ * In this case, we selected 3600s as the time range.
+ */
+#define __MAX_CONVERT_SECS     3600UL
+
 /* How many nanoseconds per second */
 #define NSEC_PER_SEC ukarch_time_sec_to_nsec(1)
 
 static inline uint64_t ticks_to_ns(uint64_t ticks)
 {
-       return (ns_per_tick * ticks) >> counter_shift;
+       return (ns_per_tick * ticks) >> counter_shift_to_ns;
+}
+
+static inline uint64_t ns_to_ticks(uint64_t ns)
+{
+       return (tick_per_ns * ns) >> counter_shift_to_tick;
+}
+
+/*
+ * Calculate multiplier/shift factors for scaled math.
+ */
+static void calculate_mult_shift(uint32_t *pmult, uint8_t *pshift,
+               uint64_t source, uint64_t target)
+{
+       uint64_t tmp;
+       uint32_t mult;
+       uint8_t shift = 32;
+
+       /*
+        * Get the maximum shift factor (max_shift) for the given
+        * conversion range.
+        */
+       tmp = (__MAX_CONVERT_SECS * (uint64_t)source) >> shift;
+       while (tmp) {
+               tmp >>= 1;
+               shift--;
+       }
+
+       /*
+        * Calculate shift factor (S) and scaling multiplier (M).
+        *
+        * (S) needs to be the largest shift factor (<= max_shift) where
+        * the result of the M calculation below fits into uint32_t
+        * without truncation.
+        *
+        * multiplier = (target << shift) / source
+        */
+       mult = 0;
+       do {
+               tmp = ((uint64_t)target << shift) / source;
+               if ((tmp & 0xFFFFFFFF00000000L) == 0L)
+                       mult = (uint32_t)tmp;
+               else
+                       shift--;
+       } while (shift > 0 && mult == 0L);
+
+       *pmult = mult;
+       *pshift = shift;
 }
 
 /*
@@ -140,29 +200,28 @@ static __u64  generic_timer_epochoffset(void)
 
 static int generic_timer_init(void)
 {
+       counter_freq = get_counter_frequency();
+
        /*
-        * Calculate counter shift factor and scaling multiplier.
-        *
-        * counter_shift (S) needs to be the largest (<=32) shift factor where
-        * the result of the ns_per_tick calculation below fits into uint32_t
-        * without truncation. Note that we disallow an S of zero to ensure
-        * the loop always terminates.
-        *
-        * (0.S) ns_per_tick = NSEC_PER_SEC (S.S) / counter_freq (S.0)
+        * Calculate the shift factor and scaling multiplier for
+        * cpnverting ticks to ns.
         */
-       uint64_t tmp;
+       calculate_mult_shift(&ns_per_tick, &counter_shift_to_ns,
+                               counter_freq, NSEC_PER_SEC);
 
-       counter_freq = get_counter_frequency();
-       counter_shift = 32;
-       do {
-               tmp = (NSEC_PER_SEC << counter_shift) / counter_freq;
-               if ((tmp & 0xFFFFFFFF00000000L) == 0L)
-                       ns_per_tick = (uint32_t)tmp;
-               else
-                       counter_shift--;
-       } while (counter_shift > 0 && ns_per_tick == 0L);
+       /* We disallow zero ns_per_tick */
        UK_BUGON(!ns_per_tick);
 
+       /*
+        * Calculate the shift factor and scaling multiplier for
+        * cpnverting ns to ticks.
+        */
+       calculate_mult_shift(&tick_per_ns, &counter_shift_to_tick,
+                               NSEC_PER_SEC, counter_freq);
+
+       /* We disallow zero ns_per_tick */
+       UK_BUGON(!tick_per_ns);
+
        /*
         * Monotonic time begins at boot_ticks (first read of counter
         * before calibration).
-- 
2.17.1


_______________________________________________
Minios-devel mailing list
Minios-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/minios-devel