basecmd: Improve accuracy of stats "sumsq" variable

Use a base of 256 instead of 65536 when calculating the sum of the
square of the clock differences in the stats.  This makes the
calculation more accurate.  Export the new base via DECL_CONSTANT for
the host to access.  Use DIV_ROUND_UP() when adjusting for the base to
ensure no lost ticks.  Do the division after multiplication in the
common case where the time between stats_task() invocations is less
than 64K ticks.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 13 insertions, 2 deletions

diff --git a/src/basecmd.c b/src/basecmd.c
index 2c9562b6..4902bd9d 100644
--- a/src/basecmd.c
+++ b/src/basecmd.c
@@ -191,15 +191,26 @@ command_get_status(uint32_t *args)
 }
 DECL_COMMAND_FLAGS(command_get_status, HF_IN_SHUTDOWN, "get_status");
 
+#define SUMSQ_BASE 256
+DECL_CONSTANT(STATS_SUMSQ_BASE, SUMSQ_BASE);
+
 static void
 stats_task(void)
 {
     static uint32_t last, count, sumsq;
     uint32_t cur = sched_read_time();
-    uint32_t diff = (cur - last) >> 8;
+    uint32_t diff = cur - last;
     last = cur;
     count++;
-    uint32_t nextsumsq = sumsq + diff*diff;
+    // Calculate sum of diff^2 - be careful of integer overflow
+    uint32_t nextsumsq;
+    if (diff <= 0xffff) {
+        nextsumsq = sumsq + DIV_ROUND_UP(diff * diff, SUMSQ_BASE);
+    } else if (diff <= 0xfffff) {
+        nextsumsq = sumsq + DIV_ROUND_UP(diff, SUMSQ_BASE) * diff;
+    } else {
+        nextsumsq = 0xffffffff;
+    }
     if (nextsumsq < sumsq)
         nextsumsq = 0xffffffff;
     sumsq = nextsumsq;