Introduce an adaptive delay into the main loop to prevent select buzzing.

When select(2) returns too fast with only small amounts of data, the main loop
will buzz and eat power - a significant issue on, e.g. battery-powered RasPi
devices, where it was first spotted. This code watches read volume and delays
when it sees lots of small bursts.

All regressiion tests pass.  But note: the adaptive-delay logic has to
suppress delays to arrange this, otherwise we get an all too familiar
class of spurious test failure.

1 files changed, 22 insertions, 0 deletions

diff --git a/libgpsd_core.c b/libgpsd_core.c
index 933bb4be..ac558bef 100644
--- a/libgpsd_core.c
+++ b/libgpsd_core.c
@@ -279,6 +279,9 @@ void gps_context_init(struct gps_context_t *context,
 	.shmexport      = NULL,
 #endif /* SHM_EXPORT_ENABLE */
 	.serial_write    = gpsd_serial_write,
+	.inbytesavg = 0.0,
+	.inbyteswpos = (char) 0,
+	.selectbug = false
     };
     /*@ +initallelements +nullassign +nullderef +fullinitblock @*/
     /* *INDENT-ON* */
@@ -1069,6 +1072,7 @@ gps_mask_t gpsd_poll(struct gps_device_t *session)
 {
     ssize_t newlen;
     bool driver_change = false;
+    float alpha;
 
     gps_clear_fix(&session->newdata);
 
@@ -1154,6 +1158,24 @@ gps_mask_t gpsd_poll(struct gps_device_t *session)
 	newlen = generic_get(session);
     }
 
+	 /* Calculate window average of read count */
+    session->context->inbyteswpos = session->context->inbyteswpos % WINDOW_AVG_SIZE;
+    alpha = (session->context->inbyteswpos== (unsigned char) 0) ? 0.0f:1.0f;
+    if (newlen > 0)
+       session->context->inbytesavg = alpha*session->context->inbytesavg + (float) newlen;
+    else
+       /* error reading -- counts as 0 */
+       session->context->inbytesavg = alpha*session->context->inbytesavg;
+
+    if (session->context->inbyteswpos== (unsigned char) (WINDOW_AVG_SIZE-1)) {
+       session->context->inbytesavg/=WINDOW_AVG_SIZE;
+    }
+    session->context->inbyteswpos += (unsigned char) 1;
+
+	 gpsd_report(&session->context->errout, LOG_RAW,
+		    "window average of received byte count is %f\n",
+		    session->context->inbytesavg);
+
     /* update the scoreboard structure from the GPS */
     gpsd_report(&session->context->errout, LOG_RAW + 2,
 		"%s sent %zd new characters\n",