Implement and document the buzzkill option.

All regression tests pass.

3 files changed, 48 insertions, 5 deletions

diff --git a/SConstruct b/SConstruct
index f379e624..211c9a1b 100644
--- a/SConstruct
+++ b/SConstruct
@@ -155,6 +155,7 @@ boolopts = (
     ("nofloats",      False, "float ops are expensive, suppress error estimates"),
     ("squelch",       False, "squelch gpsd_report/gpsd_hexdump to save cpu"),
     ("ncurses",       True,  "build with ncurses"),
+    ("buzzkill",      True,  "adaptive-delay logic to cope with select buzzing"),
     # Build control
     ("shared",        True,  "build shared libraries, not static"),
     ("implicit_link", imloads,"implicit linkage is supported in shared libs"),
diff --git a/build.txt b/build.txt
index 2847be60..baffd00b 100644
--- a/build.txt
+++ b/build.txt
@@ -352,6 +352,40 @@ scons minimal=yes socket_export=yes nmea0183=yes
 
 will do that.
 
+== Configuration problem cases
+
+This section describes cases in which the default build is known to be
+unavoidably broken and a custom configuration is required to do the
+right thing.
+
+Presently there is only one such case; buzzkill=yes (the default) can
+conflict with nmea2000=yes (the default), causing problems if you try
+to watch both a CAN/NMEA2000 device and a serial device.  The only
+remedy is a custom build with buzzkill=no.
+
+The buzzkill option is intended to thwart problems due to gpsd's main
+select(2) loop buzzing (spinning too fast). On battery-powered SBCs
+and mobile devices this can cause serious and unnecessary power drain;
+this has been observed on the Raspberry Pi and sporadically on
+particular Android phone hardware. There are at least two reasons it
+can happen; one is defective select(2) device-driver hooks that return
+a data-ready when they shouldn't, the other is tty layers that return
+data in single characters or small packets even when it's arriving in
+packet-sized chunks.
+
+GPSD with buzzkill=yes watches the incoming data rate from all devices
+and adaptively inserts delays in the main loop if it thinks it sees
+buzzing.  This logic is routed around if all devices have types that never
+require delays, notably pseudo-ttys and UDP packet streams.
+
+With nmea2000=yes the code tries to read NME2000 packets from a CANbus
+interface at high speed. The resulting data flow can look like
+buzzing, but the CAN driver pretends to be a pseudo-tty to avoid delay
+insertion.  The problem arises if you try to watch both a conventional
+tty and a CAN interface at the same time; this foils the buzzkill
+guard, and you will lose CAN packets due to inserted delays.  To
+prevent this, build with buzzkill=no.
+
 == Port and toolchain testing ==
 
 'scons check' will run a comprehensive regression-test suite.  You
diff --git a/gpsd.c b/gpsd.c
index 1f89d1b8..efaa8d09 100644
--- a/gpsd.c
+++ b/gpsd.c
@@ -1778,6 +1778,7 @@ static void gpsd_terminate(struct gps_context_t *context CONDITIONALLY_UNUSED)
 #endif /* PPS_ENABLE */
 }
 
+#ifdef BUZZKILL_ENABLE
 static void adaptive_delay(void)
 /* sleep a calculated time only if we have a buggy select() */
 {
@@ -1826,6 +1827,7 @@ static void adaptive_delay(void)
     }
     return;
 }
+#endif /* BUZZKILL_ENABLE */
 
 /*@ -mustfreefresh @*/
 int main(int argc, char *argv[])
@@ -2206,14 +2208,20 @@ int main(int argc, char *argv[])
     while (0 == signalled) {
 	fd_set efds;
 
+#ifdef BUZZKILL_ENABLE
 	/*
-	 * Adaptive delay to prevent buzzing if the tty layer returns data
-	 * one character at a time and too fast. This pushes CPU usage up and
-	 * eats power.  This has been directly observed as a problem on the
-	 * Raspberry Pi, and is probably behind occasional reports of
-	 * GPSD-related excessive power drain on Android phones.
+	 * Adaptive delay to prevent buzzing if the tty layer returns
+	 * data one character at a time and too fast. Buzzing pushes
+	 * CPU usage up and eats power.
+	 *
+	 * This has been directly observed as a problem on the
+	 * Raspberry Pi. A buzzing problem with outright defective
+	 * select(2) implementations is probably behind occasional
+	 * reports of GPSD-related excessive power drain on Android
+	 * phones.
 	 */
 	(void) adaptive_delay();
+#endif /* BUZZKILL_ENABLE */
 
 	switch(gpsd_await_data(&rfds, &efds, maxfd, &all_fds, &context.errout))
 	{