UAVCAN performance audit (#6829)

* UAVCAN ESC output: removing ESC output channels from published message that are always zero. This allows the UAVCAN stack to always transfer only the minimum number of output values, avoiding redundant zeroes and the associated increase in bus load and CPU time

* Added a separate mixer file for CAN quadrotor

* Sampling profiler improvements

* PMSP: Output more endpoints

* Matrix update

* libc usage workaround

* Removed UAVCAN perfcounters

src/modules/uavcan/actuators/esc.cpp

@@ -136,6 +136,25 @@ void UavcanEscController::update_outputs(float *outputs, unsigned num_outputs)
 		}
 	}
 
+	/*
+	 * Remove channels that are always zero.
+	 * The objective of this optimization is to avoid broadcasting multi-frame transfers when a single frame
+	 * transfer would be enough. This is a valid optimization as the UAVCAN specification implies that all
+	 * non-specified ESC setpoints should be considered zero.
+	 * The positive outcome is a (marginally) lower bus traffic and lower CPU load.
+	 *
+	 * From the standpoint of the PX4 architecture, however, this is a hack. It should be investigated why
+	 * the mixer returns more outputs than are actually used.
+	 */
+	for (int index = int(msg.cmd.size()) - 1; index >= _max_number_of_nonzero_outputs; index--) {
+		if (msg.cmd[index] != 0) {
+			_max_number_of_nonzero_outputs = index + 1;
+			break;
+		}
+	}
+
+	msg.cmd.resize(_max_number_of_nonzero_outputs);
+
 	/*
 	 * Publish the command message to the bus
 	 * Note that for a quadrotor it takes one CAN frame

src/modules/uavcan/actuators/esc.hpp

@@ -107,6 +107,7 @@ private:
 	 * ESC states
 	 */
 	uint32_t 			_armed_mask = 0;
+	uint8_t				_max_number_of_nonzero_outputs = 0;
 
 	/*
 	 * Perf counters