Merge branch 'master' of github.com:PX4/Firmware into mpu6k_queue

11 files changed, 329 insertions, 114 deletions

diff --git a/src/drivers/device/cdev.cpp b/src/drivers/device/cdev.cpp
index 422321850..47ebcd40a 100644
--- a/src/drivers/device/cdev.cpp
+++ b/src/drivers/device/cdev.cpp
@@ -111,21 +111,21 @@ CDev::~CDev()
 int
 CDev::init()
 {
-	int ret = OK;
-
 	// base class init first
-	ret = Device::init();
+	int ret = Device::init();
 
 	if (ret != OK)
 		goto out;
 
 	// now register the driver
-	ret = register_driver(_devname, &fops, 0666, (void *)this);
+	if (_devname != nullptr) {
+		ret = register_driver(_devname, &fops, 0666, (void *)this);
 
-	if (ret != OK)
-		goto out;
+		if (ret != OK)
+			goto out;
 
-	_registered = true;
+		_registered = true;
+	}
 
 out:
 	return ret;
@@ -395,4 +395,4 @@ cdev_poll(struct file *filp, struct pollfd *fds, bool setup)
 	return cdev->poll(filp, fds, setup);
 }
 
-} // namespace device
\ No newline at end of file
+} // namespace device
diff --git a/src/drivers/device/device.cpp b/src/drivers/device/device.cpp
index 04a5222c3..c3ee77b1c 100644
--- a/src/drivers/device/device.cpp
+++ b/src/drivers/device/device.cpp
@@ -223,5 +223,22 @@ interrupt(int irq, void *context)
 	return OK;
 }
 
+int
+Device::read(unsigned offset, void *data, unsigned count)
+{
+	return -ENODEV;
+}
+
+int
+Device::write(unsigned offset, void *data, unsigned count)
+{
+	return -ENODEV;
+}
+
+int
+Device::ioctl(unsigned operation, unsigned &arg)
+{
+	return -ENODEV;
+}
 
-} // namespace device
\ No newline at end of file
+} // namespace device
diff --git a/src/drivers/device/device.h b/src/drivers/device/device.h
index 7d375aab9..a9ed5d77c 100644
--- a/src/drivers/device/device.h
+++ b/src/drivers/device/device.h
@@ -69,10 +69,61 @@ class __EXPORT Device
 {
 public:
 	/**
+	 * Destructor.
+	 *
+	 * Public so that anonymous devices can be destroyed.
+	 */
+	virtual ~Device();
+
+	/**
 	 * Interrupt handler.
 	 */
 	virtual void	interrupt(void *ctx);	/**< interrupt handler */
 
+	/*
+	 * Direct access methods.
+	 */
+
+	/**
+	 * Initialise the driver and make it ready for use.
+	 *
+	 * @return	OK if the driver initialized OK, negative errno otherwise;
+	 */
+	virtual int	init();
+
+	/**
+	 * Read directly from the device.
+	 *
+	 * The actual size of each unit quantity is device-specific.
+	 *
+	 * @param offset	The device address at which to start reading
+	 * @param data		The buffer into which the read values should be placed.
+	 * @param count		The number of items to read.
+	 * @return		The number of items read on success, negative errno otherwise.
+	 */
+	virtual int	read(unsigned address, void *data, unsigned count);
+
+	/**
+	 * Write directly to the device.
+	 *
+	 * The actual size of each unit quantity is device-specific.
+	 *
+	 * @param address	The device address at which to start writing.
+	 * @param data		The buffer from which values should be read.
+	 * @param count		The number of items to write.
+	 * @return		The number of items written on success, negative errno otherwise.
+	 */	 
+	virtual int	write(unsigned address, void *data, unsigned count);
+
+	/**
+	 * Perform a device-specific operation.
+	 *
+	 * @param operation	The operation to perform.
+	 * @param arg		An argument to the operation.
+	 * @return		Negative errno on error, OK or positive value on success.
+	 */
+	virtual int	ioctl(unsigned operation, unsigned &arg);
+
 protected:
 	const char	*_name;			/**< driver name */
 	bool		_debug_enabled;		/**< if true, debug messages are printed */
@@ -85,14 +136,6 @@ protected:
 	 */
 	Device(const char *name,
 	       int irq = 0);
-	~Device();
-
-	/**
-	 * Initialise the driver and make it ready for use.
-	 *
-	 * @return	OK if the driver initialised OK.
-	 */
-	virtual int	init();
 
 	/**
 	 * Enable the device interrupt
@@ -189,7 +232,7 @@ public:
 	/**
 	 * Destructor
 	 */
-	~CDev();
+	virtual ~CDev();
 
 	virtual int	init();
 
@@ -282,6 +325,7 @@ public:
 	 * Test whether the device is currently open.
 	 *
 	 * This can be used to avoid tearing down a device that is still active.
+	 * Note - not virtual, cannot be overridden by a subclass.
 	 *
 	 * @return              True if the device is currently open.
 	 */
@@ -396,9 +440,9 @@ public:
 	    const char *devname,
 	    uint32_t base,
 	    int irq = 0);
-	~PIO();
+	virtual ~PIO();
 
-	int		init();
+	virtual int	init();
 
 protected:
 
@@ -407,7 +451,7 @@ protected:
 	 *
 	 * @param offset	Register offset in bytes from the base address.
 	 */
-	uint32_t reg(uint32_t offset) {
+	uint32_t	reg(uint32_t offset) {
 		return *(volatile uint32_t *)(_base + offset);
 	}
 
@@ -444,4 +488,4 @@ private:
 
 } // namespace device
 
-#endif /* _DEVICE_DEVICE_H */
\ No newline at end of file
+#endif /* _DEVICE_DEVICE_H */
diff --git a/src/drivers/device/i2c.h b/src/drivers/device/i2c.h
index b4a9cdd53..549879352 100644
--- a/src/drivers/device/i2c.h
+++ b/src/drivers/device/i2c.h
@@ -55,13 +55,11 @@ class __EXPORT I2C : public CDev
 
 public:
 
-    /**
-     * Get the address
-     */
-    uint16_t get_address() {
-      return _address;
-    }
-
+	/**
+	 * Get the address
+	 */
+	int16_t		get_address() { return _address; }
+	
 protected:
 	/**
 	 * The number of times a read or write operation will be retried on
@@ -90,7 +88,7 @@ protected:
 	    uint16_t address,
 	    uint32_t frequency,
 	    int irq = 0);
-	~I2C();
+	virtual ~I2C();
 
 	virtual int	init();
 
diff --git a/src/drivers/device/spi.h b/src/drivers/device/spi.h
index d2d01efb3..e0122372a 100644
--- a/src/drivers/device/spi.h
+++ b/src/drivers/device/spi.h
@@ -71,7 +71,7 @@ protected:
 	    enum spi_mode_e mode,
 	    uint32_t frequency,
 	    int irq = 0);
-	~SPI();
+	virtual ~SPI();
 
 	virtual int	init();
 
diff --git a/src/drivers/meas_airspeed/meas_airspeed.cpp b/src/drivers/meas_airspeed/meas_airspeed.cpp
index 7a2e22c01..ebae21cf7 100644
--- a/src/drivers/meas_airspeed/meas_airspeed.cpp
+++ b/src/drivers/meas_airspeed/meas_airspeed.cpp
@@ -34,6 +34,7 @@
 /**
  * @file meas_airspeed.cpp
  * @author Lorenz Meier
+ * @author Sarthak Kaingade
  * @author Simon Wilks
  *
  * Driver for the MEAS Spec series connected via I2C.
@@ -92,9 +93,6 @@
 
 /* Register address */
 #define ADDR_READ_MR			0x00	/* write to this address to start conversion */
-#define ADDR_READ_DF2			0x00	/* read from this address to read pressure only */
-#define ADDR_READ_DF3			0x01
-#define ADDR_READ_DF4			0x02	/* read from this address to read pressure and temp */
 
 /* Measurement rate is 100Hz */
 #define CONVERSION_INTERVAL	(1000000 / 100)	/* microseconds */
@@ -122,7 +120,7 @@ protected:
 extern "C" __EXPORT int meas_airspeed_main(int argc, char *argv[]);
 
 MEASAirspeed::MEASAirspeed(int bus, int address) : Airspeed(bus, address,
-	CONVERSION_INTERVAL)
+			CONVERSION_INTERVAL)
 {
 
 }
@@ -160,7 +158,7 @@ MEASAirspeed::collect()
 
 	perf_begin(_sample_perf);
 
-	ret = transfer(nullptr, 0, &val[0], 2);
+	ret = transfer(nullptr, 0, &val[0], 4);
 
 	if (ret < 0) {
 		log("error reading from sensor: %d", ret);
@@ -171,25 +169,37 @@ MEASAirspeed::collect()
 
 	if (status == 2) {
 		log("err: stale data");
+
 	} else if (status == 3) {
 		log("err: fault");
 	}
 
-	uint16_t diff_pres_pa = (val[1]) | ((val[0] & ~(0xC0)) << 8);
+	//uint16_t diff_pres_pa = (val[1]) | ((val[0] & ~(0xC0)) << 8);
 	uint16_t temp = (val[3] & 0xE0) << 8 | val[2];
 
-	diff_pres_pa = abs(diff_pres_pa - (16384 / 2.0f));
-	diff_pres_pa -= _diff_pres_offset;
+	// XXX leaving this in until new calculation method has been cross-checked
+	//diff_pres_pa = abs(diff_pres_pa - (16384 / 2.0f));
+	//diff_pres_pa -= _diff_pres_offset;
+	int16_t dp_raw = 0, dT_raw = 0;
+	dp_raw = (val[0] << 8) + val[1];
+	dp_raw = 0x3FFF & dp_raw;
+	dT_raw = (val[2] << 8) + val[3];
+	dT_raw = (0xFFE0 & dT_raw) >> 5;
+	float temperature = ((200 * dT_raw) / 2047) - 50;
 
 	// XXX we may want to smooth out the readings to remove noise.
 
+	// Calculate differential pressure. As its centered around 8000
+	// and can go positive or negative, enforce absolute value
+	uint16_t diff_press_pa = abs(dp_raw - (16384 / 2.0f));
+
 	_reports[_next_report].timestamp = hrt_absolute_time();
-	_reports[_next_report].temperature = temp;
-	_reports[_next_report].differential_pressure_pa = diff_pres_pa;
+	_reports[_next_report].temperature = temperature;
+	_reports[_next_report].differential_pressure_pa = diff_press_pa;
 
 	// Track maximum differential pressure measured (so we can work out top speed).
-	if (diff_pres_pa > _reports[_next_report].max_differential_pressure_pa) {
-		_reports[_next_report].max_differential_pressure_pa = diff_pres_pa;
+	if (diff_press_pa > _reports[_next_report].max_differential_pressure_pa) {
+		_reports[_next_report].max_differential_pressure_pa = diff_press_pa;
 	}
 
 	/* announce the airspeed if needed, just publish else */
diff --git a/src/drivers/stm32/drv_pwm_servo.c b/src/drivers/stm32/drv_pwm_servo.c
index 7b060412c..dbb45a138 100644
--- a/src/drivers/stm32/drv_pwm_servo.c
+++ b/src/drivers/stm32/drv_pwm_servo.c
@@ -88,6 +88,7 @@
 #define rCCR4(_tmr)   	REG(_tmr, STM32_GTIM_CCR4_OFFSET)
 #define rDCR(_tmr)    	REG(_tmr, STM32_GTIM_DCR_OFFSET)
 #define rDMAR(_tmr)   	REG(_tmr, STM32_GTIM_DMAR_OFFSET)
+#define rBDTR(_tmr)	REG(_tmr, STM32_ATIM_BDTR_OFFSET)
 
 static void		pwm_timer_init(unsigned timer);
 static void		pwm_timer_set_rate(unsigned timer, unsigned rate);
@@ -110,6 +111,11 @@ pwm_timer_init(unsigned timer)
 	rCCER(timer) = 0;
 	rDCR(timer) = 0;
 
+	if ((pwm_timers[timer].base == STM32_TIM1_BASE) || (pwm_timers[timer].base == STM32_TIM8_BASE)) {
+		/* master output enable = on */
+		rBDTR(timer) = ATIM_BDTR_MOE;
+	}
+
 	/* configure the timer to free-run at 1MHz */
 	rPSC(timer) = (pwm_timers[timer].clock_freq / 1000000) - 1;
 
diff --git a/src/modules/systemlib/hx_stream.c b/src/modules/systemlib/hx_stream.c
index 8d77f14a8..8e9c2bfcf 100644
--- a/src/modules/systemlib/hx_stream.c
+++ b/src/modules/systemlib/hx_stream.c
@@ -53,14 +53,26 @@
 
 
 struct hx_stream {
-	uint8_t			buf[HX_STREAM_MAX_FRAME + 4];
-	unsigned		frame_bytes;
-	bool			escaped;
-	bool			txerror;
-
+	/* RX state */
+	uint8_t			rx_buf[HX_STREAM_MAX_FRAME + 4];
+	unsigned		rx_frame_bytes;
+	bool			rx_escaped;
+	hx_stream_rx_callback	rx_callback;
+	void			*rx_callback_arg;
+
+	/* TX state */
 	int			fd;
-	hx_stream_rx_callback	callback;
-	void			*callback_arg;
+	bool			tx_error;
+	uint8_t			*tx_buf;
+	unsigned		tx_resid;
+	uint32_t		tx_crc;
+	enum {
+		TX_IDLE = 0,
+		TX_SEND_START,
+		TX_SEND_DATA,
+		TX_SENT_ESCAPE,
+		TX_SEND_END
+	}			tx_state;
 
 	perf_counter_t		pc_tx_frames;
 	perf_counter_t		pc_rx_frames;
@@ -81,21 +93,7 @@ static void
 hx_tx_raw(hx_stream_t stream, uint8_t c)
 {
 	if (write(stream->fd, &c, 1) != 1)
-		stream->txerror = true;
-}
-
-static void
-hx_tx_byte(hx_stream_t stream, uint8_t c)
-{
-	switch (c) {
-	case FBO:
-	case CEO:
-		hx_tx_raw(stream, CEO);
-		c ^= 0x20;
-		break;
-	}
-
-	hx_tx_raw(stream, c);
+		stream->tx_error = true;
 }
 
 static int
@@ -105,11 +103,11 @@ hx_rx_frame(hx_stream_t stream)
 		uint8_t	b[4];
 		uint32_t w;
 	} u;
-	unsigned length = stream->frame_bytes;
+	unsigned length = stream->rx_frame_bytes;
 
 	/* reset the stream */
-	stream->frame_bytes = 0;
-	stream->escaped = false;
+	stream->rx_frame_bytes = 0;
+	stream->rx_escaped = false;
 
 	/* not a real frame - too short */
 	if (length < 4) {
@@ -122,11 +120,11 @@ hx_rx_frame(hx_stream_t stream)
 	length -= 4;
 
 	/* compute expected CRC */
-	u.w = crc32(&stream->buf[0], length);
+	u.w = crc32(&stream->rx_buf[0], length);
 
 	/* compare computed and actual CRC */
 	for (unsigned i = 0; i < 4; i++) {
-		if (u.b[i] != stream->buf[length + i]) {
+		if (u.b[i] != stream->rx_buf[length + i]) {
 			perf_count(stream->pc_rx_errors);
 			return 0;
 		}
@@ -134,7 +132,7 @@ hx_rx_frame(hx_stream_t stream)
 
 	/* frame is good */
 	perf_count(stream->pc_rx_frames);
-	stream->callback(stream->callback_arg, &stream->buf[0], length);
+	stream->rx_callback(stream->rx_callback_arg, &stream->rx_buf[0], length);
 	return 1;
 }
 
@@ -150,8 +148,8 @@ hx_stream_init(int fd,
 	if (stream != NULL) {
 		memset(stream, 0, sizeof(struct hx_stream));
 		stream->fd = fd;
-		stream->callback = callback;
-		stream->callback_arg = arg;
+		stream->rx_callback = callback;
+		stream->rx_callback_arg = arg;
 	}
 
 	return stream;
@@ -179,49 +177,112 @@ hx_stream_set_counters(hx_stream_t stream,
 	stream->pc_rx_errors = rx_errors;
 }
 
+void
+hx_stream_reset(hx_stream_t stream)
+{
+	stream->rx_frame_bytes = 0;
+	stream->rx_escaped = false;
+
+	stream->tx_buf = NULL;
+	stream->tx_resid = 0;
+	stream->tx_state = TX_IDLE;
+}
+
 int
-hx_stream_send(hx_stream_t stream,
+hx_stream_start(hx_stream_t stream,
 	       const void *data,
 	       size_t count)
 {
-	union {
-		uint8_t	b[4];
-		uint32_t w;
-	} u;
-	const uint8_t *p = (const uint8_t *)data;
-	unsigned resid = count;
-
-	if (resid > HX_STREAM_MAX_FRAME)
+	if (count > HX_STREAM_MAX_FRAME)
 		return -EINVAL;
 
-	/* start the frame */
-	hx_tx_raw(stream, FBO);
+	stream->tx_buf = data;
+	stream->tx_resid = count;
+	stream->tx_state = TX_SEND_START;
+	stream->tx_crc = crc32(data, count);
+	return OK;
+}
+
+int
+hx_stream_send_next(hx_stream_t stream)
+{
+	int c;
+
+	/* sort out what we're going to send */
+	switch (stream->tx_state) {
 
-	/* transmit the data */
-	while (resid--)
-		hx_tx_byte(stream, *p++);
+	case TX_SEND_START:
+		stream->tx_state = TX_SEND_DATA;
+		return FBO;
 
-	/* compute the CRC */
-	u.w = crc32(data, count);
+	case TX_SEND_DATA:
+		c = *stream->tx_buf;
 
-	/* send the CRC */
-	p = &u.b[0];
-	resid = 4;
+		switch (c) {
+		case FBO:
+		case CEO:
+			stream->tx_state = TX_SENT_ESCAPE;
+			return CEO;
+		}
+		break;
+
+	case TX_SENT_ESCAPE:
+		c = *stream->tx_buf ^ 0x20;
+		stream->tx_state = TX_SEND_DATA;
+		break;
+
+	case TX_SEND_END:
+		stream->tx_state = TX_IDLE;
+		return FBO;
+
+	case TX_IDLE:
+	default:
+		return -1;
+	}
+
+	/* if we are here, we have consumed a byte from the buffer */
+	stream->tx_resid--;
+	stream->tx_buf++;
+
+	/* buffer exhausted */
+	if (stream->tx_resid == 0) {
+		uint8_t *pcrc = (uint8_t *)&stream->tx_crc;
+
+		/* was the buffer the frame CRC? */
+		if (stream->tx_buf == (pcrc + sizeof(stream->tx_crc))) {
+			stream->tx_state = TX_SEND_END;
+		} else {
+			/* no, it was the payload - switch to sending the CRC */
+			stream->tx_buf = pcrc;
+			stream->tx_resid = sizeof(stream->tx_crc);
+		}
+	}
+	return c;
+}
+
+int
+hx_stream_send(hx_stream_t stream,
+	       const void *data,
+	       size_t count)
+{
+	int result;
 
-	while (resid--)
-		hx_tx_byte(stream, *p++);
+	result = hx_stream_start(stream, data, count);
+	if (result != OK)
+		return result;
 
-	/* and the trailing frame separator */
-	hx_tx_raw(stream, FBO);
+	int c;
+	while ((c = hx_stream_send_next(stream)) >= 0)
+		hx_tx_raw(stream, c);
 
 	/* check for transmit error */
-	if (stream->txerror) {
-		stream->txerror = false;
+	if (stream->tx_error) {
+		stream->tx_error = false;
 		return -EIO;
 	}
 
 	perf_count(stream->pc_tx_frames);
-	return 0;
+	return OK;
 }
 
 void
@@ -234,17 +295,17 @@ hx_stream_rx(hx_stream_t stream, uint8_t c)
 	}
 
 	/* escaped? */
-	if (stream->escaped) {
-		stream->escaped = false;
+	if (stream->rx_escaped) {
+		stream->rx_escaped = false;
 		c ^= 0x20;
 
 	} else if (c == CEO) {
-		/* now escaped, ignore the byte */
-		stream->escaped = true;
+		/* now rx_escaped, ignore the byte */
+		stream->rx_escaped = true;
 		return;
 	}
 
 	/* save for later */
-	if (stream->frame_bytes < sizeof(stream->buf))
-		stream->buf[stream->frame_bytes++] = c;
+	if (stream->rx_frame_bytes < sizeof(stream->rx_buf))
+		stream->rx_buf[stream->rx_frame_bytes++] = c;
 }
diff --git a/src/modules/systemlib/hx_stream.h b/src/modules/systemlib/hx_stream.h
index 128689953..1f3927222 100644
--- a/src/modules/systemlib/hx_stream.h
+++ b/src/modules/systemlib/hx_stream.h
@@ -58,7 +58,8 @@ __BEGIN_DECLS
  * Allocate a new hx_stream object.
  *
  * @param fd		The file handle over which the protocol will
- *			communicate.
+ *			communicate, or -1 if the protocol will use
+ *			hx_stream_start/hx_stream_send_next.
  * @param callback	Called when a frame is received.
  * @param callback_arg	Passed to the callback.
  * @return		A handle to the stream, or NULL if memory could
@@ -80,6 +81,7 @@ __EXPORT extern void		hx_stream_free(hx_stream_t stream);
  *
  * Any counter may be set to NULL to disable counting that datum.
  *
+ * @param stream	A handle returned from hx_stream_init.
  * @param tx_frames	Counter for transmitted frames.
  * @param rx_frames	Counter for received frames.
  * @param rx_errors	Counter for short and corrupt received frames.
@@ -90,6 +92,44 @@ __EXPORT extern void		hx_stream_set_counters(hx_stream_t stream,
 		perf_counter_t rx_errors);
 
 /**
+ * Reset a stream.
+ *
+ * Forces the local stream state to idle.
+ *
+ * @param stream	A handle returned from hx_stream_init.
+ */
+__EXPORT extern void		hx_stream_reset(hx_stream_t stream);
+
+/**
+ * Prepare to send a frame.
+ *
+ * Use this in conjunction with hx_stream_send_next to 
+ * set the frame to be transmitted.
+ *
+ * Use hx_stream_send() to write to the stream fd directly.
+ *
+ * @param stream	A handle returned from hx_stream_init.
+ * @param data		Pointer to the data to send.
+ * @param count		The number of bytes to send.
+ * @return		Zero on success, -errno on error.
+ */
+__EXPORT extern int		hx_stream_start(hx_stream_t stream,
+		const void *data,
+		size_t count);
+
+/**
+ * Get the next byte to send for a stream.
+ *
+ * This requires that the stream be prepared for sending by
+ * calling hx_stream_start first.
+ *
+ * @param stream	A handle returned from hx_stream_init.
+ * @return		The byte to send, or -1 if there is 
+ *			nothing left to send.
+ */
+__EXPORT extern int		hx_stream_send_next(hx_stream_t stream);
+
+/**
  * Send a frame.
  *
  * This function will block until all frame bytes are sent if
diff --git a/src/modules/systemlib/perf_counter.c b/src/modules/systemlib/perf_counter.c
index 879f4715a..3c1e10287 100644
--- a/src/modules/systemlib/perf_counter.c
+++ b/src/modules/systemlib/perf_counter.c
@@ -201,23 +201,50 @@ perf_end(perf_counter_t handle)
 	switch (handle->type) {
 	case PC_ELAPSED: {
 			struct perf_ctr_elapsed *pce = (struct perf_ctr_elapsed *)handle;
-			hrt_abstime elapsed = hrt_absolute_time() - pce->time_start;
 
-			pce->event_count++;
-			pce->time_total += elapsed;
+			if (pce->time_start != 0) {
+				hrt_abstime elapsed = hrt_absolute_time() - pce->time_start;
 
-			if ((pce->time_least > elapsed) || (pce->time_least == 0))
-				pce->time_least = elapsed;
+				pce->event_count++;
+				pce->time_total += elapsed;
 
-			if (pce->time_most < elapsed)
-				pce->time_most = elapsed;
+				if ((pce->time_least > elapsed) || (pce->time_least == 0))
+					pce->time_least = elapsed;
+
+				if (pce->time_most < elapsed)
+					pce->time_most = elapsed;
+
+				pce->time_start = 0;
+			}
+		}
+		break;
+
+	default:
+		break;
+	}
+}
+
+void
+perf_cancel(perf_counter_t handle)
+{
+	if (handle == NULL)
+		return;
+
+	switch (handle->type) {
+	case PC_ELAPSED: {
+			struct perf_ctr_elapsed *pce = (struct perf_ctr_elapsed *)handle;
+
+			pce->time_start = 0;
 		}
+		break;
 
 	default:
 		break;
 	}
 }
 
+
+
 void
 perf_reset(perf_counter_t handle)
 {
diff --git a/src/modules/systemlib/perf_counter.h b/src/modules/systemlib/perf_counter.h
index 5c2cb15b2..4cd8b67a1 100644
--- a/src/modules/systemlib/perf_counter.h
+++ b/src/modules/systemlib/perf_counter.h
@@ -92,13 +92,25 @@ __EXPORT extern void		perf_begin(perf_counter_t handle);
  * End a performance event.
  *
  * This call applies to counters that operate over ranges of time; PC_ELAPSED etc.
+ * If a call is made without a corresopnding perf_begin call, or if perf_cancel
+ * has been called subsequently, no change is made to the counter.
  *
  * @param handle		The handle returned from perf_alloc.
  */
 __EXPORT extern void		perf_end(perf_counter_t handle);
 
 /**
- * Reset a performance event.
+ * Cancel a performance event.
+ *
+ * This call applies to counters that operate over ranges of time; PC_ELAPSED etc.
+ * It reverts the effect of a previous perf_begin.
+ *
+ * @param handle		The handle returned from perf_alloc.
+ */
+__EXPORT extern void		perf_cancel(perf_counter_t handle);
+
+/**
+ * Reset a performance counter.
  *
  * This call resets performance counter to initial state
  *