Add a sample mixer definition and documentation.

Add support for comments in mixer definitions.

ROMFS/Makefile

@@ -20,7 +20,8 @@ ROMFS_FSSPEC	:= $(SRCROOT)/scripts/rcS~init.d/rcS \
 		   $(SRCROOT)/scripts/rc.logging~init.d/rc.logging \
 		   $(SRCROOT)/scripts/rc.standalone~init.d/rc.standalone \
 		   $(SRCROOT)/scripts/rc.PX4IO~init.d/rc.PX4IO \
-		   $(SRCROOT)/scripts/rc.PX4IOAR~init.d/rc.PX4IOAR
+		   $(SRCROOT)/scripts/rc.PX4IOAR~init.d/rc.PX4IOAR \
+		   $(SRCROOT)/mixers/FMU_delta.mix~mixers/FMU_delta.mix
 
 #
 # Add the PX4IO firmware to the spec if someone has dropped it into the

ROMFS/mixers/FMU_delta.mix

@@ -0,0 +1,51 @@
+Delta-wing mixer for PX4FMU
+===========================
+
+Lines in this file that begin with a capital letter and a colon are interpreted
+as mixer commands.  All other lines are ignored.
+
+This delta-wing mixer assumes the elevon servos are connected to PX4FMU servo
+outputs 0 and 1 and the motor speed control to output 2. Output 3 is assumed to
+be unused.
+
+Inputs to the mixer come from channel group 0 (vehicle attitude), channels 0
+(roll), 1 (pitch) and 3 (thrust).
+
+See the README for more information on the scaler format.
+
+Elevon mixers
+-------------
+Three scalers total (output, roll, pitch).
+
+On the assumption that the two elevon servos are physically reversed, the pitch
+input is inverted between the two servos.
+
+The scaling factor for roll inputs is adjusted to implement differential travel
+for the elevons.
+
+M: 3
+S: 0 0  10000  10000      0 -10000  10000
+S: 0 0   3000   5000      0 -10000  10000
+S: 0 1   5000   5000      0 -10000  10000
+
+M: 3
+S: 0 0  10000  10000      0 -10000  10000
+S: 0 0   5000   3000      0 -10000  10000
+S: 0 1  -5000  -5000      0 -10000  10000
+
+
+Motor speed mixer
+-----------------
+Two scalers total (output, thrust).
+
+This mixer generates a full-range output (-1 to 1) from an input in the (0 - 1)
+range.  Inputs below zero are treated as zero.
+
+M: 2
+S: 0 0  10000  10000      0 -10000  10000
+S: 0 2      0  20000 -10000 -10000  10000
+
+
+We leave the fourth mixer empty.
+
+M: 0

ROMFS/mixers/README

@@ -0,0 +1,90 @@
+PX4 mixer definitions
+=====================
+
+Files in this directory implement example mixers that can be used as a basis
+for customisation, or for general testing purposes.
+
+Mixer basics
+------------
+
+Mixers combine control values from various sources (control tasks, user inputs,
+etc.) and produce output values suitable for controlling actuators; servos,
+motors, switches and so on.
+
+An actuator derives its value from the combination of one or more control
+values. Each of the control values is scaled according to the actuator's
+configuration and then combined to produce the actuator value, which may then be
+further scaled to suit the specific output type.
+
+Internally, all scaling is performed using floating point values. Inputs and
+outputs are clamped to the range -1.0 to 1.0.
+
+control    control   control
+   |          |         |
+   v          v         v
+ scale      scale     scale
+   |          |         |
+   |          v         |
+   +-------> mix <------+
+              |
+            scale
+              |
+              v
+             out
+
+Scaling
+-------
+
+Basic scalers provide linear scaling of the input to the output.
+
+Each scaler allows the input value to be scaled independently for inputs
+greater/less than zero. An offset can be applied to the output, and lower and
+upper boundary constraints can be applied. Negative scaling factors cause the
+output to be inverted (negative input produces positive output).
+
+Scaler pseudocode:
+
+if (input < 0)
+    output = (input * NEGATIVE_SCALE) + OFFSET
+else
+    output = (input * POSITIVE_SCALE) + OFFSET
+
+if (output < LOWER_LIMIT)
+    output = LOWER_LIMIT
+if (output > UPPER_LIMIT)
+    output = UPPER_LIMIT
+
+Syntax
+------
+
+Mixer definitions are text files; lines beginning with a single capital letter
+followed by a colon are significant. All other lines are ignored, meaning that
+explanatory text can be freely mixed with the definitions.
+
+Each file may define more than one mixer; the allocation of mixers to actuators
+is specific to the device reading the mixer definition.
+
+A mixer begins with a line of the form
+
+	M: <scaler count>
+
+If the scaler count is zero, the mixer is a placeholder and the device will not
+allocate a mixer for this position. Otherwise, this line is followed by scaler
+definitions matching the given count.
+
+A scaler definition is a line of the form:
+
+	S: <group> <index> <-ve scale> <+ve scale> <offset> <lower limit> <upper limit>
+
+The first scaler definition following the M: line configures the output scaler.
+The <group> and <index> fields are ignored in this case.
+
+For the remaining scalers, the <group> value identifies the control group from
+which the scaler will read. Control group 0 is the vehicle attitude control
+group; other group numbers may be assigned for other purposes. The <index> value
+selects the  control within the group that will be scaled.
+
+The remaining fields on the line represent the scaler parameters as discussed
+above. Whilst the calculations are performed as floating-point operations, the
+values stored in the definition file are scaled by a factor of 10000; i.e. an
+offset of -0.5 is encoded as -5000.

apps/systemlib/mixer.c

@@ -163,29 +163,43 @@ mixer_mix(struct mixer_s *mixer, float **controls)
 static int
 mixer_getline(int fd, char *line, unsigned maxlen)
 {
-	int	ret;
-	char	c;
-
-	while (--maxlen) {
-		ret = read(fd, &c, 1);
-
-		if (ret <= 0)
-			return ret;
-
-		if (c == '\r')
-			continue;
-
-		if (c == '\n') {
-			*line = '\0';
-			return 1;
+	/* reduce line budget by 1 to account for terminal NUL */
+	maxlen--;
+
+	/* loop looking for a non-comment line */
+	for (;;) {
+		int	ret;
+		char	c;
+		char	*p = line;
+
+		/* loop reading characters for this line */
+		for (;;) {
+			ret = read(fd, &c, 1);
+
+			/* on error or EOF, return same */
+			if (ret <= 0)
+				return ret;
+
+			/* ignore carriage returns */
+			if (c == '\r')
+				continue;
+
+			/* line termination */
+			if (c == '\n') {
+				/* ignore malformed lines */
+				if (line[1] != ':')
+					break;
+
+				/* terminate line as string and return */
+				*p = '\0';
+				return 1;
+			}
+
+			/* if we have space, accumulate the byte and go on */
+			if ((p - line) < maxlen)
+				*p++ = c;
 		}
-
-		*line++ = c;
 	}
-
-	/* line too long */
-	puts("line too long");
-	return -1;
 }
 
 static int
@@ -214,7 +228,7 @@ mixer_load(int fd, struct mixer_s **mp)
 {
 	int		ret, result = -1;
 	struct mixer_s *mixer = NULL;
-	char		buf[100];
+	char		buf[60];
 	unsigned	scalers;
 
 	ret = mixer_getline(fd, buf, sizeof(buf));