Plane: add support for generating a PX4 mixer for failsafe

this creates APM/MIXER.MIX which will be used if the FMU dies to
provide manual control over RC

ArduPlane/px4_mixer.pde

@@ -0,0 +1,235 @@
+// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+
+/*
+  handle creation of PX4 mixer file, for failover to direct RC control
+  on failure of FMU
+
+  This will create APM/MIXER.MIX on the microSD card. The user may
+  also create APM/CUSTOM.MIX, and if it exists that will be used
+  instead. That allows the user to setup more complex failsafe mixes
+  that include flaps, landing gear, ignition cut etc
+ */
+
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <drivers/drv_pwm_output.h>
+#include <systemlib/mixer/mixer.h>
+
+
+/*
+  create a mixer file given key fixed wing parameters
+ */
+static bool create_mixer_file(const char *filename)
+{
+    int mix_fd = open(filename, O_WRONLY|O_CREAT|O_TRUNC, 0644);
+    if (mix_fd == -1) {
+        hal.console->printf("Unable to create mixer file\n");
+        return false;
+    }
+    dprintf(mix_fd, "Auto-generated mixer file for ArduPilot\n\n");
+
+    /*
+      this is the equivalent of channel_output_mixer()
+     */
+    const uint16_t mix_max = 10000 * g.mixing_gain;
+    const int8_t mixmul[5][2] = { { 0, 0 }, { 1, 1 }, { 1, -1 }, { -1, 1 }, { -1, -1 }};
+
+    for (uint8_t i=0; i<8; i++) {
+        int16_t c1, c2, mix=0;
+        bool rev = false;
+        RC_Channel_aux::Aux_servo_function_t function = RC_Channel_aux::channel_function(i);
+        if (i == rcmap.pitch()-1 && g.vtail_output > MIXING_DISABLED && g.vtail_output <= MIXING_DNDN) {
+            // first channel of VTAIL mix
+            c1 = rcmap.yaw()-1;
+            c2 = i;
+            rev = false;
+            mix = -mix_max*mixmul[g.vtail_output][0];
+        } else if (i == rcmap.yaw()-1 && g.vtail_output > MIXING_DISABLED && g.vtail_output <= MIXING_DNDN) {
+            // second channel of VTAIL mix
+            c1 = rcmap.pitch()-1;
+            c2 = i;
+            rev = true;
+            mix = mix_max*mixmul[g.vtail_output][1];
+        } else if (i == rcmap.roll()-1 && g.elevon_output > MIXING_DISABLED && g.elevon_output <= MIXING_DNDN) {
+            // first channel of ELEVON mix
+            c1 = i;
+            c2 = rcmap.pitch()-1;
+            rev = true;
+            mix = mix_max*mixmul[g.elevon_output][1];
+        } else if (i == rcmap.pitch()-1 && g.elevon_output > MIXING_DISABLED && g.elevon_output <= MIXING_DNDN) {
+            // second channel of ELEVON mix
+            c1 = i;
+            c2 = rcmap.roll()-1;
+            rev = false;
+            mix = mix_max*mixmul[g.elevon_output][0];
+        } else if (function == RC_Channel_aux::k_aileron || 
+                   function == RC_Channel_aux::k_flaperon1 || 
+                   function == RC_Channel_aux::k_flaperon2) {
+            // a secondary aileron. We don't mix flap input in yet for flaperons
+            c1 = rcmap.roll()-1;
+        } else if (function == RC_Channel_aux::k_elevator) {
+            // a secondary elevator
+            c1 = rcmap.pitch()-1;
+        } else if (function == RC_Channel_aux::k_rudder || 
+                   function == RC_Channel_aux::k_steering) {
+            // a secondary rudder or wheel
+            c1 = rcmap.yaw()-1;
+        } else if (g.flapin_channel > 0 &&
+                   (function == RC_Channel_aux::k_flap ||
+                    function == RC_Channel_aux::k_flap_auto)) {
+            // a flap output channel, and we have a manual flap input channel
+            c1 = g.flapin_channel-1;
+        } else if (i < 4 ||
+                   function == RC_Channel_aux::k_elevator_with_input ||
+                   function == RC_Channel_aux::k_aileron_with_input ||
+                   function == RC_Channel_aux::k_manual) {
+            // a pass-thru channel
+            c1 = i;
+        } else {
+            // a empty output
+            dprintf(mix_fd, "Z:\n\n");
+            continue;
+        }
+        if (mix == 0) {
+            // pass thru channel, possibly with reversal. We also
+            // adjust the gain based on the range of input and output
+            // channels. We don't yet adjust the offset based on trim
+            // positions.
+            const RC_Channel *chan1 = RC_Channel::rc_channel(i);
+            const RC_Channel *chan2 = RC_Channel::rc_channel(c1);
+            int8_t rev = (chan1->get_reverse() == chan2->get_reverse())?1:-1;
+            float gain = 1.0;
+            if (chan1->radio_max > chan1->radio_min) {
+                gain = (chan2->radio_max - chan2->radio_min) / (chan1->radio_max - chan1->radio_min);
+            }
+            dprintf(mix_fd, "M: 1\n");
+            dprintf(mix_fd, "O:   %d %d      0 -10000  10000\n", (int)(rev*10000*gain), (int)(rev*10000*gain));
+            dprintf(mix_fd, "S: 0 %u 10000 10000    0 -10000  10000\n\n", c1);
+        } else {
+            // mix of two input channels to give an output channel
+            dprintf(mix_fd, "M: 2\n");
+            dprintf(mix_fd, "O:   %d  %d      0 -10000  10000\n", mix, mix);
+            dprintf(mix_fd, "S: 0 %u  10000  10000    0 -10000  10000\n", c1);
+            if (rev) {
+                dprintf(mix_fd, "S: 0 %u  10000  10000   0 -10000  10000\n\n", c2);
+            } else {
+                dprintf(mix_fd, "S: 0 %u -10000 -10000   0 -10000  10000\n\n", c2);
+            }
+        }
+    }    
+    close(mix_fd);
+    return true;
+}
+
+
+/*
+  setup mixer on PX4 so that if FMU dies the pilot gets manual control
+ */
+static bool setup_failsafe_mixing(void)
+{
+    // we create MIXER.MIX regardless of whether we will be using it,
+    // as it gives a template for the user to modify to create their
+    // own CUSTOM.MIX file
+    const char *mixer_filename = "/fs/microsd/APM/MIXER.MIX";
+    const char *custom_mixer_filename = "/fs/microsd/APM/CUSTOM.MIX";
+    bool ret = false;
+
+    if (!create_mixer_file(mixer_filename)) {
+        return false;
+    }
+
+    struct stat st;
+    const char *filename;
+    if (stat(custom_mixer_filename, &st) == 0) {
+        filename = custom_mixer_filename;
+    } else {
+        filename = mixer_filename;
+    }
+
+    enum AP_HAL::Util::safety_state old_state = hal.util->safety_switch_state();
+    struct pwm_output_values pwm_values = {.values = {0}, .channel_count = 8};
+
+    int px4io_fd = open("/dev/px4io", 0);
+    if (px4io_fd == -1) {
+        // px4io isn't started, no point in setting up a mixer
+        return false;
+    }
+
+    // we need to force safety on to allow us to load a mixer
+    hal.rcout->force_safety_on();
+
+    /* reset any existing mixer in px4io. This shouldn't be needed,
+     * but is good practice */
+    if (ioctl(px4io_fd, MIXERIOCRESET, 0) != 0) {
+        hal.console->printf("Unable to reset mixer\n");
+        goto failed;
+    }
+
+    char buf[2048];
+    if (load_mixer_file(filename, &buf[0], sizeof(buf)) != 0) {
+        hal.console->printf("Unable to load %s\n", filename);
+        goto failed;
+    }
+
+	/* pass the buffer to the device */
+    if (ioctl(px4io_fd, MIXERIOCLOADBUF, (unsigned long)buf) != 0) {
+        hal.console->printf("Unable to send mixer to IO\n");
+        goto failed;        
+    }
+
+    // setup RC config for each channel based on user specified mix/max/trim
+    for (uint8_t i=0; i<RC_MAX_CHANNELS; i++) {
+        RC_Channel *ch = RC_Channel::rc_channel(i);
+        if (ch == NULL) {
+            continue;
+        }
+        struct pwm_output_rc_config config;
+        config.channel = i;
+        config.rc_min = 900;
+        config.rc_max = 2100;
+        config.rc_trim = 1500;
+        config.rc_dz = 0; // zero for the purposes of manual takeover
+        config.rc_assignment = i;
+        // we set reverse as false, as users of ArduPilot will have
+        // input reversed on transmitter, so from the point of view of
+        // the mixer the input is never reversed. The one exception is
+        // the 2nd channel, which is reversed inside the PX4IO code,
+        // so needs to be unreversed here to give sane behaviour.
+        if (i == 1) {
+            config.rc_reverse = true;
+        } else {
+            config.rc_reverse = false;
+        }
+        ioctl(px4io_fd, PWM_SERVO_SET_RC_CONFIG, (unsigned long)&config);
+    }
+
+    for (uint8_t i = 0; i < pwm_values.channel_count; i++) {
+        pwm_values.values[i] = 900;
+    }
+    ioctl(px4io_fd, PWM_SERVO_SET_MIN_PWM, (long unsigned int)&pwm_values);
+
+    for (uint8_t i = 0; i < pwm_values.channel_count; i++) {
+        pwm_values.values[i] = 2100;
+    }
+    ioctl(px4io_fd, PWM_SERVO_SET_MAX_PWM, (long unsigned int)&pwm_values);
+    ioctl(px4io_fd, PWM_SERVO_SET_OVERRIDE_OK, 0);
+
+    ret = true;
+
+failed:
+    if (px4io_fd != -1) {
+        close(px4io_fd);
+    }
+    // restore safety state if it was previously armed
+    if (old_state == AP_HAL::Util::SAFETY_ARMED) {
+        hal.rcout->force_safety_off();
+    }
+    return ret;
+}
+
+
+#endif // CONFIG_HAL_BOARD

ArduPlane/system.pde

@@ -87,6 +87,12 @@ static void init_ardupilot()
     //
     load_parameters();
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4
+    // this must be before BoardConfig.init() so if
+    // BRD_SAFETYENABLE==0 then we don't have safety off yet
+    setup_failsafe_mixing();
+#endif
+
     BoardConfig.init();
 
     // allow servo set on all channels except first 4