AP_MSP: make MSP packing smaller and more efficient

this changes two MSP items (GPS and home pos) to use packed structures
insteaf of individual sbuf writes. This makes for faster and smaller
code

libraries/AP_MSP/AP_MSP_Telem_Backend.cpp

@@ -185,28 +185,20 @@ void AP_MSP_Telem_Backend::update_home_pos(home_state_t &home_state)
 void AP_MSP_Telem_Backend::update_gps_state(gps_state_t &gps_state)
 {
     AP_GPS& gps = AP::gps();
+
+    memset(&gps_state, 0, sizeof(gps_state));
+
     WITH_SEMAPHORE(gps.get_semaphore());
-    gps_state.gps_fix_type = gps.status();
+    gps_state.fix_type = gps.status() >= AP_GPS::GPS_Status::GPS_OK_FIX_3D? 2:0;
+    gps_state.num_sats = gps.num_sats();
 
-    if (gps_state.gps_fix_type >= AP_GPS::GPS_Status::GPS_OK_FIX_2D) {
-        gps_state.gps_num_sats = gps.num_sats();
-    } else {
-        gps_state.gps_num_sats = 0;
-    }
-
-    if (gps_state.gps_fix_type >= AP_GPS::GPS_Status::GPS_OK_FIX_3D) {
+    if (gps_state.fix_type > 0) {
         const Location &loc = AP::gps().location(); //get gps instance 0
-        gps_state.gps_latitude = loc.lat;
-        gps_state.gps_longitude = loc.lng;
-        gps_state.gps_altitude_cm = loc.alt;
-        gps_state.gps_speed_ms = gps.ground_speed();
-        gps_state.gps_ground_course_cd = gps.ground_course_cd();
-    } else {
-        gps_state.gps_latitude = 0;
-        gps_state.gps_longitude = 0;
-        gps_state.gps_altitude_cm = 0;
-        gps_state.gps_speed_ms = 0;
-        gps_state.gps_ground_course_cd = 0;
+        gps_state.lat = loc.lat;
+        gps_state.lon = loc.lng;
+        gps_state.alt_m = loc.alt/100; // 1m resolution
+        gps_state.speed_cms = gps.ground_speed() * 100;
+        gps_state.ground_course_cd = gps.ground_course_cd();
     }
 }
 
@@ -505,11 +497,6 @@ MSPCommandResult AP_MSP_Telem_Backend::msp_process_out_raw_gps(sbuf_t *dst)
     gps_state_t gps_state;
     update_gps_state(gps_state);
 
-    sbuf_write_u8(dst, gps_state.gps_fix_type >= 3 ? 2 : 0);   // bitmask 1 << 1 is GPS FIX
-    sbuf_write_u8(dst, gps_state.gps_num_sats);
-    sbuf_write_u32(dst, gps_state.gps_latitude);
-    sbuf_write_u32(dst, gps_state.gps_longitude);
-    sbuf_write_u16(dst, (uint16_t)constrain_int32(gps_state.gps_altitude_cm / 100, 0, UINT16_MAX));   // alt changed from 1m to 0.01m per lsb since MSP API 1.39 by RTH. To maintain backwards compatibility compensate to 1m per lsb in MSP again.
     // handle airspeed override
     bool airspeed_en = false;
 #if OSD_ENABLED
@@ -518,11 +505,10 @@ MSPCommandResult AP_MSP_Telem_Backend::msp_process_out_raw_gps(sbuf_t *dst)
     if (airspeed_en) {
         airspeed_state_t airspeed_state;
         update_airspeed(airspeed_state);
-        sbuf_write_u16(dst, airspeed_state.airspeed_estimate_ms * 100);           // airspeed in cm/s
-    } else {
-        sbuf_write_u16(dst, (uint16_t)roundf(gps_state.gps_speed_ms * 100));      // speed in cm/s
+        gps_state.speed_cms = airspeed_state.airspeed_estimate_ms * 100;           // airspeed in cm/s
     }
-    sbuf_write_u16(dst, gps_state.gps_ground_course_cd * 1000);                   // degrees * 10 == centideg * 1000
+
+    sbuf_write_data(dst, &gps_state, sizeof(gps_state));
     return MSP_RESULT_ACK;
 }
 
@@ -538,9 +524,18 @@ MSPCommandResult AP_MSP_Telem_Backend::msp_process_out_comp_gps(sbuf_t *dst)
         home_angle_deg = 0;
     }
 
-    sbuf_write_u16(dst, home_state.home_distance_m);
-    sbuf_write_u16(dst, (uint16_t)home_angle_deg); //deg
-    sbuf_write_u8(dst, 1); // 1 is toggle GPS position update
+    struct PACKED {
+        uint16_t dist_home_m;
+        uint16_t home_angle_deg;
+        uint8_t toggle_gps;
+    } p;
+
+    p.dist_home_m = home_state.home_distance_m;
+    p.home_angle_deg = home_angle_deg;
+    p.toggle_gps = 1;
+
+    sbuf_write_data(dst, &p, sizeof(p));
+
     return MSP_RESULT_ACK;
 }
 
@@ -903,7 +898,7 @@ void AP_MSP_Telem_Backend::hide_osd_items(void)
         // flash satcount when no 3D Fix
         gps_state_t gps_state;
         update_gps_state(gps_state);
-        if (gps_state.gps_fix_type < AP_GPS::GPS_Status::GPS_OK_FIX_3D) {
+        if (gps_state.fix_type == 0) {
             BIT_SET(osd_hidden_items_bitmask, OSD_GPS_SATS);
         }
         // flash home dir and distance if home is not set

libraries/AP_MSP/AP_MSP_Telem_Backend.h

@@ -47,14 +47,14 @@ public:
         battery_state_e batt_state;
     } battery_state_t;
 
-    typedef struct gps_state_s {
-        int32_t gps_latitude;
-        int32_t gps_longitude;
-        uint8_t gps_num_sats;
-        int32_t gps_altitude_cm;
-        float gps_speed_ms;
-        uint16_t gps_ground_course_cd;
-        uint8_t gps_fix_type;
+    typedef struct PACKED gps_state_s {
+        uint8_t fix_type;
+        uint8_t num_sats;
+        int32_t lat;
+        int32_t lon;
+        uint16_t alt_m;
+        uint16_t speed_cms;
+        int16_t ground_course_cd;
     } gps_state_t;
 
     typedef struct airspeed_state_s {