AP_Generator: mask MaintenanceRequired out from error bits in prearm

Turns out the generator sets this bit when it reaches the
maintenance-required state.

Mask it out from the error bits so the user can still fly.

Add some periodically-run code to complain about maintenance-required,
separate from the prearm checks.

libraries/AP_Generator/AP_Generator_RichenPower.cpp

@@ -194,6 +194,23 @@ constexpr float AP_Generator_RichenPower::heat_required_for_run()
 }
 
 
+void AP_Generator_RichenPower::check_maintenance_required()
+{
+    // don't bother the user while flying:
+    if (hal.util->get_soft_armed()) {
+        return;
+    }
+
+    const uint32_t now = AP_HAL::millis();
+
+    if (last_reading.errors & (1U<<uint16_t(Errors::MaintenanceRequired))) {
+        if (now - last_maintenance_warning_ms > 60000) {
+            gcs().send_text(MAV_SEVERITY_NOTICE, "Generator: requires maintenance");
+            last_maintenance_warning_ms = now;
+        }
+    }
+}
+
 /*
   update the state of the sensor
 */
@@ -205,6 +222,7 @@ void AP_Generator_RichenPower::update(void)
 
     if (last_reading_ms != 0) {
         update_runstate();
+        check_maintenance_required();
     }
 
     (void)get_reading();
@@ -335,17 +353,22 @@ bool AP_Generator_RichenPower::pre_arm_check(char *failmsg, uint8_t failmsg_len)
         hal.util->snprintf(failmsg, failmsg_len, "no messages in %ums", unsigned(now - last_reading_ms));
         return false;
     }
-    if (last_reading.seconds_until_maintenance == 0) {
-        hal.util->snprintf(failmsg, failmsg_len, "requires maintenance");
-    }
     if (SRV_Channels::get_channel_for(SRV_Channel::k_generator_control) == nullptr) {
         hal.util->snprintf(failmsg, failmsg_len, "need a servo output channel");
         return false;
     }
 
-    if (last_reading.errors) {
+    uint16_t errors = last_reading.errors;
+
+    // requiring maintenance isn't something that should stop
+    // people flying - they have work to do.  But we definitely
+    // complain about it - a lot.
+    errors &= ~(1U << uint16_t(Errors::MaintenanceRequired));
+
+    if (errors) {
+
         for (uint16_t i=0; i<16; i++) {
-            if (last_reading.errors & (1U << (uint16_t)i)) {
+            if (errors & (1U << i)) {
                 if (i < (uint16_t)Errors::LAST) {
                     hal.util->snprintf(failmsg, failmsg_len, "error: %s", error_strings[i]);
                 } else {

libraries/AP_Generator/AP_Generator_RichenPower.h

@@ -204,5 +204,11 @@ private:
         }
         return AP_HAL::millis() - idle_state_start_ms;
     }
+
+    // check if the generator requires maintenance and send a message if it does:
+    void check_maintenance_required();
+    // if we are emitting warnings about the generator requiring
+    // maintenamce, this is the last time we sent the warning:
+    uint32_t last_maintenance_warning_ms;
 };
 #endif