AP_Compass: switch to a general method of heater compensation

this allows any board to easily setup heater compensation for an
internal compass. The offsets are in body frame (previously in sensor
frame) and are sensor specific using bus device IDs

libraries/AP_Compass/AP_Compass_AK09916.cpp

@@ -324,19 +324,6 @@ void AP_Compass_AK09916::_update()
     _make_adc_sensitivity_adjustment(raw_field);
     raw_field *= AK09916_MILLIGAUSS_SCALE;
 
-#ifdef HAL_AK09916_HEATER_OFFSET
-    /*
-      the internal AK09916 can be impacted by the magnetic field from
-      a heater. We use the heater duty cycle to correct for the error
-     */
-    if (AP_HAL::Device::devid_get_bus_type(_bus->get_bus_id()) == AP_HAL::Device::BUS_TYPE_SPI) {
-        auto *bc = AP::boardConfig();
-        if (bc) {
-            raw_field += HAL_AK09916_HEATER_OFFSET * bc->get_heater_duty_cycle() * 0.01;
-        }
-    }
-#endif
-
     accumulate_sample(raw_field, _compass_instance, 10);
 
 check_registers:

libraries/AP_Compass/AP_Compass_Backend.cpp

@@ -5,6 +5,7 @@
 
 #include <AP_BattMonitor/AP_BattMonitor.h>
 #include <AP_Vehicle/AP_Vehicle_Type.h>
+#include <AP_BoardConfig/AP_BoardConfig.h>
 
 extern const AP_HAL::HAL& hal;
 
@@ -21,6 +22,30 @@ void AP_Compass_Backend::rotate_field(Vector3f &mag, uint8_t instance)
     }
     mag.rotate(state.rotation);
 
+#ifdef HAL_HEATER_MAG_OFFSET
+    /*
+      apply compass compensations for boards that have a heater which
+      interferes with an internal compass. This needs to be applied
+      before the board orientation so it is independent of
+      AHRS_ORIENTATION
+     */
+    if (!is_external(instance)) {
+        const uint32_t dev_id = uint32_t(_compass._state[Compass::StateIndex(instance)].dev_id);
+        static const struct offset {
+            uint32_t dev_id;
+            Vector3f ofs;
+        } offsets[] = HAL_HEATER_MAG_OFFSET;
+        const auto *bc = AP::boardConfig();
+        if (bc) {
+            for (const auto &o : offsets) {
+                if (o.dev_id == dev_id) {
+                    mag += o.ofs * bc->get_heater_duty_cycle() * 0.01;
+                }
+            }
+        }
+    }
+#endif
+
     if (!state.external) {
         mag.rotate(_compass._board_orientation);
     } else {

libraries/AP_Compass/AP_Compass_IST8310.cpp

@@ -229,19 +229,6 @@ void AP_Compass_IST8310::timer()
     /* Resolution: 0.3 µT/LSB - already convert to milligauss */
     Vector3f field = Vector3f{x * 3.0f, y * 3.0f, z * 3.0f};
 
-#ifdef HAL_IST8310_I2C_HEATER_OFFSET
-    /*
-      the internal IST8310 can be impacted by the magnetic field from
-      a heater. We use the heater duty cycle to correct for the error
-     */
-    if (!is_external(_instance) && AP_HAL::Device::devid_get_bus_type(_dev->get_bus_id()) == AP_HAL::Device::BUS_TYPE_I2C) {
-        const auto *bc = AP::boardConfig();
-        if (bc) {
-            field += HAL_IST8310_I2C_HEATER_OFFSET * bc->get_heater_duty_cycle() * 0.01;
-        }
-    }
-#endif
-    
     accumulate_sample(field, _instance);
 }