diff --git a/libraries/AP_Compass/AP_Compass.cpp b/libraries/AP_Compass/AP_Compass.cpp
index 0815029e33..c1631ceef0 100644
--- a/libraries/AP_Compass/AP_Compass.cpp
+++ b/libraries/AP_Compass/AP_Compass.cpp
@@ -1065,7 +1065,7 @@ void Compass::_probe_external_i2c_compasses(void)
 #if !defined(HAL_SKIP_AUTO_INTERNAL_I2C_PROBE)
     bool all_external = (AP_BoardConfig::get_board_type() == AP_BoardConfig::PX4_BOARD_PIXHAWK2);
 #endif
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) || defined(HAL_USE_I2C_MAG_HMC5843)
+#if AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED || defined(HAL_USE_I2C_MAG_HMC5843)
     // external i2c bus
     FOREACH_I2C_EXTERNAL(i) {
         ADD_BACKEND(DRIVER_HMC5843, AP_Compass_HMC5843::probe(GET_I2C_DEVICE(i, HAL_COMPASS_HMC5843_I2C_ADDR),
@@ -1084,7 +1084,7 @@ void Compass::_probe_external_i2c_compasses(void)
 #endif
 #endif
 
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) || defined(HAL_USE_I2C_MAG_QMC5883L)
+#if AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED || defined(HAL_USE_I2C_MAG_QMC5883L)
     //external i2c bus
     FOREACH_I2C_EXTERNAL(i) {
         ADD_BACKEND(DRIVER_QMC5883L, AP_Compass_QMC5883L::probe(GET_I2C_DEVICE(i, HAL_COMPASS_QMC5883L_I2C_ADDR),
@@ -1127,7 +1127,7 @@ void Compass::_probe_external_i2c_compasses(void)
 #endif
 #endif // HAL_BUILD_AP_PERIPH
 
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) || defined(HAL_USE_I2C_MAG_LIS3MDL)
+#if AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED || defined(HAL_USE_I2C_MAG_LIS3MDL)
     // lis3mdl on bus 0 with default address
 #if !defined(HAL_SKIP_AUTO_INTERNAL_I2C_PROBE)
     FOREACH_I2C_INTERNAL(i) {
@@ -1154,7 +1154,7 @@ void Compass::_probe_external_i2c_compasses(void)
     }
 #endif
 
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) || defined(HAL_USE_I2C_MAG_AK09916)
+#if AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED || defined(HAL_USE_I2C_MAG_AK09916)
     // AK09916. This can be found twice, due to the ICM20948 i2c bus pass-thru, so we need to be careful to avoid that
     FOREACH_I2C_EXTERNAL(i) {
         ADD_BACKEND(DRIVER_AK09916, AP_Compass_AK09916::probe(GET_I2C_DEVICE(i, HAL_COMPASS_AK09916_I2C_ADDR),
@@ -1168,7 +1168,7 @@ void Compass::_probe_external_i2c_compasses(void)
 #endif
 #endif
 
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) || defined(HAL_USE_I2C_MAG_IST8310)
+#if AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED || defined(HAL_USE_I2C_MAG_IST8310)
     // IST8310 on external and internal bus
     if (AP_BoardConfig::get_board_type() != AP_BoardConfig::PX4_BOARD_FMUV5 &&
         AP_BoardConfig::get_board_type() != AP_BoardConfig::PX4_BOARD_FMUV6) {
@@ -1197,7 +1197,7 @@ void Compass::_probe_external_i2c_compasses(void)
     }
 #endif
 
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) || defined(HAL_USE_I2C_MAG_IST8308)
+#if AP_COMPASS_IST8308_ENABLED
     // external i2c bus
     FOREACH_I2C_EXTERNAL(i) {
         ADD_BACKEND(DRIVER_IST8308, AP_Compass_IST8308::probe(GET_I2C_DEVICE(i, HAL_COMPASS_IST8308_I2C_ADDR),
@@ -1209,9 +1209,9 @@ void Compass::_probe_external_i2c_compasses(void)
                     all_external, ROTATION_NONE));
     }
 #endif
-#endif
+#endif  // AP_COMPASS_IST8308_ENABLED
 
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) || defined(HAL_USE_I2C_MAG_MMC3416)
+#if AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED || defined(HAL_USE_I2C_MAG_MMC3416)
     // external i2c bus
     FOREACH_I2C_EXTERNAL(i) {
         ADD_BACKEND(DRIVER_MMC3416, AP_Compass_MMC3416::probe(GET_I2C_DEVICE(i, HAL_COMPASS_MMC3416_I2C_ADDR),
@@ -1225,7 +1225,7 @@ void Compass::_probe_external_i2c_compasses(void)
 #endif
 #endif
 
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) || defined(HAL_USE_I2C_MAG_RM3100)
+#if AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED || defined(HAL_USE_I2C_MAG_RM3100)
 #ifdef HAL_COMPASS_RM3100_I2C_ADDR
     const uint8_t rm3100_addresses[] = { HAL_COMPASS_RM3100_I2C_ADDR };
 #else
@@ -1251,7 +1251,7 @@ void Compass::_probe_external_i2c_compasses(void)
 #endif
 #endif
 
-#if !defined(HAL_DISABLE_I2C_MAGS_BY_DEFAULT) && !defined(STM32F1)
+#if AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED && !defined(STM32F1)
     // BMM150 on I2C, not on F1 to save flash
     FOREACH_I2C_EXTERNAL(i) {
         for (uint8_t addr=BMM150_I2C_ADDR_MIN; addr <= BMM150_I2C_ADDR_MAX; addr++) {
diff --git a/libraries/AP_Compass/AP_Compass_Backend.h b/libraries/AP_Compass/AP_Compass_Backend.h
index 784bf0081d..0faae41500 100644
--- a/libraries/AP_Compass/AP_Compass_Backend.h
+++ b/libraries/AP_Compass/AP_Compass_Backend.h
@@ -29,6 +29,8 @@
 #include <AP_MSP/msp.h>
 #endif
 
+#include <AP_Math/AP_Math.h>
+
 class Compass;  // forward declaration
 class AP_Compass_Backend
 {
diff --git a/libraries/AP_Compass/AP_Compass_IST8308.cpp b/libraries/AP_Compass/AP_Compass_IST8308.cpp
index 9aa883c9e8..c4c3ea499f 100644
--- a/libraries/AP_Compass/AP_Compass_IST8308.cpp
+++ b/libraries/AP_Compass/AP_Compass_IST8308.cpp
@@ -16,6 +16,8 @@
  */
 #include "AP_Compass_IST8308.h"
 
+#if AP_COMPASS_IST8308_ENABLED
+
 #include <stdio.h>
 #include <utility>
 
@@ -224,3 +226,5 @@ void AP_Compass_IST8308::read()
 {
     drain_accumulated_samples(_instance);
 }
+
+#endif  // AP_COMPASS_IST8308_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_IST8308.h b/libraries/AP_Compass/AP_Compass_IST8308.h
index 002158247b..94d4899dd9 100644
--- a/libraries/AP_Compass/AP_Compass_IST8308.h
+++ b/libraries/AP_Compass/AP_Compass_IST8308.h
@@ -16,12 +16,14 @@
  */
 #pragma once
 
+#include "AP_Compass_config.h"
+
+#if AP_COMPASS_IST8308_ENABLED
+
 #include <AP_Common/AP_Common.h>
 #include <AP_HAL/AP_HAL.h>
 #include <AP_HAL/I2CDevice.h>
-#include <AP_Math/AP_Math.h>
 
-#include "AP_Compass.h"
 #include "AP_Compass_Backend.h"
 
 #ifndef HAL_COMPASS_IST8308_I2C_ADDR
@@ -53,3 +55,5 @@ private:
     uint8_t _instance;
     bool _force_external;
 };
+
+#endif  // AP_COMPASS_IST8308_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_config.h b/libraries/AP_Compass/AP_Compass_config.h
index 448687b0d0..d10af7e9d8 100644
--- a/libraries/AP_Compass/AP_Compass_config.h
+++ b/libraries/AP_Compass/AP_Compass_config.h
@@ -9,18 +9,31 @@
 #endif
 
 // Backend support
+#ifndef AP_COMPASS_BACKEND_DEFAULT_ENABLED
+#define AP_COMPASS_BACKEND_DEFAULT_ENABLED 1
+#endif
+
 #ifndef AP_COMPASS_EXTERNALAHRS_ENABLED
-#define AP_COMPASS_EXTERNALAHRS_ENABLED HAL_EXTERNAL_AHRS_ENABLED
+#define AP_COMPASS_EXTERNALAHRS_ENABLED (AP_COMPASS_BACKEND_DEFAULT_ENABLED && HAL_EXTERNAL_AHRS_ENABLED)
 #endif
 
 #ifndef AP_COMPASS_MSP_ENABLED
-#define AP_COMPASS_MSP_ENABLED HAL_MSP_SENSORS_ENABLED
+#define AP_COMPASS_MSP_ENABLED (AP_COMPASS_BACKEND_DEFAULT_ENABLED && HAL_MSP_SENSORS_ENABLED)
 #endif
 
 #ifndef AP_COMPASS_SITL_ENABLED
-#define AP_COMPASS_SITL_ENABLED AP_SIM_ENABLED
+#define AP_COMPASS_SITL_ENABLED (AP_COMPASS_BACKEND_DEFAULT_ENABLED && AP_SIM_ENABLED)
 #endif
 
 #ifndef AP_COMPASS_UAVCAN_ENABLED
-#define AP_COMPASS_UAVCAN_ENABLED HAL_ENABLE_LIBUAVCAN_DRIVERS
+#define AP_COMPASS_UAVCAN_ENABLED (AP_COMPASS_BACKEND_DEFAULT_ENABLED && HAL_ENABLE_LIBUAVCAN_DRIVERS)
+#endif
+
+// i2c-based compasses:
+#ifndef AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED
+#define AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED AP_COMPASS_BACKEND_DEFAULT_ENABLED
+#endif
+
+#ifndef AP_COMPASS_IST8308_ENABLED
+#define AP_COMPASS_IST8308_ENABLED AP_COMPASS_I2C_BACKEND_DEFAULT_ENABLED
 #endif