AP_InertialSensor: only increase FIFO buffer when compiled with fast rate

libraries/AP_InertialSensor/AP_InertialSensor_Invensensev3.cpp

@@ -171,7 +171,18 @@ struct PACKED FIFODataHighRes {
 static_assert(sizeof(FIFOData) == 16, "FIFOData must be 16 bytes");
 static_assert(sizeof(FIFODataHighRes) == 20, "FIFODataHighRes must be 20 bytes");
 
-#define INV3_FIFO_BUFFER_LEN 16
+/*
+    Ideally we would like the fifo buffer to be big enough to hold all of the packets that might have been
+    accumulated between reads. This is so that they can all be read in a single SPI transaction and avoid
+    the overhead of multiple reads. The maximum number of samples for 20-bit high res that can be store in the
+    fifo is 2k/20, or 105 samples. The likely maximum required for dynamic fifo is the output data rate / loop rate,
+    4k/200 or 20 samples in the extreme case we are trying to support.
+ */
+#if AP_INERTIALSENSOR_FAST_SAMPLE_WINDOW_ENABLED
+#define INV3_FIFO_BUFFER_LEN 24
+#else
+#define INV3_FIFO_BUFFER_LEN 8
+#endif
 
 AP_InertialSensor_Invensensev3::AP_InertialSensor_Invensensev3(AP_InertialSensor &imu,
                                                                AP_HAL::OwnPtr<AP_HAL::Device> _dev,