diff --git a/src/include/cpu/x86/tsc.h b/src/include/cpu/x86/tsc.h
index 6ce7f5f..8e49a66 100644
--- a/src/include/cpu/x86/tsc.h
+++ b/src/include/cpu/x86/tsc.h
@@ -40,4 +40,8 @@
 }
 #endif
 
+#if CONFIG_TSC_CONSTANT_RATE
+unsigned long tsc_freq_mhz(void);
+#endif
+
 #endif /* CPU_X86_TSC_H */