diff --git a/sys/dev/amdsmu/amdsmu.h b/sys/dev/amdsmu/amdsmu.h
--- a/sys/dev/amdsmu/amdsmu.h
+++ b/sys/dev/amdsmu/amdsmu.h
@@ -77,6 +77,7 @@
 
 	bus_space_handle_t	metrics_space;
 	struct amdsmu_metrics	metrics;
+	uint32_t		idlemask;
 };
 
 static inline uint32_t
diff --git a/sys/dev/amdsmu/amdsmu.c b/sys/dev/amdsmu/amdsmu.c
--- a/sys/dev/amdsmu/amdsmu.c
+++ b/sys/dev/amdsmu/amdsmu.c
@@ -281,6 +281,14 @@
 	return (0);
 }
 
+static void
+amdsmu_fetch_idlemask(device_t dev)
+{
+	struct amdsmu_softc *sc = device_get_softc(dev);
+
+	sc->idlemask = amdsmu_read4(sc, SMU_REG_IDLEMASK);
+}
+
 static int
 amdsmu_attach(device_t dev)
 {
@@ -407,6 +415,13 @@
 	if (err != 0)
 		goto err_dump;
 
+	/* Get idlemask & add sysctl. */
+	amdsmu_fetch_idlemask(dev);
+	SYSCTL_ADD_U32(sc->sysctlctx, SYSCTL_CHILDREN(sc->sysctlnode), OID_AUTO,
+	    "idlemask", CTLFLAG_RD, &sc->idlemask, 0, "SMU idlemask. This "
+	    "value is not documented - only used to help AMD internally debug "
+	    "issues");
+
 	return (0);
 err_dump:
 	bus_space_unmap(sc->bus_tag, sc->reg_space, SMU_MEM_SIZE);
diff --git a/sys/dev/amdsmu/amdsmu_reg.h b/sys/dev/amdsmu/amdsmu_reg.h
--- a/sys/dev/amdsmu/amdsmu_reg.h
+++ b/sys/dev/amdsmu/amdsmu_reg.h
@@ -32,6 +32,7 @@
 #define SMU_REG_MESSAGE		0x538
 #define SMU_REG_RESPONSE	0x980
 #define SMU_REG_ARGUMENT	0x9BC
+#define SMU_REG_IDLEMASK	0xD14
 
 enum amdsmu_res {
 	SMU_RES_WAIT		= 0x00,