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
@@ -29,6 +29,7 @@
 #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,
@@ -133,6 +134,9 @@
 	bus_space_handle_t	metrics_space;
 	bool			added_metrics_sysctl;
 	struct amdsmu_metrics	metrics;
+
+	bool			added_idlemask_sysctl;
+	uint32_t		idlemask;
 };
 
 static bool
@@ -471,6 +475,24 @@
 	    0, "Total time spent in software DRIPS (SW DRIPS) (us)");
 }
 
+static void
+amdsmu_dump_idlemask(device_t dev)
+{
+	struct amdsmu_softc	*sc = device_get_softc(dev);
+
+	sc->idlemask = bus_space_read_4(sc->bus_tag, sc->reg_space,
+	    SMU_REG_IDLEMASK);
+
+	/* Add sysctl node for idlemask. */
+	if (sc->added_idlemask_sysctl)
+		return;
+	sc->added_idlemask_sysctl = true;
+
+	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 debug issues");
+}
+
 static int
 amdsmu_enter(device_t dev)
 {
@@ -491,6 +513,7 @@
 		return (-1);
 	}
 	amdsmu_dump_metrics(dev);
+	amdsmu_dump_idlemask(dev);
 	return (0);
 }
 
@@ -554,6 +577,10 @@
 	amdsmu_init_metrics(dev);
 	amdsmu_dump_metrics(dev);
 
+	/* Set up for getting idlemask. */
+	sc->added_idlemask_sysctl = false;
+	amdsmu_dump_idlemask(dev);
+
 	return (0);
 }