Index: share/man/man4/smsc.4
===================================================================
--- share/man/man4/smsc.4
+++ share/man/man4/smsc.4
@@ -72,6 +72,23 @@
 .It
 SMSC LAN951x Ethernet adapters with integrated USB hub
 .El
+.Sh LOADER TUNABLES
+Tunables can be set at the
+.Xr loader 8
+prompt before booting the kernel or stored in
+.Xr loader.conf 5 .
+.Bl -tag -width indent
+.It Va hw.usb.smsc.turbo_mode
+Disable or enable
+.Dq turbo mode
+which allows multiple frames to be received
+in each transaction.
+This increases throughput at the cost of latency and jitter.
+Disabling it by be useful for timekeeping or VoIP applications.
+Default is
+.Dq Li 1
+(enabled).
+.El
 .Sh SEE ALSO
 .Xr arp 4 ,
 .Xr intro 4 ,
Index: sys/dev/usb/net/if_smsc.c
===================================================================
--- sys/dev/usb/net/if_smsc.c
+++ sys/dev/usb/net/if_smsc.c
@@ -112,14 +112,19 @@
 
 #include <dev/usb/net/if_smscreg.h>
 
+SYSCTL_NODE(_hw_usb, OID_AUTO, smsc, CTLFLAG_RW, 0, "USB smsc");
+
 #ifdef USB_DEBUG
 static int smsc_debug = 0;
 
-SYSCTL_NODE(_hw_usb, OID_AUTO, smsc, CTLFLAG_RW, 0, "USB smsc");
 SYSCTL_INT(_hw_usb_smsc, OID_AUTO, debug, CTLFLAG_RWTUN, &smsc_debug, 0,
     "Debug level");
 #endif
 
+static int smsc_turbo_mode = 1;
+SYSCTL_INT(_hw_usb_smsc, OID_AUTO, turbo_mode, CTLFLAG_RDTUN, &smsc_turbo_mode, 1,
+    "Turbo mode");
+
 /*
  * Various supported device vendors/products.
  */
@@ -1398,10 +1403,13 @@
 	 * Burst capability is the number of URBs that can be in a burst of data/
 	 * ethernet frames.
 	 */
-	if (usbd_get_speed(sc->sc_ue.ue_udev) == USB_SPEED_HIGH)
-		burst_cap = 37;
-	else
-		burst_cap = 128;
+	if (smsc_turbo_mode) {
+		if (usbd_get_speed(sc->sc_ue.ue_udev) == USB_SPEED_HIGH)
+			burst_cap = 37;
+		else
+			burst_cap = 128;
+	} else
+		burst_cap = 0;
 
 	smsc_write_reg(sc, SMSC_BURST_CAP, burst_cap);
 
@@ -1427,7 +1435,8 @@
 	/* The following setings are used for 'turbo mode', a.k.a multiple frames
 	 * per Rx transaction (again info taken form Linux driver).
 	 */
-	reg_val |= (SMSC_HW_CFG_MEF | SMSC_HW_CFG_BCE);
+	if (smsc_turbo_mode)
+		reg_val |= (SMSC_HW_CFG_MEF | SMSC_HW_CFG_BCE);
 
 	smsc_write_reg(sc, SMSC_HW_CFG, reg_val);