diff --git a/hw/usb-xhci.c b/hw/usb-xhci.c
index 0c5007b..28fe9de 100644
--- a/hw/usb-xhci.c
+++ b/hw/usb-xhci.c
@@ -177,7 +177,11 @@ typedef enum TRBType {
     ER_DOORBELL,
     ER_HOST_CONTROLLER,
     ER_DEVICE_NOTIFICATION,
-    ER_MFINDEX_WRAP
+    ER_MFINDEX_WRAP,
+    /* vendor specific bits */
+    CR_VENDOR_VIA_CHALLENGE_RESPONSE = 48,
+    CR_VENDOR_NEC_FIRMWARE_REVISION  = 49,
+    CR_VENDOR_NEC_CHALLENGE_RESPONSE = 50,
 } TRBType;
 
 #define CR_LINK TR_LINK
@@ -2140,18 +2144,15 @@ static void xhci_process_commands(XHCIState *xhci)
         case CR_GET_PORT_BANDWIDTH:
             event.ccode = xhci_get_port_bandwidth(xhci, trb.parameter);
             break;
-        case 48:
-            /* VIA challenge response */
+        case CR_VENDOR_VIA_CHALLENGE_RESPONSE:
             xhci_via_challenge(trb.parameter);
             break;
-        case 49:
-            /* NEC get FW revision */
+        case CR_VENDOR_NEC_FIRMWARE_REVISION:
             event.type = 48; /* NEC reply */
             event.length = 0x3025;
             break;
-        case 50:
+        case CR_VENDOR_NEC_CHALLENGE_RESPONSE:
         {
-            /* NEC challenge response */
             uint32_t chi = trb.parameter >> 32;
             uint32_t clo = trb.parameter;
             uint32_t val = xhci_nec_challenge(chi, clo);