diff --git a/include/usb/xhci.h b/include/usb/xhci.h
index a3e5914..15926eb 100644
--- a/include/usb/xhci.h
+++ b/include/usb/xhci.h
@@ -850,6 +850,8 @@
 /* transfer_len bitmasks - bits 0:16 */
 #define	TRB_LEN(p)			((p) & 0x1ffff)
 #define	TRB_LEN_MASK			(0x1ffff)
+/* TD Size, packets remaining in this TD, bits 21:17 (5 bits, so max 31) */
+#define TRB_TD_SIZE(p)          (min((p), (u32)31) << 17)
 /* Interrupter Target - which MSI-X vector to target the completion event at */
 #define	TRB_INTR_TARGET_SHIFT		(22)
 #define	TRB_INTR_TARGET_MASK		(0x3ff)