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diff --git a/Documentation/networking/phy.rst b/Documentation/networking/phy.rst
index 704f31da5167..712e44caebd0 100644
--- a/Documentation/networking/phy.rst
+++ b/Documentation/networking/phy.rst
@@ -308,6 +308,15 @@ Some of the interface modes are described below:
rate of 125Mpbs using a 4B/5B encoding scheme, resulting in an underlying
data rate of 100Mpbs.
+``PHY_INTERFACE_MODE_QUSGMII``
+ This defines the Cisco the Quad USGMII mode, which is the Quad variant of
+ the USGMII (Universal SGMII) link. It's very similar to QSGMII, but uses
+ a Packet Control Header (PCH) instead of the 7 bytes preamble to carry not
+ only the port id, but also so-called "extensions". The only documented
+ extension so-far in the specification is the inclusion of timestamps, for
+ PTP-enabled PHYs. This mode isn't compatible with QSGMII, but offers the
+ same capabilities in terms of link speed and negociation.
+
Pause frames / flow control
===========================