net: phy: Introduce QUSGMII PHY mode

The QUSGMII mode is a derivative of Cisco's USXGMII standard. This
standard is pretty similar to SGMII, but allows for faster speeds, and
has the build-in bits for Quad and Octa variants (like QSGMII).

The main difference with SGMII/QSGMII is that USXGMII/QUSGMII re-uses
the preamble to carry various information, named 'Extensions'.

As of today, the USXGMII standard only mentions the "PCH" extension,
which is used to convey timestamps, allowing in-band signaling of PTP
timestamps without having to modify the frame itself.

This commit adds support for that mode. When no extension is in use, it
behaves exactly like QSGMII, although it's not compatible with QSGMII.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 0 deletions

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
 ===========================