diff options
authorRussell King <>2019-06-21 15:49:21 +0100
committerRussell King <>2019-07-09 11:44:26 +0100
commit290696a412e9ae4decb0bd123e3a2ff6241d9845 (patch)
parent0a993241b9b9563b4968f558e941d240cddbcf22 (diff)
doc: phy: document some PHY_INTERFACE_MODE_xxx settings
There seems to be some confusion surrounding three PHY interface modes, specifically 1000BASE-X, 2500BASE-X and SGMII. Add some documentation to phylib detailing precisely what these interface modes refer to. Signed-off-by: Russell King <>
1 files changed, 44 insertions, 1 deletions
diff --git a/Documentation/networking/phy.rst b/Documentation/networking/phy.rst
index 0dd90d7df5ec..a689966bc4be 100644
--- a/Documentation/networking/phy.rst
+++ b/Documentation/networking/phy.rst
@@ -202,7 +202,8 @@ the PHY/controller, of which the PHY needs to be aware.
*interface* is a u32 which specifies the connection type used
between the controller and the PHY. Examples are GMII, MII,
-RGMII, and SGMII. For a full list, see include/linux/phy.h
+RGMII, and SGMII. See "PHY interface mode" below. For a full
+list, see include/linux/phy.h
Now just make sure that phydev->supported and phydev->advertising have any
values pruned from them which don't make sense for your controller (a 10/100
@@ -225,6 +226,48 @@ When you want to disconnect from the network (even if just briefly), you call
phy_stop(phydev). This function also stops the phylib state machine and
disables PHY interrupts.
+PHY interface modes
+The PHY interface mode supplied in the phy_connect() family of functions
+defines the initial operating mode of the PHY interface. This is not
+guaranteed to remain constant; there are PHYs which dynamically change
+their interface mode without software interaction depending on the
+negotiation results.
+Some of the interface modes are described below:
+ This defines the 1000BASE-X single-lane serdes link as defined by the
+ 802.3 standard section 36. The link operates at a fixed bit rate of
+ 1.25Gbaud using a 10B/8B encoding scheme, resulting in an underlying
+ data rate of 1Gbps. Embedded in the data stream is a 16-bit control
+ word which is used to negotiate the duplex and pause modes with the
+ remote end. This does not include "up-clocked" variants such as 2.5Gbps
+ speeds (see below.)
+ This defines a variant of 1000BASE-X which is clocked 2.5 times faster,
+ than the 802.3 standard giving a fixed bit rate of 3.125Gbaud.
+ This is used for Cisco SGMII, which is a modification of 1000BASE-X
+ as defined by the 802.3 standard. The SGMII link consists of a single
+ serdes lane running at a fixed bit rate of 1.25Gbaud with 10B/8B
+ encoding. The underlying data rate is 1Gbps, with the slower speeds of
+ 100Mbps and 10Mbps being achieved through replication of each data symbol.
+ The 802.3 control word is re-purposed to send the negotiated speed and
+ duplex information from to the MAC, and for the MAC to acknowledge
+ receipt. This does not include "up-clocked" variants such as 2.5Gbps
+ speeds.
+ Note: mismatched SGMII vs 1000BASE-X configuration on a link can
+ successfully pass data in some circumstances, but the 16-bit control
+ word will not be correctly interpreted, which may cause mismatches in
+ duplex, pause or other settings. This is dependent on the MAC and/or
+ PHY behaviour.
Pause frames / flow control