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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 <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Russell King 2019-06-21 15:59:09 +01:00 committed by David S. Miller
parent 08003d0b63
commit 8c25c0cb5b
1 changed files with 44 additions and 1 deletions
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45
Documentation/networking/phy.rst
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@ -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:
``PHY_INTERFACE_MODE_1000BASEX``
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.)
``PHY_INTERFACE_MODE_2500BASEX``
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.
``PHY_INTERFACE_MODE_SGMII``
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
===========================