net: phy: Add Edge-rate driver for Microsemi PHYs.

Edge-rate:
As system and networking speeds increase, a signal's output transition,
also know as the edge rate or slew rate (V/ns), takes on greater importance
because high-speed signals come with a price. That price is an assortment of
interference problems like ringing on the line, signal overshoot and
undershoot, extended signal settling times, crosstalk noise, transmission
line reflections, false signal detection by the receiving device and
electromagnetic interference (EMI) -- all of which can negate the potential
gains designers are seeking when they try to increase system speeds through
the use of higher performance logic devices. The fact is, faster signaling
edge rates can cause a higher level of electrical noise or other type of
interference that can actually lead to slower line speeds and lower maximum
system frequencies. This parameter allow the board designers to change the
driving strange, and thereby change the EMI behavioral.

Edge-rate parameters (vddmac, edge-slowdown) get from Device Tree.

Tested on Beaglebone Black with VSC 8531 PHY.

Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microsemi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 58 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/net/mscc-phy-vsc8531.txt b/Documentation/devicetree/bindings/net/mscc-phy-vsc8531.txt
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index 00000000000000..99c7eb0a00c8f3
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+++ b/Documentation/devicetree/bindings/net/mscc-phy-vsc8531.txt
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+* Microsemi - vsc8531 Giga bit ethernet phy
+
+Required properties:
+- compatible	: Should contain phy id as "ethernet-phy-idAAAA.BBBB"
+		  The PHY device uses the binding described in
+		  Documentation/devicetree/bindings/net/phy.txt
+
+Optional properties:
+- vsc8531,vddmac	: The vddmac in mV.
+- vsc8531,edge-slowdown	: % the edge should be slowed down relative to
+			  the fastest possible edge time. Native sign
+			  need not enter.
+			  Edge rate sets the drive strength of the MAC
+			  interface output signals.  Changing the drive
+			  strength will affect the edge rate of the output
+			  signal.  The goal of this setting is to help
+			  reduce electrical emission (EMI) by being able
+			  to reprogram drive strength and in effect slow
+			  down the edge rate if desired.  Table 1 shows the
+			  impact to the edge rate per VDDMAC supply for each
+			  drive strength setting.
+			  Ref: Table:1 - Edge rate change below.
+
+Note: see dt-bindings/net/mscc-phy-vsc8531.h for applicable values
+
+Table: 1 - Edge rate change
+----------------------------------------------------------------|
+| 		Edge Rate Change (VDDMAC)			|
+|								|
+| 3300 mV	2500 mV		1800 mV		1500 mV		|
+|---------------------------------------------------------------|
+| Default	Deafult		Default		Default		|
+| (Fastest)			(recommended)	(recommended)	|
+|---------------------------------------------------------------|
+| -2%		-3%		-5%		-6%		|
+|---------------------------------------------------------------|
+| -4%		-6%		-9%		-14%		|
+|---------------------------------------------------------------|
+| -7%		-10%		-16%		-21%		|
+|(recommended)	(recommended)					|
+|---------------------------------------------------------------|
+| -10%		-14%		-23%		-29%		|
+|---------------------------------------------------------------|
+| -17%		-23%		-35%		-42%		|
+|---------------------------------------------------------------|
+| -29%		-37%		-52%		-58%		|
+|---------------------------------------------------------------|
+| -53%		-63%		-76%		-77%		|
+| (slowest)							|
+|---------------------------------------------------------------|
+
+Example:
+
+        vsc8531_0: ethernet-phy@0 {
+                compatible = "ethernet-phy-id0007.0570";
+                vsc8531,vddmac		= <3300>;
+                vsc8531,edge-slowdown	= <21>;
+        };