net: phy: micrel: add documentation for Micrel KSZ90x1 binding

Add the DTS documentation for the Micrel KSZ90x1 binding.

The original document was from:
[commit 4b405efbe12de28b26289282b431323d73992381 from the Linux kernel]

This takes the original document and adds a clarification on how the skew
values are represented in the code.

References:

Micrel ksz9021rl/rn Data Sheet, Revision 1.2. Dated 2/13/2014.
http://www.micrel.com/_PDF/Ethernet/datasheets/ksz9021rl-rn_ds.pdf

Micrel ksz9031rnx Data Sheet, Revision 2.1. Dated 11/20/2014.
http://www.micrel.com/_PDF/Ethernet/datasheets/KSZ9031RNX.pdf

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

doc/device-tree-bindings/net/micrel-ksz90x1.txt

@@ -0,0 +1,165 @@
+Micrel KSZ9021/KSZ9031 Gigabit Ethernet PHY
+
+Some boards require special tuning values, particularly when it comes to
+clock delays. You can specify clock delay values by adding
+micrel-specific properties to an Ethernet OF device node.
+
+Note that these settings are applied after any phy-specific fixup from
+phy_fixup_list (see phy_init_hw() from drivers/net/phy/phy_device.c),
+and therefore may overwrite them.
+
+KSZ9021:
+
+  All skew control options are specified in picoseconds. The minimum
+  value is 0, the maximum value is 1800, and it is incremented by 120ps
+  steps.
+
+  Optional properties:
+
+    - rxc-skew-ps : Skew control of RXC pad
+    - rxdv-skew-ps : Skew control of RX CTL pad
+    - txc-skew-ps : Skew control of TXC pad
+    - txen-skew-ps : Skew control of TX CTL pad
+    - rxd0-skew-ps : Skew control of RX data 0 pad
+    - rxd1-skew-ps : Skew control of RX data 1 pad
+    - rxd2-skew-ps : Skew control of RX data 2 pad
+    - rxd3-skew-ps : Skew control of RX data 3 pad
+    - txd0-skew-ps : Skew control of TX data 0 pad
+    - txd1-skew-ps : Skew control of TX data 1 pad
+    - txd2-skew-ps : Skew control of TX data 2 pad
+    - txd3-skew-ps : Skew control of TX data 3 pad
+
+KSZ9031:
+
+  All skew control options are specified in picoseconds. The minimum
+  value is 0, and the maximum is property-dependent. The increment
+  step is 60ps.
+
+  The KSZ9031 hardware supports a range of skew values from negative to
+  positive, where the specific range is property dependent. All values
+  specified in the devicetree are offset by the minimum value so they
+  can be represented as positive integers in the devicetree since it's
+  difficult to represent a negative number in the devictree.
+
+  The following 5-bit values table apply to rxc-skew-ps and txc-skew-ps.
+
+  Pad Skew Value	Delay (ps)	Devicetree Value
+  ------------------------------------------------------
+  0_0000		-900ps		0
+  0_0001		-840ps		60
+  0_0010		-780ps		120
+  0_0011		-720ps		180
+  0_0100		-660ps		240
+  0_0101		-600ps		300
+  0_0110		-540ps		360
+  0_0111		-480ps		420
+  0_1000		-420ps		480
+  0_1001		-360ps		540
+  0_1010		-300ps		600
+  0_1011		-240ps		660
+  0_1100		-180ps		720
+  0_1101		-120ps		780
+  0_1110		-60ps		840
+  0_1111		0ps		900
+  1_0000		60ps		960
+  1_0001		120ps		1020
+  1_0010		180ps		1080
+  1_0011		240ps		1140
+  1_0100		300ps		1200
+  1_0101		360ps		1260
+  1_0110		420ps		1320
+  1_0111		480ps		1380
+  1_1000		540ps		1440
+  1_1001		600ps		1500
+  1_1010		660ps		1560
+  1_1011		720ps		1620
+  1_1100		780ps		1680
+  1_1101		840ps		1740
+  1_1110		900ps		1800
+  1_1111		960ps		1860
+
+  The following 4-bit values table apply to the txdX-skew-ps, rxdX-skew-ps
+  data pads, and the rxdv-skew-ps, txen-skew-ps control pads.
+
+  Pad Skew Value	Delay (ps)	Devicetree Value
+  ------------------------------------------------------
+  0000			-420ps		0
+  0001			-360ps		60
+  0010			-300ps		120
+  0011			-240ps		180
+  0100			-180ps		240
+  0101			-120ps		300
+  0110			-60ps		360
+  0111			0ps		420
+  1000			60ps		480
+  1001			120ps		540
+  1010			180ps		600
+  1011			240ps		660
+  1100			300ps		720
+  1101			360ps		780
+  1110			420ps		840
+  1111			480ps		900
+
+  Optional properties:
+
+    Maximum value of 1860:
+
+      - rxc-skew-ps : Skew control of RX clock pad
+      - txc-skew-ps : Skew control of TX clock pad
+
+    Maximum value of 900:
+
+      - rxdv-skew-ps : Skew control of RX CTL pad
+      - txen-skew-ps : Skew control of TX CTL pad
+      - rxd0-skew-ps : Skew control of RX data 0 pad
+      - rxd1-skew-ps : Skew control of RX data 1 pad
+      - rxd2-skew-ps : Skew control of RX data 2 pad
+      - rxd3-skew-ps : Skew control of RX data 3 pad
+      - txd0-skew-ps : Skew control of TX data 0 pad
+      - txd1-skew-ps : Skew control of TX data 1 pad
+      - txd2-skew-ps : Skew control of TX data 2 pad
+      - txd3-skew-ps : Skew control of TX data 3 pad
+
+Examples:
+
+	/* Attach to an Ethernet device with autodetected PHY */
+	&enet {
+		rxc-skew-ps = <1800>;
+		rxdv-skew-ps = <0>;
+		txc-skew-ps = <1800>;
+		txen-skew-ps = <0>;
+		status = "okay";
+	};
+
+	/* Attach to an explicitly-specified PHY */
+	mdio {
+		phy0: ethernet-phy@0 {
+			rxc-skew-ps = <1800>;
+			rxdv-skew-ps = <0>;
+			txc-skew-ps = <1800>;
+			txen-skew-ps = <0>;
+			reg = <0>;
+		};
+	};
+	ethernet@70000 {
+		status = "okay";
+		phy = <&phy0>;
+		phy-mode = "rgmii-id";
+	};
+
+References
+
+  Micrel ksz9021rl/rn Data Sheet, Revision 1.2. Dated 2/13/2014.
+  http://www.micrel.com/_PDF/Ethernet/datasheets/ksz9021rl-rn_ds.pdf
+
+  Micrel ksz9031rnx Data Sheet, Revision 2.1. Dated 11/20/2014.
+  http://www.micrel.com/_PDF/Ethernet/datasheets/KSZ9031RNX.pdf
+
+Notes:
+
+  Note that a previous version of the Micrel ksz9021rl/rn Data Sheet
+  was missing extended register 106 (transmit data pad skews), and
+  incorrectly specified the ps per step as 200ps/step instead of
+  120ps/step. The latest update to this document reflects the latest
+  revision of the Micrel specification even though usage in the kernel
+  still reflects that incorrect document.