dt-bindings: i3c: Document core bindings

A new I3C subsystem has been added and a generic description has been
created to represent the I3C bus and the devices connected on it.

Document this generic representation.

Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/i3c/i3c.txt

+Generic device tree bindings for I3C busses
+===========================================
+
+This document describes generic bindings that should be used to describe I3C
+busses in a device tree.
+
+Required properties
+-------------------
+
+- #address-cells  - should be <3>. Read more about addresses below.
+- #size-cells     - should be <0>.
+- compatible      - name of the I3C master controller driving the I3C bus
+
+For other required properties e.g. to describe register sets,
+clocks, etc. check the binding documentation of the specific driver.
+The node describing an I3C bus should be named i3c-master.
+
+Optional properties
+-------------------
+
+These properties may not be supported by all I3C master drivers. Each I3C
+master bindings should specify which of them are supported.
+
+- i3c-scl-hz: frequency of the SCL signal used for I3C transfers.
+	      When undefined the core sets it to 12.5MHz.
+
+- i2c-scl-hz: frequency of the SCL signal used for I2C transfers.
+	      When undefined, the core looks at LVR (Legacy Virtual Register)
+	      values of I2C devices described in the device tree to determine
+	      the maximum I2C frequency.
+
+I2C devices
+===========
+
+Each I2C device connected to the bus should be described in a subnode. All
+properties described in Documentation/devicetree/bindings/i2c/i2c.txt are
+valid here, but several new properties have been added.
+
+New constraint on existing properties:
+--------------------------------------
+- reg: contains 3 cells
+  + first cell : still encoding the I2C address
+
+  + second cell: shall be 0
+
+  + third cell: shall encode the I3C LVR (Legacy Virtual Register)
+	bit[31:8]: unused/ignored
+	bit[7:5]: I2C device index. Possible values
+	* 0: I2C device has a 50 ns spike filter
+	* 1: I2C device does not have a 50 ns spike filter but supports high
+	     frequency on SCL
+	* 2: I2C device does not have a 50 ns spike filter and is not tolerant
+	     to high frequencies
+	* 3-7: reserved
+
+	bit[4]: tell whether the device operates in FM (Fast Mode) or FM+ mode
+	* 0: FM+ mode
+	* 1: FM mode
+
+	bit[3:0]: device type
+	* 0-15: reserved
+
+The I2C node unit-address should always match the first cell of the reg
+property: <device-type>@<i2c-address>.
+
+I3C devices
+===========
+
+All I3C devices are supposed to support DAA (Dynamic Address Assignment), and
+are thus discoverable. So, by default, I3C devices do not have to be described
+in the device tree.
+This being said, one might want to attach extra resources to these devices,
+and those resources may have to be described in the device tree, which in turn
+means we have to describe I3C devices.
+
+Another use case for describing an I3C device in the device tree is when this
+I3C device has a static I2C address and we want to assign it a specific I3C
+dynamic address before the DAA takes place (so that other devices on the bus
+can't take this dynamic address).
+
+The I3C device should be names <device-type>@<static-i2c-address>,<i3c-pid>,
+where device-type is describing the type of device connected on the bus
+(gpio-controller, sensor, ...).
+
+Required properties
+-------------------
+- reg: contains 3 cells
+  + first cell : encodes the static I2C address. Should be 0 if the device does
+		 not have one (0 is not a valid I2C address).
+
+  + second and third cells: should encode the ProvisionalID. The second cell
+			    contains the manufacturer ID left-shifted by 1.
+			    The third cell contains ORing of the part ID
+			    left-shifted by 16, the instance ID left-shifted
+			    by 12 and the extra information. This encoding is
+			    following the PID definition provided by the I3C
+			    specification.
+
+Optional properties
+-------------------
+- assigned-address: dynamic address to be assigned to this device. This
+		    property is only valid if the I3C device has a static
+		    address (first cell of the reg property != 0).
+
+
+Example:
+
+	i3c-master@d040000 {
+		compatible = "cdns,i3c-master";
+		clocks = <&coreclock>, <&i3csysclock>;
+		clock-names = "pclk", "sysclk";
+		interrupts = <3 0>;
+		reg = <0x0d040000 0x1000>;
+		#address-cells = <3>;
+		#size-cells = <0>;
+		i2c-scl-hz = <100000>;
+
+		/* I2C device. */
+		nunchuk: nunchuk@52 {
+			compatible = "nintendo,nunchuk";
+			reg = <0x52 0x0 0x10>;
+		};
+
+		/* I3C device with a static I2C address. */
+		thermal_sensor: sensor@68,39200144004 {
+			reg = <0x68 0x392 0x144004>;
+			assigned-address = <0xa>;
+		};
+
+		/*
+		 * I3C device without a static I2C address but requiring
+		 * resources described in the DT.
+		 */
+		sensor@0,39200154004 {
+			reg = <0x0 0x392 0x154004>;
+			clocks = <&clock_provider 0>;
+		};
+	};