4. Device Bindings

This chapter contains requirements, known as bindings, for how specific types and classes of devices are represented in the devicetree. The compatible property of a device node describes the specific binding (or bindings) to which the node complies.

Bindings may be defined as extensions of other each. For example a new bus type could be defined as an extension of the simple-bus binding. In this case, the compatible property would contain several strings identifying each binding—from the most specific to the most general (see section 2.3.1, compatible).

4.1. Binding Guidelines

4.1.1. General Principles

When creating a new devicetree representation for a device, a binding should be created that fully describes the required properties and value of the device. This set of properties shall be sufficiently descriptive to provide device drivers with needed attributes of the device.

Some recommended practices include:

1. Define a compatible string using the conventions described in section 2.3.1.

2. Use the standard properties (defined in sections 2.3 and 2.4) as applicable for the new device. This usage typically includes the reg and interrupts properties at a minimum.

3. Use the conventions specified in section 4 (Device Bindings) if the new device fits into one the DTSpec defined device classes.

4. Use the miscellaneous property conventions specified in section 4.1.2, if applicable.

5. If new properties are needed by the binding, the recommended format for property names is: "<company>,<property-name>", where <company> is an OUI or short unique string like a stock ticker that identifies the creator of the binding.

   Example: "ibm,ppc-interrupt-server#s"

4.1.2. Miscellaneous Properties

This section defines a list of helpful properties that might be applicable to many types of devices and device classes. They are defined here to facilitate standardization of names and usage.

4.1.2.1. clock-frequency Property

Table 4.1 clock-frequency Property

Property	clock-frequency
Value type	<prop-encoded-array>
Description	Specifies the frequency of a clock in Hz. The value is a <prop-encoded-array> in one of two forms: a 32-bit integer consisting of one <u32> specifying the frequency a 64-bit integer represented as a <u64> specifying the frequency

4.1.2.2. reg-shift Property

Table 4.2 reg-shift Property

Property	reg-shift
Value type	<u32>
Description	The reg-shift property provides a mechanism to represent devices that are identical in most respects except for the number of bytes between registers. The reg-shift property specifies in bytes how far the discrete device registers are separated from each other. The individual register location is calculated by using following formula: “registers address” << reg-shift. If unspecified, the default value is 0. For example, in a system where 16540 UART registers are located at addresses 0x0, 0x4, 0x8, 0xC, 0x10, 0x14, 0x18, and 0x1C, a reg-shift = <2> property would be used to specify register locations.

4.1.2.3. label Property

Table 4.3 label Property

Property	label
Value type	<string>
Description	The label property defines a human readable string describing a device. The binding for a given device specifies the exact meaning of the property for that device.

4.2. Serial devices

4.2.1. Serial Class Binding

The class of serial devices consists of various types of point to point serial line devices. Examples of serial line devices include the 8250 UART, 16550 UART, HDLC device, and BISYNC device. In most cases hardware compatible with the RS-232 standard fit into the serial device class.

I²C and SPI (Serial Peripheral Interface) devices shall not be represented as serial port devices because they have their own specific representation.

4.2.1.1. clock-frequency Property

Table 4.4 clock-frequecy Property

Property	clock-frequency
Value type	<u32>
Description	Specifies the frequency in Hertz of the baud rate generator’s input clock.
Example	clock-frequency = <100000000>;

4.2.1.2. current-speed Property

Table 4.5 current-speed Property

Property	current-speed
Value type	<u32>
Description	Specifies the current speed of a serial device in bits per second. A boot program should set this property if it has initialized the serial device.
Example	115,200 Baud: current-speed = <115200>;

4.2.2. National Semiconductor 16450/16550 Compatible UART Requirements

Serial devices compatible to the National Semiconductor 16450/16550 UART (Universal Asynchronous Receiver Transmitter) should be represented in the devicetree using following properties.

Table 4.6 ns16550 UART Properties

Property Name	Usage	Value Type	Definition
compatible	R	<string list>	Value shall include “ns16550”.
clock-frequency	R	<u32>	Specifies the frequency (in Hz) of the baud rate generator’s input clock
current-speed	OR	<u32>	Specifies current serial device speed in bits per second
reg	R	<prop encoded array>	Specifies the physical address of the registers device within the address space of the parent bus
interrupts	OR	<prop encoded array>	Specifies the interrupts generated by this device. The value of the interrupts property consists of one or more interrupt specifiers. The format of an interrupt specifier is defined by the binding document describing the node’s interrupt parent.
reg-shift	O	<u32>	Specifies in bytes how far the discrete device registers are separated from each other. The individual register location is calculated by using following formula: "registers address" << reg-shift. If unspecified, the default value is 0.
virtual-reg	SD	<u32> or <u64>	See section 2.3.7. Specifies an effective address that maps to the first physical address specified in the reg property. This property is required if this device node is the system’s console.
Usage legend: R=Required, O=Optional, OR=Optional but Recommended, SD=See Definition

Note

All other standard properties (section 2.3) are allowed but are optional.

4.3. Network devices

Network devices are packet oriented communication devices. Devices in this class are assumed to implement the data link layer (layer 2) of the seven-layer OSI model and use Media Access Control (MAC) addresses. Examples of network devices include Ethernet, FDDI, 802.11, and Token-Ring.

4.3.1. Network Class Binding

4.3.1.1. address-bits Property

Table 4.7 address-bits Property

Property	address-bits
Value type	<u32>
Description	Specifies number of address bits required to address the device described by this node. This property specifies number of bits in MAC address. If unspecified, the default value is 48.
Example	address-bits = <48>;

4.3.1.2. local-mac-address Property

Table 4.8 local-mac-address Property

Property	local-mac-address
Value type	<prop-encoded-array> encoded as an array of hex numbers
Description	Specifies MAC address that was assigned to the network device described by the node containing this property.
Example	local-mac-address = [ 00 00 12 34 56 78 ];

4.3.1.3. mac-address Property

Table 4.9 mac-address Property

Property	mac-address
Value type	<prop-encoded-array> encoded as an array of hex numbers
Description	Specifies the MAC address that was last used by the boot program. This property should be used in cases where the MAC address assigned to the device by the boot program is different from the local-mac-address property. This property shall be used only if the value differs from local-mac-address property value.
Example	mac-address = [ 01 02 03 04 05 06 ];

4.3.1.4. max-frame-size Property

Table 4.10 max-frame-size Property

Property	max-frame-size
Value type	<u32>
Description	Specifies maximum packet length in bytes that the physical interface can send and receive.
Example	max-frame-size = <1518>;

4.3.2. Ethernet specific considerations

Network devices based on the IEEE 802.3 collections of LAN standards (collectively referred to as Ethernet) may be represented in the devicetree using following properties, in addition to properties specified of the network device class.

The properties listed in this section augment the properties listed in the network device class.

4.3.2.1. max-speed Property

Table 4.11 max-speed Property

Property	max-speed
Value type	<u32>
Description	Specifies maximum speed (specified in megabits per second) supported the device.
Example	max-speed = <1000>;

4.3.2.2. phy-connection-type Property

Table 4.12 phy-connection-type Property

Property	phy-connection-type
Value type	<string>
Description	Specifies interface type between the Ethernet device and a physical layer (PHY) device. The value of this property is specific to the implementation. Recommended values are shown in the following table.
Example	phy-connection-type = "mii";

Table 4.13 Defined values for the phy-connection-type Property

Connection type	Value
Media Independent Interface	mii
Reduced Media Independent Interface	rmii
Gigabit Media Independent Interface	gmii
Reduced Gigabit Media Independent	rgmii
rgmii with internal delay	rgmii-id
rgmii with internal delay on TX only	rgmii-txid
rgmii with internal delay on RX only	rgmii-rxid
Ten Bit Interface	tbi
Reduced Ten Bit Interface	rtbi
Serial Media Independent Interface	smii
Serial Gigabit Media Independent Interface	sgmii
Reverse Media Independent Interface	rev-mii
10 Gigabits Media Independent Interface	xgmii
Multimedia over Coaxial	moca
Quad Serial Gigabit Media Independent Interface	qsgmii
Turbo Reduced Gigabit Media Independent Interface	trgmii

4.3.2.3. phy-handle Property

Table 4.14 phy-handle Property

Property	phy-handle
Value type	<phandle>
Description	Specifies a reference to a node representing a physical layer (PHY) device connected to this Ethernet device. This property is required in case where the Ethernet device is connected a physical layer device.
Example	phy-handle = <&PHY0>;

4.4. Power ISA Open PIC Interrupt Controllers

This section specifies the requirements for representing Open PIC compatible interrupt controllers. An Open PIC interrupt controller implements the Open PIC architecture (developed jointly by AMD and Cyrix) and specified in The Open Programmable Interrupt Controller (PIC) Register Interface Specification Revision 1.2 [b18].

Interrupt specifiers in an Open PIC interrupt domain are encoded with two cells. The first cell defines the interrupt number. The second cell defines the sense and level information.

Sense and level information shall be encoded as follows in interrupt specifiers:

0 = low to high edge sensitive type enabled
1 = active low level sensitive type enabled
2 = active high level sensitive type enabled
3 = high to low edge sensitive type enabled

Table 4.15 Open-PIC properties

Property Name	Usage	Value Type	Definition
compatible	R	<string>	Value shall include "open-pic"
reg	R	<prop encoded array>	Specifies the physical address of the registers device within the address space of the parent bus
interrupt-controller	R	<empty>	Specifies that this node is an interrupt controller
#interrupt-cells	R	<u32>	Shall be 2.
#address-cells	R	<u32>	Shall be 0.
Usage legend: R=Required, O=Optional, OR=Optional but Recommended, SD=See Definition

Note

All other standard properties (section 2.3) are allowed but are optional.

4.5. simple-bus Compatible Value

System-on-a-chip processors may have an internal I/O bus that cannot be probed for devices. The devices on the bus can be accessed directly without additional configuration required. This type of bus is represented as a node with a compatible value of “simple-bus”.

Table 4.16 simple-bus Compatible Node Properties

Property Name	Usage	Value Type	Definition
compatible	R	<string>	Value shall include “simple-bus”.
ranges	R	<prop encoded array>	This property represents the mapping between parent address to child address spaces (see section 2.3.8, ranges).
Usage legend: R=Required, O=Optional, OR=Optional but Recommended, SD=See Definition