===========================================
CPU topology binding description
===========================================

===========================================
1 - Introduction
===========================================

In a SMP system, the hierarchy of CPUs is defined through three entities that
are used to describe the layout of physical CPUs in the system:

- socket
- cluster
- core
- thread

The bottom hierarchy level sits at core or thread level depending on whether
symmetric multi-threading (SMT) is supported or not.

For instance in a system where CPUs support SMT, "cpu" nodes represent all
threads existing in the system and map to the hierarchy level "thread" above.
In systems where SMT is not supported "cpu" nodes represent all cores present
in the system and map to the hierarchy level "core" above.

CPU topology bindings allow one to associate cpu nodes with hierarchical groups
corresponding to the system hierarchy; syntactically they are defined as device
tree nodes.

Currently, only ARM/RISC-V intend to use this cpu topology binding but it may be
used for any other architecture as well.

The cpu nodes, as per bindings defined in [4], represent the devices that
correspond to physical CPUs and are to be mapped to the hierarchy levels.

A topology description containing phandles to cpu nodes that are not compliant
with bindings standardized in [4] is therefore considered invalid.

===========================================
2 - cpu-map node
===========================================

The ARM/RISC-V CPU topology is defined within the cpu-map node, which is a direct
child of the cpus node and provides a container where the actual topology
nodes are listed.

- cpu-map node

	Usage: Optional - On SMP systems provide CPUs topology to the OS.
			  Uniprocessor systems do not require a topology
			  description and therefore should not define a
			  cpu-map node.

	Description: The cpu-map node is just a container node where its
		     subnodes describe the CPU topology.

	Node name must be "cpu-map".

	The cpu-map node's parent node must be the cpus node.

	The cpu-map node's child nodes can be:

	- one or more cluster nodes or
	- one or more socket nodes in a multi-socket system

	Any other configuration is considered invalid.

The cpu-map node can only contain 4 types of child nodes:

- socket node
- cluster node
- core node
- thread node

whose bindings are described in paragraph 3.

The nodes describing the CPU topology (socket/cluster/core/thread) can
only be defined within the cpu-map node and every core/thread in the
system must be defined within the topology.  Any other configuration is
invalid and therefore must be ignored.

===========================================
2.1 - cpu-map child nodes naming convention
===========================================

cpu-map child nodes must follow a naming convention where the node name
must be "socketN", "clusterN", "coreN", "threadN" depending on the node type
(ie socket/cluster/core/thread) (where N = {0, 1, ...} is the node number; nodes
which are siblings within a single common parent node must be given a unique and
sequential N value, starting from 0).
cpu-map child nodes which do not share a common parent node can have the same
name (ie same number N as other cpu-map child nodes at different device tree
levels) since name uniqueness will be guaranteed by the device tree hierarchy.

===========================================
3 - socket/cluster/core/thread node bindings
===========================================

Bindings for socket/cluster/cpu/thread nodes are defined as follows:

- socket node

	 Description: must be declared within a cpu-map node, one node
		      per physical socket in the system. A system can
		      contain single or multiple physical socket.
		      The association of sockets and NUMA nodes is beyond
		      the scope of this bindings, please refer [2] for
		      NUMA bindings.

	This node is optional for a single socket system.

	The socket node name must be "socketN" as described in 2.1 above.
	A socket node can not be a leaf node.

	A socket node's child nodes must be one or more cluster nodes.

	Any other configuration is considered invalid.

- cluster node

	 Description: must be declared within a cpu-map node, one node
		      per cluster. A system can contain several layers of
		      clustering within a single physical socket and cluster
		      nodes can be contained in parent cluster nodes.

	The cluster node name must be "clusterN" as described in 2.1 above.
	A cluster node can not be a leaf node.

	A cluster node's child nodes must be:

	- one or more cluster nodes; or
	- one or more core nodes

	Any other configuration is considered invalid.

- core node

	Description: must be declared in a cluster node, one node per core in
		     the cluster. If the system does not support SMT, core
		     nodes are leaf nodes, otherwise they become containers of
		     thread nodes.

	The core node name must be "coreN" as described in 2.1 above.

	A core node must be a leaf node if SMT is not supported.

	Properties for core nodes that are leaf nodes:

	- cpu
		Usage: required
		Value type: <phandle>
		Definition: a phandle to the cpu node that corresponds to the
			    core node.

	If a core node is not a leaf node (CPUs supporting SMT) a core node's
	child nodes can be:

	- one or more thread nodes

	Any other configuration is considered invalid.

- thread node

	Description: must be declared in a core node, one node per thread
		     in the core if the system supports SMT. Thread nodes are
		     always leaf nodes in the device tree.

	The thread node name must be "threadN" as described in 2.1 above.

	A thread node must be a leaf node.

	A thread node must contain the following property:

	- cpu
		Usage: required
		Value type: <phandle>
		Definition: a phandle to the cpu node that corresponds to
			    the thread node.

===========================================
4 - Example dts
===========================================

Example 1 (ARM 64-bit, 16-cpu system, two clusters of clusters in a single
physical socket):

cpus {
	#size-cells = <0>;
	#address-cells = <2>;

	cpu-map {
		socket0 {
			cluster0 {
				cluster0 {
					core0 {
						thread0 {
							cpu = <&CPU0>;
						};
						thread1 {
							cpu = <&CPU1>;
						};
					};

					core1 {
						thread0 {
							cpu = <&CPU2>;
						};
						thread1 {
							cpu = <&CPU3>;
						};
					};
				};

				cluster1 {
					core0 {
						thread0 {
							cpu = <&CPU4>;
						};
						thread1 {
							cpu = <&CPU5>;
						};
					};

					core1 {
						thread0 {
							cpu = <&CPU6>;
						};
						thread1 {
							cpu = <&CPU7>;
						};
					};
				};
			};

			cluster1 {
				cluster0 {
					core0 {
						thread0 {
							cpu = <&CPU8>;
						};
						thread1 {
							cpu = <&CPU9>;
						};
					};
					core1 {
						thread0 {
							cpu = <&CPU10>;
						};
						thread1 {
							cpu = <&CPU11>;
						};
					};
				};

				cluster1 {
					core0 {
						thread0 {
							cpu = <&CPU12>;
						};
						thread1 {
							cpu = <&CPU13>;
						};
					};
					core1 {
						thread0 {
							cpu = <&CPU14>;
						};
						thread1 {
							cpu = <&CPU15>;
						};
					};
				};
			};
		};
	};

	CPU0: cpu@0 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x0 0x0>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU1: cpu@1 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x0 0x1>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU2: cpu@100 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x0 0x100>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU3: cpu@101 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x0 0x101>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU4: cpu@10000 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x0 0x10000>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU5: cpu@10001 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x0 0x10001>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU6: cpu@10100 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x0 0x10100>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU7: cpu@10101 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x0 0x10101>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU8: cpu@100000000 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x1 0x0>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU9: cpu@100000001 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x1 0x1>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU10: cpu@100000100 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x1 0x100>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU11: cpu@100000101 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x1 0x101>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU12: cpu@100010000 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x1 0x10000>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU13: cpu@100010001 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x1 0x10001>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU14: cpu@100010100 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x1 0x10100>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};

	CPU15: cpu@100010101 {
		device_type = "cpu";
		compatible = "arm,cortex-a57";
		reg = <0x1 0x10101>;
		enable-method = "spin-table";
		cpu-release-addr = <0 0x20000000>;
	};
};

Example 2 (ARM 32-bit, dual-cluster, 8-cpu system, no SMT):

cpus {
	#size-cells = <0>;
	#address-cells = <1>;

	cpu-map {
		cluster0 {
			core0 {
				cpu = <&CPU0>;
			};
			core1 {
				cpu = <&CPU1>;
			};
			core2 {
				cpu = <&CPU2>;
			};
			core3 {
				cpu = <&CPU3>;
			};
		};

		cluster1 {
			core0 {
				cpu = <&CPU4>;
			};
			core1 {
				cpu = <&CPU5>;
			};
			core2 {
				cpu = <&CPU6>;
			};
			core3 {
				cpu = <&CPU7>;
			};
		};
	};

	CPU0: cpu@0 {
		device_type = "cpu";
		compatible = "arm,cortex-a15";
		reg = <0x0>;
	};

	CPU1: cpu@1 {
		device_type = "cpu";
		compatible = "arm,cortex-a15";
		reg = <0x1>;
	};

	CPU2: cpu@2 {
		device_type = "cpu";
		compatible = "arm,cortex-a15";
		reg = <0x2>;
	};

	CPU3: cpu@3 {
		device_type = "cpu";
		compatible = "arm,cortex-a15";
		reg = <0x3>;
	};

	CPU4: cpu@100 {
		device_type = "cpu";
		compatible = "arm,cortex-a7";
		reg = <0x100>;
	};

	CPU5: cpu@101 {
		device_type = "cpu";
		compatible = "arm,cortex-a7";
		reg = <0x101>;
	};

	CPU6: cpu@102 {
		device_type = "cpu";
		compatible = "arm,cortex-a7";
		reg = <0x102>;
	};

	CPU7: cpu@103 {
		device_type = "cpu";
		compatible = "arm,cortex-a7";
		reg = <0x103>;
	};
};

Example 3: HiFive Unleashed (RISC-V 64 bit, 4 core system)

{
	#address-cells = <2>;
	#size-cells = <2>;
	compatible = "sifive,fu540g", "sifive,fu500";
	model = "sifive,hifive-unleashed-a00";

	...
	cpus {
		#address-cells = <1>;
		#size-cells = <0>;
		cpu-map {
			socket0 {
				cluster0 {
					core0 {
						cpu = <&CPU1>;
					};
					core1 {
						cpu = <&CPU2>;
					};
					core2 {
						cpu0 = <&CPU2>;
					};
					core3 {
						cpu0 = <&CPU3>;
					};
				};
			};
		};

		CPU1: cpu@1 {
			device_type = "cpu";
			compatible = "sifive,rocket0", "riscv";
			reg = <0x1>;
		}

		CPU2: cpu@2 {
			device_type = "cpu";
			compatible = "sifive,rocket0", "riscv";
			reg = <0x2>;
		}
		CPU3: cpu@3 {
			device_type = "cpu";
			compatible = "sifive,rocket0", "riscv";
			reg = <0x3>;
		}
		CPU4: cpu@4 {
			device_type = "cpu";
			compatible = "sifive,rocket0", "riscv";
			reg = <0x4>;
		}
	}
};
===============================================================================
[1] ARM Linux kernel documentation
    Documentation/devicetree/bindings/arm/cpus.yaml
[2] Devicetree NUMA binding description
    Documentation/devicetree/bindings/numa.txt
[3] RISC-V Linux kernel documentation
    Documentation/devicetree/bindings/riscv/cpus.yaml
[4] https://www.devicetree.org/specifications/