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RISC-V CPU Bindings
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The device tree allows to describe the layout of CPUs in a system through
the “cpus” node, which in turn contains a number of subnodes (ie “cpu”)
defining properties for every cpu.
Bindings for CPU nodes follow the Devicetree Specification, available from:
https://www.devicetree.org/specifications/
with updates for 32-bit and 64-bit RISC-V systems provided in this document.
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Terminology
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This document uses some terminology common to the RISC-V community that is not
widely used, the definitions of which are listed here:
- hart: A hardware execution context, which contains all the state mandated by
the RISC-V ISA: a PC and some registers. This terminology is designed to
disambiguate software’s view of execution contexts from any particular
microarchitectural implementation strategy. For example, my Intel laptop is
described as having one socket with two cores, each of which has two hyper
threads. Therefore this system has four harts.
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cpus and cpu node bindings definition
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The RISC-V architecture, in accordance with the Devicetree Specification,
requires the cpus and cpu nodes to be present and contain the properties
described below.
cpus node
Description: Container of cpu nodes The node name must be "cpus". A cpus node must define the following properties: - #address-cells Usage: required Value type: <u32> Definition: must be set to 1 - #size-cells Usage: required Value type: <u32> Definition: must be set to 0cpu node
Description: Describes a hart context PROPERTIES - device_type Usage: required Value type: <string> Definition: must be "cpu" - reg Usage: required Value type: <u32> Definition: The hart ID of this CPU node - compatible: Usage: required Value type: <stringlist> Definition: must contain "riscv", may contain one of "sifive,rocket0" - mmu-type: Usage: optional Value type: <string> Definition: Specifies the CPU's MMU type. Possible values are "riscv,sv32" "riscv,sv39" "riscv,sv48" - riscv,isa: Usage: required Value type: <string> Definition: Contains the RISC-V ISA string of this hart. These ISA strings are defined by the RISC-V ISA manual.
Example: SiFive Freedom U540G Development Kit
This system contains two harts: a hart marked as disabled that’s used for
low-level system tasks and should be ignored by Linux, and a second hart that
Linux is allowed to run on.
cpus {
#address-cells = <1>;
#size-cells = <0>;
timebase-frequency = <1000000>;
cpu@0 {
clock-frequency = <1600000000>;
compatible = "sifive,rocket0", "riscv";
device_type = "cpu";
i-cache-block-size = <64>;
i-cache-sets = <128>;
i-cache-size = <16384>;
next-level-cache = <&L15 &L0>;
reg = <0>;
riscv,isa = "rv64imac";
status = "disabled";
L10: interrupt-controller {
#interrupt-cells = <1>;
compatible = "riscv,cpu-intc";
interrupt-controller;
};
};
cpu@1 {
clock-frequency = <1600000000>;
compatible = "sifive,rocket0", "riscv";
d-cache-block-size = <64>;
d-cache-sets = <64>;
d-cache-size = <32768>;
d-tlb-sets = <1>;
d-tlb-size = <32>;
device_type = "cpu";
i-cache-block-size = <64>;
i-cache-sets = <64>;
i-cache-size = <32768>;
i-tlb-sets = <1>;
i-tlb-size = <32>;
mmu-type = "riscv,sv39";
next-level-cache = <&L15 &L0>;
reg = <1>;
riscv,isa = "rv64imafdc";
status = "okay";
tlb-split;
L13: interrupt-controller {
#interrupt-cells = <1>;
compatible = "riscv,cpu-intc";
interrupt-controller;
};
};
};
Example: Spike ISA Simulator with 1 Hart
This device tree matches the Spike ISA golden model as run with spike -p1.
cpus {
cpu@0 {
device_type = "cpu";
reg = <0x00000000>;
status = "okay";
compatible = "riscv";
riscv,isa = "rv64imafdc";
mmu-type = "riscv,sv48";
clock-frequency = <0x3b9aca00>;
interrupt-controller {
#interrupt-cells = <0x00000001>;
interrupt-controller;
compatible = "riscv,cpu-intc";
}
}
}
