1. Article purpose[edit source]
The purpose of this article is to explain how to configure SCMI agents in the system. These agents are used by Linux® OS and U-Boot bootloader as described in SCMI overview.
The SCMI services device tree bindings are defined in the Linux kernel source tree in Arm SCMI DT bindings[1] documentation.
2. SCMI agent nodes[edit source]
Each SCMI agent is a subnode of the firmware node in the device tree description.
SCMI devices are described in the firmware node of the device tree, with compatible = "arm,scmi". The node define the SCMI transport used, described in the next section.
Last, the SCMI agent node lists all supported SCMI protocols by procol instance node protocol@X.
scmi-agent {
compatible = "arm,scmi";
(...) /* SCMI transport properties */
/* Below are the discovered SCMI protocols */
protocol@X {
reg = <0xX>;
(...)
};
protocol@Y {
reg = <0xY>;
(...)
};
};
3. SCMI Transport[edit source]
SCMI framework support several transport layers for message exchange as mailbox or SMCCC. Both use mmio-sram devices as shared memory for message exchange. Device Tree defines generic bindings for Mailbox and SRAM memory devices.
For example below and piece and internal RAM used as device memory for SCMI message transfer.
sram@2ffff000 {
compatible = "mmio-sram";
reg = <0x2ffff000 0x1000>;
#address-cells = <1>;
#size-cells = <1>;
ranges = <0 0x2ffff000 0x1000>;
scmi0_shm: scmi_shm@0 {
reg = <0 0x80>;
};
};
Mailbox device: ad-hoc arm,smc-mbox.
STM32MP software release uses a SMC mailbox described with a "arm,smc-mbox" compatible node. Property arm,func-id defines the function ID used to invoke secure world for a specific SCMI agent/server interface.
scmi0_mbox: mailbox-0 {
#mbox-cells = <0>;
compatible = "arm,smc-mbox";
arm,func-id = <0x82002000>;
};
In example below, SCMI agent 0 uses scmi0_mbox mailbox and scmi0_shm as shared memory.
firmware {
scmi0: scmi-0 {
compatible = "arm,scmi";
(...)
mboxes = <&scmi0_mbox 0>;
mbox-names = "txrx";
shmem = <&scmi0_shm>;
(...)
};
};
4. SCMI phandles in the Device Tree[edit source]
This section show how the devices exposed to SCMI agents can be used in the Device Tree description of the system.
Consider a SCMI agent enabling SCMI protocol 0x14 (Clocks) to expose a 2 clocks (clock_id values are 0 and 1). Consider another device that depends on the SCMI agent's clock number 1. This will translate in the Device Tree Source implementation:
firmware {
scmi {
compatible = "arm,scmi";
(...)
scmi_clk: protocol@14 {
reg = <0x14>;
#clock-cells = <1>;
}; }; };
some_device {
(...)
clocks = <&scmi_clk 1>; /* phandle to clock 1 exposed by scmi_clk */
(...)
}
5. Platform Specific Information[edit source]
5.1. STM32MP15 SCMI device tree nodes[edit source]
STM32MP15 exposes SCMI over 2 agent devices in the device tree description. One device per supported SCMI agents, hence two: one for the clocks and reset controllers related to RCC TZEN hardening configuration, and one for the clock controllers related to RCC MCKPROT hardening configuration.
As defined in Arm SCMI DT bindings[2], a SCMI agent node can contain one or more supported SCMI protocols. Each are described in a specific subnode of the agent node. STM32MP15 implements SCMI Clock protocol (ID 0x14) that is a clock provider device and SCMI Reset Domain protocol (ID 0x16) that is a reset controller provider device.
Example below shows a configuration supported in STM32MP15 software release.
/* Shared memory used for SCMI agent/server communication */ scmi_sram: sram@2ffff000 { compatible = "mmio-sram"; reg = <0x2ffff000 0x1000>; #address-cells = <1>; #size-cells = <1>; ranges = <0 0x2ffff000 0x1000>; scmi0_shm: scmi_shm@0 { reg = <0 0x80>; }; scmi1_shm: scmi_shm@200 { reg = <0x200 0x80>; }; }; /* Mailbox device used for SCMI agent/server communication */ scmi0_mbox: mailbox-0 { #mbox-cells = <0>; compatible = "arm,smc-mbox"; arm,func-id = <0x82002000>; }; scmi1_mbox: mailbox-1 { #mbox-cells = <0>; compatible = "arm,smc-mbox"; arm,func-id = <0x82002001>; }; /* SCMI agent nodes, in the firmware node */ firmware { scmi0: scmi-0 { compatible = "arm,scmi"; #address-cells = <1>; #size-cells = <0>; status = "okay"; /* To enable upon RCC[TZEN] */ mboxes = <&scmi0_mbox 0>; mbox-names = "txrx"; shmem = <&scmi0_shm>; scmi0_clk: protocol@14 { reg = <0x14>; #clock-cells = <1>; }; scmi0_reset: protocol@16 { reg = <0x16>; #reset-cells = <1>; }; }; scmi1: scmi-1 { compatible = "arm,scmi"; #address-cells = <1>; #size-cells = <0>; status = "disabled"; /* To enable upon RCC[MCKPROT] */ mboxes = <&scmi1_mbox 0>; mbox-names = "txrx"; shmem = <&scmi1_shm>; scmi1_clk: protocol@14 { reg = <0x14>; #clock-cells = <1>; }; };
6. References[edit source]
- ↑ Documentation/devicetree/bindings/arm/arm,scmi.txt Arm SCMI DT bindings
- ↑ Documentation/devicetree/bindings/arm/arm,scmi.txt Arm SCMI DT bindings
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