1. Article purpose[edit source]
The purpose of this article is to explain how the SCMI resources are configured in U-Boot bootloader and Linux® kernel. The configuration is performed using the device tree mechanism[1].
SCMI services provided in STM32 MPU platforms are described in the SCMI overview article.
2. DT bindings documentation[edit source]
Generic information about SCMI is available in the SCMI overview article.
The device tree binding documents for SCMI are stored either in the given applicable components listed below, or in the Linux kernel repository:
- U-Boot, Linux® OS:
- Arm SCMI DT bindings: Documentation/devicetree/bindings/firmware/arm,scmi.yaml
SCMI agent is only present in U-Boot and Linux® OS , while SCMI server is integrated in TF-A or OP-TEE. The DT bindings documentation only defines the SCMI agent configuration, there is no bindings for SCMI server configuration part.
The SCMI agent DT node must reflect the services exposed by SCMI server embedded in TF-A or OP-TEE.
2.1. DT bindings documentation - additional information - scmi node basics[edit source]
An SCMI agent, in U-Boot or Linux kernel is represented by a node named scmi in the firmware node of the device tree. Property compatible in the scmi node defines the SCMI transport used. Valid values for compatible property are listed in the SCMI DT bindings [2]. Each SCMI protocol used is represented by a subnode of the scmi node.
firmware {
scmi {
compatible = "...";
#address-cells = <1>;
#size-cells = <0>;
(...)
protocol@11 {
reg = <0x11>;
(...)
};
protocol@14 {
reg = <0x14>;
(...)
};
(...)
};
};
For example, the SCMI specification[3] defines the SCMI power domain management protocol as identified with protocol ID 0x11. Device tree represents this SCMI protocol as a power domain controller in the Linux® kernel, also called a "PM domain provider" in [4] documentation, that exposes power domains to Linux® kernel drivers. From SCMI communication perspective, agent and server for that platform use well defined ID numbers for each power domain, called domain IDs in SCMI litterature.
As shown in the source code snipet below, the Linux® power domain provider is a subnode named protocol@11 of the scmi node. Each power domain exposed through this protocol is identified by well known ID number. A device consuming the SCMI agent's power domain ID 2 would refer to that power domain using the SCMI protocol node phandle, scmi_devpd1 here, and use the SCMI power domain ID 2 as phandle argument: e.g. power-domains = <&scmi_devpd1 2>;. The below DTSI source code snipet is extracted from the SCMI DT bindings documentation[2].
firmware {
scmi {
compatible = "arm,scmi-smc";
shmem = <&cpu_scp_lpri0 &cpu_scp_lpri1>;
arm,smc-id = <0xc3000001>;
#address-cells = <1>;
#size-cells = <0>;
scmi_devpd1: protocol@11 {
reg = <0x11>;
#power-domain-cells = <1>;
};
};
};
[edit source]
When the platform uses a predefined area of sram compatible memory for exchanging SCMI messages between non-secure and secure worlds, the platform must set property shmem = <SOME_PHANDLE> in the scmi node, and possibly in the SCMI protocol subnodes, in case such procotol uses a specifc channel. When using compatible="linaro,scmi-optee", SCMI communication can use OP-TEE dynamically allocated shared memory which is represented by the absence of property shmem in the related node.
The below example describes an OP-TEE SCMI firmware that exposes 2 channels. Channel 0 uses pre-defined sram shared memory and is used for SCMI base communication and for SCMI reset controls messaging. Channel 1 uses dynamically allocated OP-TEE shared memory and is used for a voltage regulator controller.
scmi_sram: sram@2ffff000 {
compatible = "mmio-sram";
reg = <0x2ffff000 0x1000>;
#address-cells = <1>;
#size-cells = <1>;
ranges = <0 0x2ffff000 0x1000>;
scmi_shm: scmi_shm@0 {
compatible = "arm,scmi-shmem";
reg = <0 0x80>;
};
};
firmware {
scmi: scmi {
compatible = "linaro,scmi-optee";
linaro,optee-channel-id = <0>;
shmem = <&scmi_shm>;
#address-cells = <1>;
#size-cells = <0>;
scmi_clk: protocol@14 {
reg = <0x14>;
#clock-cells = <1>;
};
protocol@17 {
reg = <0x17>;
linaro,optee-channel-id = <1>;
regulators {
#address-cells = <1>;
#size-cells = <0>;
scmi_reg11: reg11@0 {
reg = <0>;
regulator-name = "reg11";
regulator-min-microvolt = <1100000>;
};
};
};
};
};
3. DT configuration[edit source]
This hardware description is a combination of the STM32 microprocessor device tree files (.dtsi extension) and board device tree files (.dts extension). See the Device tree for an explanation of the device-tree file organization.
STM32CubeMX can be used to generate the board device tree. Refer to How to configure the DT using STM32CubeMX for more details.
3.1. DT configuration (STM32/SoC level)[edit source]
SoC level device tree data defines the SCMI node that represents SCMI communication means and SCMI protocols exposed by the SCMI server embedded in system secure world.
SCMI agent is described in the device tree by a node named scmi. This node uses property compatible to define the SCMI transport layer being used, either "arm,scmi-smc" or "linaro,scmi-optee" whether embedding TF-A or OP-TEE respectively. The SCMI protocols exposed by the server and used by the agent are represented by subnodes in the scmi node, each subnode defining the related SCMI protocol ID number using reg bindings rules.
Configuration when using an SCMI server embedded in TF-A services is described in below section SCMI agent when TF-A in secure world
Configuration when using an SCMI server embedded in OP-TEE services is described in below section SCMI agent when OP-TEE in secure world. SCMI firmware embedded in OP-TEE exposes service to the SCMI agent through communication channel number 0, hence property linaro,optee-channel-id shall be set to 0.
Note that for STM32MP15x lines 
, ecosystem release ≥ unknown revision 5.0.0 
, only OP-TEE firmware is supported in secure world.
When the using a predefined area of sram compatible memory for exchanging SCMI messages between non-secure and secure worlds, the platform must have a shmem property in the scmi node that refers to the phandle of a related "arm,scmi-shmem" compatible nodes in the device tree to describe the shared memory. For ecosystem release ≥ unknown revision 5.0.0 , SCMI communication only uses OP-TEE native shared memory, not internal RAM (sram in device tree bindings) hence there is no 'shmem' property in the related SCMI node.
SoC level device tree configuration data defines the SCMI protocols exposed by the SCMI server related to SoC resrouces. These are:
- clock and reset controllers controlled by the SoC RCC subsystem and managed respectively through the SCMI clock protocol (protocol ID 0x14) and SCMI reset domain (protocol ID 0x16);
- voltage regulators controller by SoC PWR subsystem and managed through the SCMI voltage domain protocol (protocol ID 0x17);
- possibly CPU operating point exposed through the SCMI performance management protocol (protocol ID 0x13).
Description of how devices consume SCMI services is described in below section DT_configuration_for_SCMI_resource_consumers. .
3.2. DT configuration (board level)[edit source]
Board level device tree configuration data defines the controls exposed by the SCMI server related to board specific configuration, as voltage regulators (exposed through the SCMI voltage domain protocol) for regulators controlled from the Power Management Integrated Circuit (PMIC) device STPMIC1 on STM32MP13x lines boards.
Description of how device node binds to SCMI services is described in below section DT configuration for SCMI resource consumers.
Remember that SCMI agent (Linux® kernel and U-Boot bootloader) device tree configuration data shall reflect the SCMI firmware services exposed by the SCMI server embedded in system secure world.
3.3. DT configuration examples[edit source]
3.3.1. SCMI agent when TF-A in secure world[edit source]
Compatible "arm,scmi-smc" requires node property shmem and a specific property: arm,smc-id = <FUNC_ID>; that defines the platform SMC function ID used for SCMI messages notification. When the platform defines a specific communication (couple SMC function ID and shared memory), the node of the SCMI protocol shall define both arm,smc-id and shmem properties in the SCMI protocol subnode.
Note that STM32MP15 platforms currently use a single channel with pre-allocated SYSRAM shared memory for SCMI, to expose clock and reset controllers, as show in the DTS extract below:
scmi_sram: sram@2ffff000 {
compatible = "mmio-sram";
reg = <0x2ffff000 0x1000>;
#address-cells = <1>;
#size-cells = <1>;
ranges = <0 0x2ffff000 0x1000>;
scmi_shm: scmi_shm@0 {
compatible = "arm,scmi-shmem";
reg = <0 0x80>;
};
};
firmware {
scmi: scmi {
compatible = "arm,scmi-smc";
arm,smc-id = <0x82002000>;
shmem = <&scmi_shm>;
#address-cells = <1>;
#size-cells = <0>;
scmi_clk: protocol@14 {
reg = <0x14>;
#clock-cells = <1>;
};
scmi_reset: protocol@16 {
reg = <0x16>;
#reset-cells = <1>;
};
};
};
3.3.2. SCMI agent when OP-TEE in secure world[edit source]
When OP-TEE is embedded in secure world, SCMI communication shall use OP-TEE transport.
Compatible "linaro,scmi-optee" requires a specific property in the device node: linaro,optee-channel-id = <CHANNEL_ID>; where CHANNEL_ID is the channel identifer defined for the SCMI communication. STM32MP15x lines and STM32MP13x lines currently exposes a single SCMI channel identified by ID value 0.
Prior ecosystem release unknown revision v5.0.0 , SCMI uses last page of internal SYSRAM for SCMI communication. This is reflected by
property shmem = <&scmi_shm>;. From ecosystem release unknown revision v5.0.0 onwards, SCMI/OP-TEE transport uses OP-TEE native shared memory for SCMI message exchange. This is represented by the absence of property shmem next to property linaro,optee-channel-id.
The below example shows STM32MP13x lines SCMI device tree data using SCMI/OP-TEE transport over channel 0 using OP-TEE native shared memory and exposing clock, reset and voltage regulator controllers.
firmware {
scmi: scmi {
compatible = "linaro,scmi-optee";
linaro,optee-channel-id = <0>;
#address-cells = <1>;
#size-cells = <0>;
scmi_clk: protocol@14 {
reg = <0x14>;
#clock-cells = <1>;
};
scmi_reset: protocol@16 {
reg = <0x16>;
#reset-cells = <1>;
};
protocol@17 {
reg = <0x17>;
linaro,optee-channel-id = <1>;
regulators {
#address-cells = <1>;
#size-cells = <0>;
scmi_reg11: voltd-reg11 {
reg = <VOLTD_SCMI_REG11>;
regulator-name = "reg11";
};
scmi_reg18: voltd-reg18 {
reg = <VOLTD_SCMI_REG18>;
regulator-name = "reg18";
};
scmi_usb33: voltd-usb33 {
reg = <VOLTD_SCMI_USB33>;
regulator-name = "usb33";
};
};
};
};
};
3.3.3. DT configuration for SCMI resource consumers[edit source]
All devices using SCMI clocks and reset controllers refer to the phandle of the related SCMI clock contoller node scmi_clk or SCMI reset controller node scmi_reset. For example, the CPU references an SCMI clock:
cpus {
#address-cells = <1>;
#size-cells = <0>;
cpu0: cpu@0 {
compatible = "arm,cortex-a7";
device_type = "cpu";
reg = <0>;
clocks = <&scmi_clk CK_SCMI_MPU>;
clock-names = "cpu";
operating-points-v2 = <&cpu0_opp_table>;
nvmem-cells = <&part_number_otp>;
nvmem-cell-names = "part_number";
#cooling-cells = <2>;
};
};
We can point also examples for I2C4 bus...
i2c4: i2c@5c002000 {
compatible = "st,stm32mp15-i2c";
reg = <0x5c002000 0x400>;
(...)
clocks = <&scmi_clk CK_SCMI_I2C4>;
resets = <&scmi_reset RST_SCMI_I2C4>;
(...)
};
... or the RTC device node.
rtc: rtc@5c004000 {
compatible = "st,stm32mp1-rtc";
reg = <0x5c004000 0x400>;
clocks = <&scmi_clk CK_SCMI_RTCAPB>,
<&scmi_clk CK_SCMI_RTC>;
clock-names = "pclk", "rtc_ck";
interrupts-extended = <&exti 19 IRQ_TYPE_LEVEL_HIGH>;
(...)
};
4. References[edit source]
Please refer to the following links for additional information:
- ↑ Device tree
- ↑ 2.0 2.1 Documentation/devicetree/bindings/firmware/arm,scmi.yaml Arm SCMI DT bindings
- ↑ https://developer.arm.com/documentation/den0056
- ↑ Documentation/devicetree/bindings/power/power-domain.yaml Power Domains DT bindings
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