Clock device tree configuration

Article purpose

This article explains how to configure the RCC internal peripheral when it is assigned to the Linux^® OS. In that case, it is controlled by the Common Clock framework.

The configuration is performed using the device tree mechanism that provides a hardware description of the RCC peripheral, used by the clk-stm32mp1 Linux driver and by the Common Clock framework.

DT bindings documentation

The RCC is a multifunction device.

Each function is represented by a separate binding document:

• generic DT bindings used by the Common Clock framework.

• vendor clock DT bindings used by the clk-stm32mp1 driver: this binding document explains how to write device tree files for clocks.

DT configuration

DT configuration (STM32 level)

The STM32MP1 Clock node is located in the stm32mp157c.dtsi. See Device tree for more explanations.

clocks node

These clocks have a fixed frequency (generally they are oscillators)

	clocks {
		clk_hse: clk-hse {
			#clock-cells = <0>;
			compatible = "fixed-clock";
			clock-frequency = <24000000>;
		};

		clk_hsi: clk-hsi {
			#clock-cells = <0>;
			compatible = "fixed-clock";
			clock-frequency = <64000000>;
		};

		clk_lse: clk-lse {
			#clock-cells = <0>;
			compatible = "fixed-clock";
			clock-frequency = <32768>;
		};

		clk_lsi: clk-lsi {
			#clock-cells = <0>;
			compatible = "fixed-clock";
			clock-frequency = <32000>;
		};

		clk_csi: clk-csi {
			#clock-cells = <0>;
			compatible = "fixed-clock";
			clock-frequency = <4000000>;
		};

		...
	};

STM32MP1 Clock node

We need to specify the number of cells in a clock specifier.
For the STM32MP1 this number shoud be 1 and is configured via 'clock-cells' property in rcc node.

rcc: rcc@50000000 {
 	compatible = "st,stm32mp1-rcc", "syscon";
	#clock-cells = <1>;
	#reset-cells = <1>;
	reg = <0x50000000 0x1000>;
	...
};

This device tree part is related to STM32MP1 microprocessors. It must be kept as is, without being modified by the end-user.

DT configuration (board level)

If a Linux driver needs a clock, it has to be added in its DT node:
clocks = <phandle> : List of phandle and clock specifier pairs, one pair

for each clock input to the device. Note: if the

clock provider specifies '0' for #clock-cells, then

only the phandle portion of the pair will appear.

• Example:

usart3: serial@4000f000 {
	compatible = "st,stm32h7-usart";
	reg = <0x4000f000 0x400>;
	interrupt-names = "event", "wakeup";
	interrupts-extended = <&intc GIC_SPI 39 IRQ_TYPE_LEVEL_HIGH>,
			      <&exti 28 1>;
	clocks = <&rcc USART3_K>;
	wakeup-source;
	power-domains = <&pd_core>;
	status = "disabled";
};

How to configure the DT using STM32CubeMX

The STM32CubeMX tool can be used to configure the STM32MPU device and get the corresponding platform configuration device tree files.
The STM32CubeMX may not support all the properties described in the above DT bindings documentation paragraph. If so, the tool inserts user sections in the generated device tree. These sections can then be edited to add some properties and they are preserved from one generation to another. Refer to STM32CubeMX user manual for further information.

References

Please refer to the following links for additional information:

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