LTDC device tree configuration

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1. Article purpose[edit source]

This article explains how to configure the LTDC ^[1] when the peripheral is assigned to the Linux^® OS.

The configuration is performed using the device tree mechanism ^[2].

The Device tree provides a hardware description of the LTDC ^[1] used by the STM32 LTDC Linux driver.

2. DT bindings documentation[edit source]

The LTDC is represented by the STM32 LTDC device tree bindings ^[3].

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 split.

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 level)[edit source]

The LTDC device tree node is declared in stm32mp151.dtsi ^[4]. The declaration (shown below) provides the hardware registers base address, the clocks, the interrupts and the reset.

		ltdc: display-controller@5a001000 {
			compatible = "st,stm32-ltdc";
			reg = <0x5a001000 0x400>;
			interrupts = <GIC_SPI 88 IRQ_TYPE_LEVEL_HIGH>,
				     <GIC_SPI 89 IRQ_TYPE_LEVEL_HIGH>;
			clocks = <&rcc LTDC_PX>;
			clock-names = "lcd";
			resets = <&rcc LTDC_R>;
			status = "disabled";
		};

Warning

This device tree part is related to STM32 microprocessors. It must be kept as is, without being modified by the end-user.

3.2. DT configuration (board level)[edit source]

The LTDC device tree related to a particular board may have the following nodes, depending on the board hardware:

ltdc node: containing the in/out port descriptions.
panel or i2cx bridge rgb node (depending of the board hardware): containing the LTDC Pinctrl and the in/out port descriptions
panel_backlight node (depending of the board hardware): related to the panel node

A full example of the LTDC pins ^[5] is available in the stm32mp15-pinctrl.dtsi ^[6]

			ltdc_pins_a: ltdc-a-0 {
				pins {
					pinmux = <STM32_PINMUX('G',  7, AF14)>, /* LCD_CLK */
						 <STM32_PINMUX('I', 10, AF14)>, /* LCD_HSYNC */
						 <STM32_PINMUX('I',  9, AF14)>, /* LCD_VSYNC */
						 <STM32_PINMUX('F', 10, AF14)>, /* LCD_DE */
						 <STM32_PINMUX('H',  2, AF14)>, /* LCD_R0 */
						 <STM32_PINMUX('H',  3, AF14)>, /* LCD_R1 */
						 <STM32_PINMUX('H',  8, AF14)>, /* LCD_R2 */
						 <STM32_PINMUX('H',  9, AF14)>, /* LCD_R3 */
						 <STM32_PINMUX('H', 10, AF14)>, /* LCD_R4 */
						 <STM32_PINMUX('C',  0, AF14)>, /* LCD_R5 */
						 <STM32_PINMUX('H', 12, AF14)>, /* LCD_R6 */
						 <STM32_PINMUX('E', 15, AF14)>, /* LCD_R7 */
						 <STM32_PINMUX('E',  5, AF14)>, /* LCD_G0 */
						 <STM32_PINMUX('E',  6, AF14)>, /* LCD_G1 */
						 <STM32_PINMUX('H', 13, AF14)>, /* LCD_G2 */
						 <STM32_PINMUX('H', 14, AF14)>, /* LCD_G3 */
						 <STM32_PINMUX('H', 15, AF14)>, /* LCD_G4 */
						 <STM32_PINMUX('I',  0, AF14)>, /* LCD_G5 */
						 <STM32_PINMUX('I',  1, AF14)>, /* LCD_G6 */
						 <STM32_PINMUX('I',  2, AF14)>, /* LCD_G7 */
						 <STM32_PINMUX('D',  9, AF14)>, /* LCD_B0 */
						 <STM32_PINMUX('G', 12, AF14)>, /* LCD_B1 */
						 <STM32_PINMUX('G', 10, AF14)>, /* LCD_B2 */
						 <STM32_PINMUX('D', 10, AF14)>, /* LCD_B3 */
						 <STM32_PINMUX('I',  4, AF14)>, /* LCD_B4 */
						 <STM32_PINMUX('A',  3, AF14)>, /* LCD_B5 */
						 <STM32_PINMUX('B',  8, AF14)>, /* LCD_B6 */
						 <STM32_PINMUX('D',  8, AF14)>; /* LCD_B7 */
					bias-disable;
					drive-push-pull;
					slew-rate = <2>;
				};
			};

			ltdc_pins_sleep_a: ltdc-a-1 {
				pins {
					pinmux = <STM32_PINMUX('G',  7, ANALOG)>, /* LCD_CLK */
						 <STM32_PINMUX('I', 10, ANALOG)>, /* LCD_HSYNC */
						 <STM32_PINMUX('I',  9, ANALOG)>, /* LCD_VSYNC */
						 <STM32_PINMUX('F', 10, ANALOG)>, /* LCD_DE */
						 <STM32_PINMUX('H',  2, ANALOG)>, /* LCD_R0 */
						 <STM32_PINMUX('H',  3, ANALOG)>, /* LCD_R1 */
						 <STM32_PINMUX('H',  8, ANALOG)>, /* LCD_R2 */
						 <STM32_PINMUX('H',  9, ANALOG)>, /* LCD_R3 */
						 <STM32_PINMUX('H', 10, ANALOG)>, /* LCD_R4 */
						 <STM32_PINMUX('C',  0, ANALOG)>, /* LCD_R5 */
						 <STM32_PINMUX('H', 12, ANALOG)>, /* LCD_R6 */
						 <STM32_PINMUX('E', 15, ANALOG)>, /* LCD_R7 */
						 <STM32_PINMUX('E',  5, ANALOG)>, /* LCD_G0 */
						 <STM32_PINMUX('E',  6, ANALOG)>, /* LCD_G1 */
						 <STM32_PINMUX('H', 13, ANALOG)>, /* LCD_G2 */
						 <STM32_PINMUX('H', 14, ANALOG)>, /* LCD_G3 */
						 <STM32_PINMUX('H', 15, ANALOG)>, /* LCD_G4 */
						 <STM32_PINMUX('I',  0, ANALOG)>, /* LCD_G5 */
						 <STM32_PINMUX('I',  1, ANALOG)>, /* LCD_G6 */
						 <STM32_PINMUX('I',  2, ANALOG)>, /* LCD_G7 */
						 <STM32_PINMUX('D',  9, ANALOG)>, /* LCD_B0 */
						 <STM32_PINMUX('G', 12, ANALOG)>, /* LCD_B1 */
						 <STM32_PINMUX('G', 10, ANALOG)>, /* LCD_B2 */
						 <STM32_PINMUX('D', 10, ANALOG)>, /* LCD_B3 */
						 <STM32_PINMUX('I',  4, ANALOG)>, /* LCD_B4 */
						 <STM32_PINMUX('A',  3, ANALOG)>, /* LCD_B5 */
						 <STM32_PINMUX('B',  8, ANALOG)>, /* LCD_B6 */
						 <STM32_PINMUX('D',  8, ANALOG)>; /* LCD_B7 */
				};
			};

A full example of the STM32MP15x Discovery board device tree is available in stm32mp157c-dk2.dts ^[7].

&ltdc {
	status = "okay";

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

		ltdc_ep0_out: endpoint@0 {
			reg = <0>;
			remote-endpoint = <&sii9022_in>;
		};
	};
};

&i2c1 {
...
	hdmi-transmitter@39 {
		compatible = "sil,sii9022";
		reg = <0x39>;
		iovcc-supply = <&v3v3_hdmi>;
		cvcc12-supply = <&v1v2_hdmi>;
		reset-gpios = <&gpioa 10 GPIO_ACTIVE_LOW>;
		interrupts = <1 IRQ_TYPE_EDGE_FALLING>;
		interrupt-parent = <&gpiog>;
		pinctrl-names = "default", "sleep";
		pinctrl-0 = <&ltdc_pins_a>;
		pinctrl-1 = <&ltdc_pins_sleep_a>;
		status = "okay";

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

			port@0 {
				reg = <0>;
				sii9022_in: endpoint {
					remote-endpoint = <&ltdc_ep0_out>;
				};
			};

			port@1 {
				reg = <1>;
				sii9022_tx_endpoint: endpoint {
					remote-endpoint = <&i2s2_endpoint>;
				};
			};
		};
	};
};

4. How to configure the DT using STM32CubeMX[edit source]

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.

5. References[edit source]

Please refer to the following links for additional information:

[LTDC-1] 1.0 ^1.1 LTDC internal peripheral

[2] Device tree

[3] Linux kernel STM32 LTDC bindings (st,stm32-ltdc.txt)

[stm32mp151_dtsi-4] Linux kernel STM32MP151 device tree (stm32mp151.dtsi)

[5] Pinctrl device tree configuration

[stm32mp15_pinctrl_dtsi-6] Linux kernel STM32MP15x pinctrl device tree (stm32mp15-pinctrl.dtsi)

[stm32mp157c_dk2_dts-7] Linux kernel STM32MP15x Discovery board device tree (stm32mp157c-dk2.dts)

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