PWM, LCD, and CTP from Kernel on STM32MP135

Linux

PWM, LCD, and CTP from Kernel on STM32MP135

Published 20 Feb 2026. By Jakob Kastelic.

This is Part 10 in the series: Linux on STM32MP135. See other articles.

Backlight with GPIO

The display backlight is controlled via a single GPIO:

panel_backlight: panel-backlight {
	compatible = "gpio-backlight";
	gpios = <&gpiob 15 GPIO_ACTIVE_HIGH>;
	default-on;
	default-brightness-level = <1>;
	status = "okay";
};

With that, the backlight can be turned on and off as follows:

# echo 1 > /sys/class/backlight/panel-backlight/brightness
# echo 0 > /sys/class/backlight/panel-backlight/brightness

There’s no tricks to this, so long as we make sure that the GPIO mentioned in the DTS matches the one on the circuit schematic.

Backlight and PWM

To control the brightness, we need to enable the PWM corresponding to this GPIO pin and make sure the panel backlight is controlled by this PWM.

We have already implemented this previously in bare-metal using the HAL driver. We can check the TIM1, GPIOB, and GPIOC registers to see what is the effect of our configuration. In the following table, “Before” and “After” are on bare metal before and after configuring the PWM output.

Register	Address	Before	After
`TIM1_CR1`	0x44000000	0x00000000	0x00000001
`TIM1_SR`	0x44000010	0x00000000	0x0003001f
`TIM1_CCMR2`	0x4400001c	0x00000000	0x00000068
`TIM1_CCER`	0x44000020	0x00000000	0x00000400
`TIM1_CNT`	0x44000024	0x00000000	0x000001eb
`TIM1_PSC`	0x44000028	0x00000000	0x00000063
`TIM1_ARR`	0x4400002c	0x00000000	0x000003e7
`TIM1_CCR3`	0x4400003c	0x00000000	0x000001f4
`TIM1_BDTR`	0x44000044	0x00000000	0x00008000
`TIM1_DMAR`	0x4400004c	0x00000000	0x00000001
`TIM1_AF1`	0x44000060	0x00000000	0x00000001
`TIM1_AF2`	0x44000064	0x00000000	0x00000001
`TIM1_VERR`	0x440003f4	0x00000000	0x00000035
`TIM1_IPIDR`	0x440003f8	0x00000000	0x00120002
`TIM1_SIDR`	0x440003fc	0x00000000	0xa3c5dd01

`GPIOB_MODER`	0x50003000	0xffbfffff	0xbfbfffff
`GPIOB_AFR[1]`	0x50003024	0x0000b000	0x1000b000

`GPIOC_IDR`	0x50004010	0x00001f00	0x00000f00

`RCC_MP_APB2ENSETR`	0x50000708	0x00000000	0x00000001
`RCC_MP_APB2ENCLRR`	0x5000070c	0x00000000	0x00000001

The bare-metal bootloader configures the PWM for 3.0489 kHz operating with 50% duty cycle.

Now let’s configure the PWM in the DTS as follows:

		timers1: timer@44000000 {
			#address-cells = <1>;
			#size-cells = <0>;
			compatible = "st,stm32-timers";
			reg = <0x44000000 0x400>;
			interrupts = <GIC_SPI 25 IRQ_TYPE_LEVEL_HIGH>,
				     <GIC_SPI 26 IRQ_TYPE_LEVEL_HIGH>,
				     <GIC_SPI 27 IRQ_TYPE_LEVEL_HIGH>,
				     <GIC_SPI 28 IRQ_TYPE_LEVEL_HIGH>;
			interrupt-names = "brk", "up", "trg-com", "cc";
			clocks = <&rcc TIM1_K>;
			clock-names = "int";
			status = "okay";

			pwm1: pwm {
				compatible = "st,stm32-pwm";
				#pwm-cells = <3>;
				status = "okay";
				pinctrl-0 = <&pwm1_pins>;
				pinctrl-names = "default";
			};

When the PWM driver module is inserted, we can control the screen brightness as follows:

modprobe pwm-stm32
cd /sys/class/pwm/pwmchip0
echo 2 > export
echo 1000000 > pwm2/period
echo 500000 > pwm2/duty_cycle
echo 1 > pwm2/enable

After that, the registers read:

Register	Value
`TIM1_CR1`	0x00000081
`TIM1_SR`	0x0003001f
`TIM1_CCMR2`	0x00000068
`TIM1_CCER`	0x00000500
`TIM1_CNT`	0x00008a35
`TIM1_PSC`	0x00000004
`TIM1_ARR`	0x0000ee33
`TIM1_CCR3`	0x0000771a
`TIM1_BDTR`	0x00008000
`TIM1_DMAR`	0x00000081
`TIM1_AF1`	0x00000001
`TIM1_AF2`	0x00000001
`TIM1_VERR`	0x00000035
`TIM1_IPIDR`	0x00120002
`TIM1_SIDR`	0xa3c5dd01
`GPIOB_MODER`	0xbfbf7fff
`GPIOB_AFR[1]`	0x1000b000
`GPIOC_IDR`	0x00001f00
`RCC_MP_APB2ENSETR`	0x00000001
`RCC_MP_APB2ENCLRR`	0x00000001

Finally, we need to make the panel backlight driver automatically talk to the PWM driver so that the backlight can be adjusted as brightness and not as PWM. We add the following to the DTS:

panel_backlight: panel-backlight {
	compatible = "pwm-backlight";
	pwms = <&pwm1 2 1000000 0>;
	brightness-levels = <0 16 32 64 128 255>;
	default-brightness-level = <4>;
	power-supply = <&v3v3_ao>;
	default-on;
	status = "okay";
};

The panel_backlight is referenced from panel_rgb, but still does not turn on automatically. Apparently the pwm-backlight driver does not have a DTS-controlled “power on” option, so we have to turn it on manually:

echo 0 > /sys/class/backlight/panel-backlight/bl_power

With that, the backlight brightness and power control work as they should via the /sys/class/backlight/ controls.

Display image on LCD

Let’s add two lines to the Buildroot configuration to make the modetest command available:

BR2_PACKAGE_LIBDRM=y
BR2_PACKAGE_LIBDRM_INSTALL_TESTS=y

I am using the Rocktech RK050HR01-CT LCD display and the following DTS fragment:

panel_rgb: panel-rgb {
	compatible = "rocktech,rk050hr18-ctg", "panel-dpi";
	enable-gpios = <&gpiog 7 GPIO_ACTIVE_HIGH>;
	backlight = <&panel_backlight>;
	power-supply = <&v3v3_ao>;
	data-mapping = "rgb565";
	status = "okay";

	width-mm = <108>;
	height-mm = <65>;

	port {
		panel_in_rgb: endpoint {
			remote-endpoint = <&ltdc_out_rgb>;
		};
	};

	panel-timing {
		clock-frequency = <24000000>;
		hactive = <800>;
		vactive = <480>;
		hsync-len = <4>;
		hfront-porch = <8>;
		hback-porch = <8>;
		vsync-len = <4>;
		vfront-porch = <16>;
		vback-porch = <16>;
		hsync-active = <0>;
		vsync-active = <0>;
		de-active = <1>;
		pixelclk-active = <1>;
	};
};

Run just modetest to determine the numbers (CRTC 41, connector 32) of the display:

# modetest | head
trying to open device '/dev/dri/card0'... done
opened device `STMicroelectronics SoC DRM` on driver `stm` (version 1.0.0 at 20170330)
Encoders:
id      crtc    type    possible crtcs  possible clones
31      41      DPI     0x00000001      0x00000001

Connectors:
id      encoder status          name            size (mm)       modes   encoders
32      31      connected       DPI-1           108x65          1       31

Modetest is now able to display a test pattern on the LCD with the following command:

# modetest -M stm -s 32@41:800x480
opened device `STMicroelectronics SoC DRM` on driver `stm` (version 1.0.0 at 20170330)
setting mode 800x480-56.72Hz on connectors 32, crtc 41

Capacitive touchpad (CTP)

Adjust the pin configuration in the DTS according to the way the PCB is wired. For completeness, here’s mine:

i2c5: i2c@4c006000 {
	compatible = "st,stm32mp13-i2c";
	reg = <0x4c006000 0x400>;
	interrupt-names = "event", "error";
	interrupts = <GIC_SPI 114 IRQ_TYPE_LEVEL_HIGH>,
		     <GIC_SPI 115 IRQ_TYPE_LEVEL_HIGH>;
	clocks = <&rcc I2C5_K>;
	resets = <&rcc I2C5_R>;
	#address-cells = <1>;
	#size-cells = <0>;
	st,syscfg-fmp = <&syscfg 0x4 0x10>;
	i2c-analog-filter;
	feature-domains = <&etzpc STM32MP1_ETZPC_I2C5_ID>;
	pinctrl-names = "default", "sleep";
	pinctrl-0 = <&i2c5_pins_a>;
	pinctrl-1 = <&i2c5_sleep_pins_a>;
	i2c-scl-rising-time-ns = <170>;
	i2c-scl-falling-time-ns = <5>;
	clock-frequency = <400000>;
	status = "okay";

	goodix: goodix-ts@5d {
		compatible = "goodix,gt911";
		reg = <0x5d>;
		pinctrl-names = "default";
		pinctrl-0 = <&goodix_pins_a>;
		interrupt-parent = <&gpioh>;
		interrupts = <12 IRQ_TYPE_EDGE_FALLING>;
		reset-gpios = <&gpiob 7 GPIO_ACTIVE_LOW>;
		AVDD28-supply = <&v3v3_ao>;
		VDDIO-supply = <&v3v3_ao>;
		touchscreen-size-x = <800>;
		touchscreen-size-y = <480>;
		status = "okay" ;
	};
};

The I2C5 pins are set as follows:

i2c5_pins_a: i2c5-0 {
	pins {
		pinmux = <STM32_PINMUX('H', 13, AF4)>, /* I2C5_SCL */
			 <STM32_PINMUX('F', 3, AF4)>; /* I2C5_SDA */
		bias-disable;
		drive-open-drain;
		slew-rate = <0>;
	};
};

Now verify that the CTP driver has been initialized correctly:

# dmesg | grep -i good
[    1.071181] Goodix-TS 0-005d: ID 911, version: 1060
[    1.074418] input: Goodix Capacitive TouchScreen as /devices/platform/soc/5c007000.etzpc/4c006000.i2c/i2c-0/0-005d/input/input0

Let’s use evtest (add BR2_PACKAGE_EVTEST=y in Buildroot) to verify that we can receive touchpad events. With evtest running, we can touch the touchpad and see that the events are streaming in:

# evtest
Event: time 54.972748, -------------- SYN_REPORT ------------
Event: time 54.976560, type 3 (EV_ABS), code 53 (ABS_MT_POSITION_X), value 575
Event: time 54.976560, type 3 (EV_ABS), code 54 (ABS_MT_POSITION_Y), value 236
Event: time 54.976560, type 3 (EV_ABS), code 0 (ABS_X), value 575
Event: time 54.976560, type 3 (EV_ABS), code 1 (ABS_Y), value 236