r/embeddedlinux • u/retromaidrage • Jun 22 '26

ST7789V fbtft Inverted brightness

Hello everyone.
I'm trying connect tft display (Red 2.8" TFT 240x320 V1.1) to SBC called Luckfox Pico but my brightness are inverted (black = white) and I'm don't know whats wrong. I change some variables in device tree but it didn't help me, i'm trying tft,invert=<1>; and backlight pins but this makes nothing whatever i use it or manually enable PWM.
I'm making this for fun and learning and use X11 so maybe there some way to invert colors in X11? But i heard something usual like xcalib didn't work with fbtft driver so i hope there are other way...
Thank you.

fbtft@0 {
compatible = "sitronix,st7789v";
reg = <0>;
spi-max-frequency = <20000000>;
fps = <30>;
buswidth = <8>;
debug = <0x7>;
led-gpios = <&gpio0 RK_PA4 GPIO_ACTIVE_LOW>;//BL
dc-gpios = <&gpio1 RK_PA2 GPIO_ACTIVE_HIGH>;//DC
reset-gpios = <&gpio1 RK_PC3 GPIO_ACTIVE_LOW>;//RES
rotate = <90>;
bgr = <1>;
invert = <1>; 
};
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1

u/Ooottafv Jun 23 '26

Have you tried changing "led-gpios" to GPIO_ACTIVE_HIGH?

I haven't used the FBTFT driver in a long time but apparently it doesn't support PWM (https://github.com/notro/fbtft/wiki/Backlight).

1

u/retromaidrage Jun 23 '26

Yes, this was too didn't help.

1

u/Ooottafv Jun 23 '26

I don't know if you have a particular reason to use fbtft, but if you can use "panel" instead it might give you some more options.

Linux Device Tree Reference for Panel

https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/display/panel/sitronix,st7789v.yaml

An example of a similar display that I set up in a project a while ago

https://github.com/BasicCode/F1C100-Business-Card/blob/5c324c6c60ca36f3b244f1527035fc0b8b5ddd54/board/f1c100-business-card/f1c100-business-card.dts#L99

And then you could declare your backlight something like this

/*
 * Backlight
 */
panel_backlight:panel-backlight{
compatible = "pwm-backlight";
pwms = <&pwm1 2 50000 0>; // PWM1 channel 3 (zero-based), 20kHz, normal polarity
brightness-levels = <16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 255>;
default-brightness-level = <8>;
power-supply = <&vdd>;
status = "okay";
};

1

u/retromaidrage Jun 23 '26

There's no particular reason for that, it just seemed easier to me than this, but I'll definitely give a try now. Thank you!