How to use timers2 as pwm-backlight?

[SChen.11]

HI, I needs use timers2 to enable the pwm function. Then it will used as pwm-backlight for LCD. I wasn't known how to write the dts file. Could you show me some example code on this function?

[PatrickF]

You could find some details in https://wiki.st.com/stm32mpu/wiki/TIM_device_tree_configuration and https://wiki.st.com/stm32mpu/wiki/PWM_overview

[SChen.11]

Hi PatrickF Thanks. I got this error message.

 Warning (pwms_property): /panel-backlight: Missing property '#pwm-cells' in node /soc/timer@40000000 or bad phandle (referred from pwms[0])

Here is my backlight dts code

187 panel_backlight: panel-backlight {
188 compatible = "pwm-backlight";
189 pwms = <&timers2 0 5000000>;
190 brightness-levels = <0 4 8 16 32 64 128 255>;
191 default-brightness-level = <6>;
192 status = "okay";
193 };
 

​It seems my "pwms" value was wrong.

[Bernard PUEL]

You have first to configure your pinctrl of your pwm output like described in this article (remove the trigger source part):

https://wiki.st.com/stm32mpu/wiki/TIM_device_tree_configuration#TIM_configured_in_PWM_mode_and_trigger_source

Then you will need to use your &pwm1_pins_a phandle defined instead of &timers2 above. see:

https://github.com/STMicroelectronics/linux/blob/v4.19-stm32mp/Documentation/devicetree/bindings/pwm/pwm.txt

As you will use in your backlight declaration 2 arguments (pwms = <&pwm1_pins_a 0 5000000>;), you will need to add this property in your pwm node:

#pwm-cells = <2>;

Hoping it is clear.

[SChen.11]

503 &timers2 {
504 status = "okay";
505 /* spare dmas for other usage */
506 /delete-property/dmas;
507 /delete-property/dma-names;
508 pwm {
509 pinctrl-0 = <&pwm2_pins_a>;
510 pinctrl-1 = <&pwm2_sleep_pins_a>;
511 pinctrl-names = "default", "sleep";
512 #pwm-cells = <2>;
513 status = "okay";
514 };
515 timer@1 {
516 status = "disabled";
517 };
518 };
 
 
187 panel_backlight: panel-backlight {
188 compatible = "pwm-backlight";
189 pwms = <&pwm2_pins_a 0 5000000>;
190 brightness-levels = <0 4 8 16 32 64 128 255>;
191 default-brightness-level = <6>;
192 status = "okay";
193 };

I add the "pwm-cell" attribute to timers2 block. Then I use pwm2_pins_a also has the warning info.

Warning (pwms_property): /panel-backlight: Missing property '#pwm-cells' in node /soc/pin-controller@50002000/pwm2-0 or bad phandle (referred from pwms[0])

[Bernard PUEL]

Could you please try this declaration and use &pwm2 phandle in your pwms declaration ( pwms = <&pwm2 0 5000000>;):

503 &timers2 {

504         status = "okay";

505         /* spare dmas for other usage */

506         /delete-property/dmas;

507         /delete-property/dma-names;

508 -        pwm {

    +        pwm2: pwm {

509                 pinctrl-0 = <&pwm2_pins_a>;

510                 pinctrl-1 = <&pwm2_sleep_pins_a>;

511                 pinctrl-names = "default", "sleep";

512                 #pwm-cells = <2>;

513                 status = "okay";

514         };

515 -        timer@1 {

516 -                status = "disabled";

517 -        };

518 };

[SChen.11]

Thanks for your help. It is working for my case.

[Bernard PUEL]

ok that's great. We will update the wiki for better clarity.