How to use Linux kernel to control LEDs connected to GPIOs on Colibri i.MX6?

Technical Support
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1

We would like to control LEDs connected to GPIOs on Colibri i.MX6 using LED support in Linux kernel. Unfortunately the kernel does not export LEDs via sysfs API (/sys/class/leds/) while the device tree seems to parsed correctly.

Related part of DTS file:

/dts-v1/;

#include <dt-bindings/input/input.h>
#include <dt-bindings/interrupt-controller/irq.h>
#include "imx6dl.dtsi"
#include "imx6qdl-colibri.dtsi"

/ {
	...

	leds {
		compatible = "gpio-leds";
		pinctrl-names = "default";
		pinctrl-0 = <&pinctrl_gpio_leds>;

		led0: heartbeat {
			label = "heartbeat";
			gpios = <&gpio1 2 GPIO_ACTIVE_HIGH>;
			default-state = "off";
			linux,default-trigger = "heartbeat";
		};		
	};
};

...

&iomuxc {
	imx6dl-colibri {
		pinctrl_gpio_leds: gpio_leds {
			fsl,pins = <
				MX6QDL_PAD_GPIO_2__GPIO1_IO02			PAD_CTRL_HYS_PD		/* heartbeat */
			>;
		};
	};
};

...

Related kernel configuration:

root@colibri-imx6:~# zcat /proc/config.gz | grep LEDS | grep -v "not set"
CONFIG_RFKILL_LEDS=y
CONFIG_RT2X00_LIB_LEDS=y
CONFIG_NEW_LEDS=y
CONFIG_LEDS_CLASS=y
CONFIG_LEDS_GPIO=y
CONFIG_LEDS_PWM=y
CONFIG_LEDS_TRIGGERS=y
CONFIG_LEDS_TRIGGER_HEARTBEAT=y
CONFIG_LEDS_TRIGGER_GPIO=y

Device tree is parsed correctly (as far as I can tell):

root@colibri-imx6:/proc/device-tree/leds# ls -l
-r--r--r--    1 root     root            10 Jan 15 14:34 compatible
dr-xr-xr-x    2 root     root             0 Jan 15 14:34 heartbeat
-r--r--r--    1 root     root             5 Jan 15 14:34 name
-r--r--r--    1 root     root             4 Jan 15 14:34 pinctrl-0
-r--r--r--    1 root     root             8 Jan 15 14:34 pinctrl-names
root@colibri-imx6:/proc/device-tree/leds# ls -l heartbeat/
-r--r--r--    1 root     root             4 Jan 15 14:34 default-state
-r--r--r--    1 root     root            12 Jan 15 14:34 gpios
-r--r--r--    1 root     root            10 Jan 15 14:34 label
-r--r--r--    1 root     root            10 Jan 15 14:34 linux,default-trigger
-r--r--r--    1 root     root            10 Jan 15 14:34 name
root@colibri-imx6:/proc/device-tree/leds# echo $(cat compatible)
gpio-leds

But the /sys/class/leds/ directory is missing expected folder (heartbeat). There is only mmc0::, which I suppose comes from MMC driver:

root@colibri-imx6:/sys/class/leds# ls -l
lrwxrwxrwx    1 root     root             0 Jan  1  1970 mmc0:: -> ../../devices/soc0/soc.0/2100000.aips-bus/2198000.usdhc/leds/mmc0::

There is nothing related to LEDs, GPIOs, etc, in the dmesg, so I really have no idea why the kernel did not export control directory for the LED.

If you know what is wrong with the configuration, please tell me.

Thanks

2

If you started with our default device tree then resp. pin will already be claimed for regular GPIO use:

imx6dl-colibri-eval-v3.dts
...
&iomuxc {
        /*
         * Mux all pins which are unused to be GPIOs
         * so they are ready for export to user space
         */
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_weim_gpio_1 &pinctrl_weim_gpio_2
                     &pinctrl_weim_gpio_3 &pinctrl_weim_gpio_4
...

So you will have to remove it from there first.

A good working example for LEDs use would be our Apalis iMX6 on Ixora combination e.g. have a look at imx6qdl-apalis-ixora.dtsi for reference.

3

You found a bug in our kernel. The regression happened when we backported the extcon subsystem which we need to switch the USB host/gadget role on the Colibri USBC port.

This patch made the gpio-leds work for me again. Likely we will have to do additional stuff to resolve the issue cleanly.

4

I opened an internal ticket how we want to handle ‘big’ comments in the future. Feel free to create an answer to workaround such limitations.

5

@max.krummenacher
Thanks for the patch, it fixed the issue for us.

Since you are talking about fixing it cleanly, is there something I should be aware of right now?

6

No.
I’m just not through with finding other places were the same bug kicks in.
I already know of drivers/of/pdt.c (unused in probably all sensible configurations for a i.MX6 module), maybe there are others.

7

Yes, I started with imx6dl-colibri-eval-v3.dts file, but I removed pinctrl for unused GPIOs. I just tried to add default pinctrl to see if the pin conflict would be reported in dmesg as I assume, and it does fail with errors:

[    1.552925] imx6dl-pinctrl 20e0000.iomuxc: pin MX6DL_PAD_GPIO_2 already requested by 20e0000.iomuxc; cannot claim for leds.25
[    1.564327] imx6dl-pinctrl 20e0000.iomuxc: pin-137 (leds.25) status -22
[    1.570972] imx6dl-pinctrl 20e0000.iomuxc: could not request pin 137 (MX6DL_PAD_GPIO_2) from group gpio_leds  on device 20e0000.iomuxc
[    1.583105] leds-gpio leds.25: Error applying setting, reverse things back

Thanks for the tip for Ixora board DTS file, I must have missed it as I only looked at Colibri related boards. Unfortunately I am afraid that it will not help me, as I copied pinctrl and leds section from that file into my DTS file, and it seems to be just same. There are parsed device tree entries for the LEDs in /proc/device-tree/leds/, but there are no directories in /sys/class/leds/.

I have attached complete DTS file for our board, maybe you will find something wrong in there which is not obvious from snippet in first post.

I suspect that the issue may not be device tree related at all and is maybe caused by some sort of kernel misconfiguration, but I have no idea what could be wrong as I just recompiled kernel with colibri-imx6_defconfig and it does not work.

Note: I am bit confused if I should add this as comment, or as answer. I am not answering the original question, but it is way too long to post it as comment.