GPIO LEDS

How to use leds-gpio driver to manage states of connected leds of your Armadeus board.

Introduction

You can manage a led connected to a GPIO pin. The LED management is similar with the standard GPIO sysfs driver, but you have some new features like triggers (e.g. "heartbeat" LED blinks like a heart at the rate oh the CPU load) . Here are the GPIO used for the user LED for each APF board:

• APF9328: PORT A / bit 2
• APF27: GPIO_PORTF | 14
• APF28: PINID_GPMI_RDY1 (Bank 0 - pin 21)
• APF51: GPIO_PORTA | 2

Configuration

First, you need to enable the leds-gpio driver in your kernel and some triggers like the "heartbeat" trigger to make the LED flash like a heartbeat.

• Configure Linux kernel:

$ make linux-menuconfig

Device Drivers  --->
     --- LED support
         [*] LED Class Support
              *** LED drivers *** 
         <*> LED Support for GPIO connected LEDs
              [*] Platform device bindings for GPIO LEDs
              *** LED Triggers ***
         [*]   LED Trigger support
         <*>     LED Timer Trigger
         <*>     LED Heartbeat Trigger
         <*>     LED backlight Trigger
         <*>     LED Default ON Trigger

Then, in your apfXX-dev.c, you would need to define your LED before the variable platform_devices[]. This code is already implemented for the APF27, APF28 and APF51 so the source code hereafter (for the APF27) is only present as a reference sample to understand how to activate a GPIO LED driver.

#include <linux/leds.h>

/* GPIO LED */
#if defined(CONFIG_LEDS_GPIO) || defined(CONFIG_LEDS_GPIO_MODULE)
static struct gpio_led apf27dev_led[] = {
	{
		.name = "apfdev:green:user",
		.default_trigger = "heartbeat",
		.gpio = (GPIO_PORTF | 14),
		.active_low = 1,
	},
};

static struct gpio_led_platform_data apf27dev_led_data = {
	.num_leds	= ARRAY_SIZE(apf27dev_led),
	.leds		= apf27dev_led
};

static struct platform_device apf27dev_led_dev = {
	.name		= "leds-gpio",
	.id		= -1,
	.dev		= {
		.platform_data	= &apf27dev_led_data,
	},
};
#endif /* CONFIG_LEDS_GPIO */

Add the LED to get it managed by the kernel.

static struct platform_device *platform_devices[] __initdata = {
#if defined(CONFIG_LEDS_GPIO) || defined(CONFIG_LEDS_GPIO_MODULE)
	&apf27dev_led_dev,
#endif
	ALSA_SOUND

};

Then rebuild and update your bard with the new kernel. Upon the next kernel boot you should see the LED flash like a heartbeat (if you have activated the "heartbeat" trigger)

Usage

# ls /sys/class/leds/apfdev\:green\:user/
brightness      max_brightness  subsystem       uevent
device          power           trigger

You can change the trigger behaviors. By default, Heartbeat is selected:

• "heatbeat": led blinks like a heart and blink frequency will change according o the CPU activity.
• "nand-disk": the led will blink each time nand access occur (try with sync command to see it blinking).

# cat /sys/class/leds/apfdev\:green\:user/trigger 
none nand-disk mmc0 timer [heartbeat] backlight gpio default-on 

# echo none > /sys/class/leds/apfdev\:green\:user/trigger 

Switch on and off the LED

# cat /sys/class/leds/apfdev\:green\:user/max_brightness > /sys/class/leds/apfde
v\:green\:user/brightness 

# echo 0 > /sys/class/leds/apfdev\:green\:user/brightness 

It is possible to switch led state using the APF28Dev user button. This button is seen as gpio17 (as seen under APF28Dev datasheet) under Linux, then configure the led trigger as gpio :

# echo "gpio" > /sys/class/leds/apfdev:green:user/trigger

New config files are available :

# ls /sys/class/leds/apfdev:green:user/
brightness          gpio                power               uevent
desired_brightness  inverted            subsystem
device              max_brightness      trigger

Then the trigger gpio can be configured with gpio file :

# echo 17 > /sys/class/leds/apfdev:green:user/gpio

Then pushing the user switch will now commute the LED state.

Links

• http://www.kernel.org/doc/Documentation/leds/leds-class.txt