Configure Parameters of LED Trigger from Kernel Space

Question

I'm working on an embedded project. Our board is using Linux kernel v3.16.7. I'm working on supporting a couple of peripheral LEDs that monitor activity. I've successfully modified the boot procedure to load the drivers and create sysfs entries in /sys/class/leds/, which is great. I've also attached a oneshot trigger to the leds so I can echo 1 > shot from within /sys/class/leds/actled1\:green/ and the led blinks. Exactly what I want.

However, I want to configure the delays for each LED when I instantiate the driver during boot, and I'm not clear on how to do that. The driver creates sysfs entries in /sys/class/leds/actled1\:green/ called delay_on and delay_off, and I can write to them from userspace to configure the delays, but it should be possible to set their initial values from from kernel space during instantiation. I also want to be able to set the invert parameter (which is just another sysfs entry just like the delays).

How can I configure the parameters of an led trigger when I instantiate the driver from kernel space?

Below is how I instantiate the LED GPIOs. First I set up the structs required:

static struct gpio_led my_leds[] __initdata = {
    {
        .name = "actled1:green",
        .default_trigger = "oneshot"
        .gpio = ACTIVITY_LED_GPIO_BASE + 0,
        .active_low = true,
    },
    {
        .name = "actled2:red",
        .default_trigger = "oneshot"
        .gpio = ACTIVITY_LED_GPIO_BASE + 1,
        .active_low = true,
    },
};

static struct gpio_led_platform_data my_leds_pdata __initdata = {
    .num_leds = ARRAY_SIZE(my_leds),
    .leds = my_leds,
};

Then, I call this function to create the platform devices:

static int __init setup_my_leds (void)
{
    struct platform_device *pdev;
    int ret;

    pdev = platform_device_alloc("leds-gpio", -1);
    if (!pdev) {
        return -ENOMEM;
    }

    ret = platform_device_add_data(pdev,
                    &my_leds_pdata,
                    sizeof(my_leds_pdata));

    if (ret < 0) {
        platform_device_put(pdev);
        return ret;
    }

    ret = platform_device_add(pdev);
    if (ret < 0) {
        platform_device_put(pdev);
        return ret;
    }

    return 0;
}

The definition for the gpio_led struct is in include/linux/leds.h line 327, and the definition for gpio_led_platform_data is in line 341 of the same file. The definition of platform_device_add_data is in drivers/base/platform.c line 284.

It may be useful to look at the source for the oneshot trigger (drivers/leds/trigger/ledtrig-oneshot.c) in order to answer the question. Also relevant is the "leds-gpio" driver (drivers/leds/leds-gpio.c).

I suspect the answer is somewhere in drivers/base/platform.c and the associated documentation, but I'm not seeing any functions that deal with the data I need.

---

To address some of the information that I inadvertently left out:

1. the boot loader sets the kernel arguments, and we can't modify the boot loader. that's fine; the values i want to set are constants and i can hard-code them in.
2. the driver is baked into the kernel at compile time (and, i presume, loaded by the bootloader) rather than loading a .ko with modprobe later.
3. i would love a general way to set arbitrary trigger parameters, not only oneshot's delay_on / delay_off. for example, oneshot's invert parameter.
4. i'm totally fine modifying oneshot / creating new triggers. in fact, once i get it working with oneshot, i'll need to create a new trigger that expands upon oneshot (which is also the reason i need to set arbitrary parameters).

Answer 1

There are a few issues and I think I've found the solutions, but even though you provided a good deal of info, there were some things missing, so I'll enumerate for all possible scenarios, so be patient ...

(1) Getting the initial values you want to set. I presume you have already figured this out, but ... You can get these from kernel cmdline parsing (e.g. you add the values to /boot/grub2/grub.cfg as myleds.delay_on=.... If you're loading via modprobe, you set a module parameter. These could also be a config file as in myleds.config_file=/etc/sysconfig/myleds.conf

(2) You could set them inside your setup_my_leds [except for the recalcitrance of oneshot_trig_activate--which we'll deal with soon enough]. From drivers/base/platform.c:

/**
 * arch_setup_pdev_archdata - Allow manipulation of archdata before its used
 * @pdev: platform device
 *
 * This is called before platform_device_add() such that any pdev_archdata may
 * be setup before the platform_notifier is called.  So if a user needs to
 * manipulate any relevant information in the pdev_archdata they can do:
 *
 *  platform_device_alloc()
 *  ... manipulate ...
 *  platform_device_add()
 *
 * And if they don't care they can just call platform_device_register() and
 * everything will just work out.
 */

So, with that in mind, let's change your setup function slightly:

static int __init setup_my_leds (void)
{
    struct platform_device *pdev;
    int ret;

    // get initial values you want to set, possibly storing away for later use
    my_leds_get_init_values(...);

    pdev = platform_device_alloc("leds-gpio", -1);
    if (!pdev) {
        return -ENOMEM;
    }

    // Choice (1): set your initial values in my_leds_pdata here
    my_leds_set_init_values(&my_leds_pdata);

    // NOTE: just does kmemdup and sets pdev->dev.platform_data
    ret = platform_device_add_data(pdev,
                    &my_leds_pdata,
                    sizeof(my_leds_pdata));

    if (ret < 0) {
        platform_device_put(pdev);
        return ret;
    }

    // Choice (2): set your initial values in pdev->dev.platform_data here
    my_leds_set_init_values(pdev->dev.platform_data);

    ret = platform_device_add(pdev);
    if (ret < 0) {
        platform_device_put(pdev);
        return ret;
    }

    return 0;
}

(3) Unfortunately, since you're using .default_trigger = "oneshot", the above data will get blasted by oneshot_trig_activate in drivers/leds/trigger/ledtrig-oneshot.c. So, we need to deal with that.

Option (A): Assuming you can rebuild the whole kernel as you choose, just modify oneshot_trig_activate in ledtrig-oneshot.c and remove the the lines that use DEFAULT_DELAY. This is only really useful if you know that it's not used by anything else in your system that might need the default values.

Option (B): If you're not allowed to modify ledtrig-oneshot.c, but are allowed to add new triggers to drivers/leds/trigger, copy the file to (e.g.) ledtrig-oneshot2.c and do the changes there. You'll need to change the .name to .name = "oneshot2". The easy way [in vi, of course :-)] is :%s/oneshot/oneshot2/g. You'll also need to add a new entry in the Kconfig and Makefile for this. Then, change your struct definition to use the new driver: .default_trigger = "oneshot2"

Option (C): Assuming you can't [or don't want to] touch the drivers/leds/trigger directory, copy ledtrig-oneshot.c to your driver directory [renaming as appropriate]. Do the edits from option (B) above there. With some trickery in your Makefile, you can get it to build both my_led_driver.ko and ledtrig-oneshot2.ko. You'll need modify your Kconfig, possibly adding a depends on LED_TRIGGERS for the led trigger driver. You could also put the two into separate subdirectories and the individual Makefile/Kconfig might be simpler: my_led/my_driver and my_led/my_trigger

Option (C) would be more work up front, but might be cleaner and more portable in the long run. Of course, you could do option (A) for proof-of-concept, then do option (B), and do the "final ship" as option (C).

An alternate way for when you set the initial values: Remember the comment for my_leds_get_init_values was possibly storing away for later use. You could change oneshot2_trig_activate to call it instead of using DEFAULT_DELAY. I don't like this quite as much and prefer the solutions that simply neuter oneshot_trig_activate's offensive behavior. But, with some testing, you may find that this is the way you have to do it.

Hopefully, the above will work. If not, edit your question with additional info and/or restrictions [and send me a comment], and I'll be glad to update my answer [I've been doing drivers for 40+].

UPDATE: Okay, herein is a fully annotated and modified LED trigger driver that you can use as a drop in replacement for drivers/led/trigger/ledtrig-oneshot.c.

Because the invert parameter can not be passed directly through any standard struct you have access to in your setup function [i.e. it's stored in a private struct inside the trigger driver], remove the "Choice (1)" and "Choice (2)". We'll set them all at once inside the [modified] oneshot_trig_activate.

Also, the init parameters you want must be set up and stored as globals by the my_leds_get_init_values so the trigger driver can find them. That is, there is no way to do this cleanly (e.g. with a pointer to a private struct that gets passed around) as the structs you have access to in setup don't have a field for this. See the top part of the trigger driver for discussion on this.

My first step was to annotate the base driver with descriptive comments. There were no comments in it, except for K&R style for copyright and a single one-liner. My annotations are ANSI ("//") comments.

If I were taking over the driver, I would add these and leave them in. However, my level of comments might be considered "over-commenting" according to the kernel style guide and might be considered "cruft", particularly for a driver that is this straightforward.

Next step was to add the necessary changes. All places that have additions/changes are marked with a comment block that starts with "C:". These are the important places to look. Note that these comments are legitimate candidates to leave in. In other more complex drivers, the level of commenting is up to the author. The "C:" is just to highlight the places for you.

With the annotations, a straight line read through might be easier now. Also, a diff -u might also help. If you've got everything under git, so much the better.

Because of all this, I'd remove the "Option (A)" [direct modification of the original file] and do "Option (B)" or "Option (C)" only.

The trigger driver uses all static definitions, so the global edit I suggested before is not needed. I did do .name = "myled_oneshot";, so you'll need to match that with .default_trigger = "myled_oneshot";. Feel free to use my_leds_whatever to be consistent with your existing naming convention. When I do this for myself, I usually use my initials, so it becomes ce_leds_whatever--YMMV

Anyway, here's the entire modified trigger driver. Note that I've done the editing, but I've not tried to compile/build it.

/*
 * One-shot LED Trigger
 *
 * Copyright 2012, Fabio Baltieri <[email protected]>
 *
 * Based on ledtrig-timer.c by Richard Purdie <[email protected]>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 */

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/device.h>
#include <linux/ctype.h>
#include <linux/slab.h>
#include <linux/leds.h>
#include "../leds.h"

// C: we need to get access to the init data populated by the setup function
// we have the "clean way" with a struct definition inside a header file and
// the "dirty way" using three separate int globals
// in either case, the externs referenced here must be defined in the "my_leds"
// driver as global

// C: the "clean way"
// (1) requires that we have a path to the .h (e.g. -I<whatever)
// (2) this would be easier/preferable for the "Option (C)"
// (3) once done, easily extensible [probably not a consideration here]
#ifdef MYLED_USESTRUCT
#include "whatever/myled_init.h"
extern struct myled_init myled_init;

// C: the "ugly way"
// (1) no need to use a separate .h file
// (2) three separate global variables is wasteful
// (3) more than three, and we really should consider the "struct"
#else
extern int myled_init_delay_on;
extern int myled_init_delay_off;
extern int myled_init_invert;
#endif

#define DEFAULT_DELAY 100

// oneshot trigger driver private data
struct oneshot_trig_data {
    unsigned int invert;                // current invert state
};

// arm oneshot sequence from sysfs write to shot file
static ssize_t led_shot(struct device *dev,
        struct device_attribute *attr, const char *buf, size_t size)
{
    struct led_classdev *led_cdev = dev_get_drvdata(dev);
    struct oneshot_trig_data *oneshot_data = led_cdev->trigger_data;

    led_blink_set_oneshot(led_cdev,
            &led_cdev->blink_delay_on, &led_cdev->blink_delay_off,
            oneshot_data->invert);

    /* content is ignored */
    return size;
}

// show invert state for "cat invert"
static ssize_t led_invert_show(struct device *dev,
        struct device_attribute *attr, char *buf)
{
    struct led_classdev *led_cdev = dev_get_drvdata(dev);
    struct oneshot_trig_data *oneshot_data = led_cdev->trigger_data;

    return sprintf(buf, "%u\n", oneshot_data->invert);
}

// set invert from sysfs write to invert file (e.g. echo 1 > invert)
static ssize_t led_invert_store(struct device *dev,
        struct device_attribute *attr, const char *buf, size_t size)
{
    struct led_classdev *led_cdev = dev_get_drvdata(dev);
    struct oneshot_trig_data *oneshot_data = led_cdev->trigger_data;
    unsigned long state;
    int ret;

    ret = kstrtoul(buf, 0, &state);
    if (ret)
        return ret;

    oneshot_data->invert = !!state;

    if (oneshot_data->invert)
        led_set_brightness_async(led_cdev, LED_FULL);
    else
        led_set_brightness_async(led_cdev, LED_OFF);

    return size;
}

// show delay_on state for "cat delay_on"
static ssize_t led_delay_on_show(struct device *dev,
        struct device_attribute *attr, char *buf)
{
    struct led_classdev *led_cdev = dev_get_drvdata(dev);

    return sprintf(buf, "%lu\n", led_cdev->blink_delay_on);
}

// set delay_on from sysfs write to delay_on file (e.g. echo 20 > delay_on)
static ssize_t led_delay_on_store(struct device *dev,
        struct device_attribute *attr, const char *buf, size_t size)
{
    struct led_classdev *led_cdev = dev_get_drvdata(dev);
    unsigned long state;
    int ret;

    ret = kstrtoul(buf, 0, &state);
    if (ret)
        return ret;

    led_cdev->blink_delay_on = state;

    return size;
}

// show delay_off state for "cat delay_off"
static ssize_t led_delay_off_show(struct device *dev,
        struct device_attribute *attr, char *buf)
{
    struct led_classdev *led_cdev = dev_get_drvdata(dev);

    return sprintf(buf, "%lu\n", led_cdev->blink_delay_off);
}

// set delay_off from sysfs write to delay_off file (e.g. echo 20 > delay_off)
static ssize_t led_delay_off_store(struct device *dev,
        struct device_attribute *attr, const char *buf, size_t size)
{
    struct led_classdev *led_cdev = dev_get_drvdata(dev);
    unsigned long state;
    int ret;

    ret = kstrtoul(buf, 0, &state);
    if (ret)
        return ret;

    led_cdev->blink_delay_off = state;

    return size;
}

// these are the "attribute" definitions -- one for each sysfs entry
// pointers to these show up in the above functions as the "attr" argument
static DEVICE_ATTR(delay_on, 0644, led_delay_on_show, led_delay_on_store);
static DEVICE_ATTR(delay_off, 0644, led_delay_off_show, led_delay_off_store);
static DEVICE_ATTR(invert, 0644, led_invert_show, led_invert_store);
static DEVICE_ATTR(shot, 0200, NULL, led_shot);

// activate the trigger device
static void oneshot_trig_activate(struct led_classdev *led_cdev)
{
    struct oneshot_trig_data *oneshot_data;
    int rc;

    // create an instance of the private data we need
    oneshot_data = kzalloc(sizeof(*oneshot_data), GFP_KERNEL);
    if (!oneshot_data)
        return;

    // save the pointer in the led class struct so it's available to other
    // functions above
    led_cdev->trigger_data = oneshot_data;

    // attach the sysfs entries
    rc = device_create_file(led_cdev->dev, &dev_attr_delay_on);
    if (rc)
        goto err_out_trig_data;
    rc = device_create_file(led_cdev->dev, &dev_attr_delay_off);
    if (rc)
        goto err_out_delayon;
    rc = device_create_file(led_cdev->dev, &dev_attr_invert);
    if (rc)
        goto err_out_delayoff;
    rc = device_create_file(led_cdev->dev, &dev_attr_shot);
    if (rc)
        goto err_out_invert;

    // C: this is what the driver used to do
#if 0
    led_cdev->blink_delay_on = DEFAULT_DELAY;
    led_cdev->blink_delay_off = DEFAULT_DELAY;
#endif

    led_cdev->activated = true;

    // C: from here to the return is what the modified driver must do

#ifdef MYLED_USESTRUCT
    led_cdev->blink_delay_on = myled_init.delay_on;
    led_cdev->blink_delay_off = myled_init.delay_off;
    oneshot_data->invert = myled_init.invert;
#else
    led_cdev->blink_delay_on = myled_init_delay_on;
    led_cdev->blink_delay_off = myled_init_delay_off;
    oneshot_data->invert = myled_init_invert;
#endif

    // C: if invert is off, nothing to do -- just like before
    // if invert is set, we implement this as if we just got an instantaneous
    // write to the sysfs "invert" file (which would call led_invert_store
    // above)

    // C: this is a direct rip-off of the above led_invert_store function which
    // we can _not_ call here directly because we don't have access to the
    // data it needs for its arguments [at least, not conveniently]
    // so, we extract the one line we actually need
    if (oneshot_data->invert)
        led_set_brightness_async(led_cdev, LED_FULL);

    return;

    // release everything if an error occurs
err_out_invert:
    device_remove_file(led_cdev->dev, &dev_attr_invert);
err_out_delayoff:
    device_remove_file(led_cdev->dev, &dev_attr_delay_off);
err_out_delayon:
    device_remove_file(led_cdev->dev, &dev_attr_delay_on);
err_out_trig_data:
    kfree(led_cdev->trigger_data);
}

// deactivate the trigger device
static void oneshot_trig_deactivate(struct led_classdev *led_cdev)
{
    struct oneshot_trig_data *oneshot_data = led_cdev->trigger_data;

    // release/destroy all the sysfs entries [and free the private data]
    if (led_cdev->activated) {
        device_remove_file(led_cdev->dev, &dev_attr_delay_on);
        device_remove_file(led_cdev->dev, &dev_attr_delay_off);
        device_remove_file(led_cdev->dev, &dev_attr_invert);
        device_remove_file(led_cdev->dev, &dev_attr_shot);
        kfree(oneshot_data);
        led_cdev->activated = false;
    }

    /* Stop blinking */
    led_set_brightness(led_cdev, LED_OFF);
}

// definition/control for trigger device registration
// C: changed the name to "myled_oneshot"
static struct led_trigger oneshot_led_trigger = {
    .name     = "myled_oneshot",
    .activate = oneshot_trig_activate,
    .deactivate = oneshot_trig_deactivate,
};

// module init function -- register the trigger device
static int __init oneshot_trig_init(void)
{
    return led_trigger_register(&oneshot_led_trigger);
}

// module exit function -- unregister the trigger device
static void __exit oneshot_trig_exit(void)
{
    led_trigger_unregister(&oneshot_led_trigger);
}

module_init(oneshot_trig_init);
module_exit(oneshot_trig_exit);

MODULE_AUTHOR("Fabio Baltieri <[email protected]>");
MODULE_DESCRIPTION("One-shot LED trigger");
MODULE_LICENSE("GPL");