树莓派 - platform总线，设备和驱动

以树莓派为例子，分析一下其中LED的 platform device 和 platform driver.

查看LED设备，被挂载在/sys/devices/platform下。

注意其中的driver指向， 

driver -> ../../../bus/platform/drivers/leds-gpio

pi@raspberrypi:/sys/devices/platform/leds $ tree
.
├── driver -> ../../../bus/platform/drivers/leds-gpio
├── driver_override
├── leds
│   ├── led0
│   │   ├── brightness
│   │   ├── device -> ../../../leds
│   │   ├── max_brightness
│   │   ├── power
│   │   │   ├── autosuspend_delay_ms
│   │   │   ├── control
│   │   │   ├── runtime_active_time
│   │   │   ├── runtime_status
│   │   │   └── runtime_suspended_time
│   │   ├── subsystem -> ../../../../../class/leds
│   │   ├── trigger
│   │   └── uevent
│   └── led1
│       ├── brightness
│       ├── device -> ../../../leds
│       ├── max_brightness
│       ├── power
│       │   ├── autosuspend_delay_ms
│       │   ├── control
│       │   ├── runtime_active_time
│       │   ├── runtime_status
│       │   └── runtime_suspended_time
│       ├── subsystem -> ../../../../../class/leds
│       ├── trigger
│       └── uevent
├── modalias
├── of_node -> ../../../firmware/devicetree/base/leds
├── power
│   ├── autosuspend_delay_ms
│   ├── control
│   ├── runtime_active_time
│   ├── runtime_status
│   └── runtime_suspended_time
├── subsystem -> ../../../bus/platform
└── uevent

同样，在/sys/bus/platform/drivers/leds-gpio下可以看到其中的leds指向了leds设备

pi@raspberrypi:/sys/bus/platform/drivers/leds-gpio $ tree
.
├── bind
├── leds -> ../../../../devices/platform/leds
├── uevent
└── unbind

这表示leds设备和leds-gpio驱动匹配

驱动leds-gpio.c

查看驱动，linux\drivers\leds\leds-gpio.c

/*
 * LEDs driver for GPIOs
 *
 * Copyright (C) 2007 8D Technologies inc.
 * Raphael Assenat <raph@8d.com>
 * Copyright (C) 2008 Freescale Semiconductor, Inc.
 *
 * 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/err.h>
#include <linux/gpio.h>
#include <linux/gpio/consumer.h>
#include <linux/kernel.h>
#include <linux/leds.h>
#include <linux/module.h>
#include <linux/of.h>
#include <linux/platform_device.h>
#include <linux/property.h>
#include <linux/slab.h>

struct gpio_led_data {
	struct led_classdev cdev;
	struct gpio_desc *gpiod;
	u8 can_sleep;
	u8 blinking;
	gpio_blink_set_t platform_gpio_blink_set;
};

static inline struct gpio_led_data *
			cdev_to_gpio_led_data(struct led_classdev *led_cdev)
{
	return container_of(led_cdev, struct gpio_led_data, cdev);
}

static void gpio_led_set(struct led_classdev *led_cdev,
	enum led_brightness value)
{
	struct gpio_led_data *led_dat = cdev_to_gpio_led_data(led_cdev);
	int level;

	if (value == LED_OFF)
		level = 0;
	else
		level = 1;

	if (led_dat->blinking) {
		led_dat->platform_gpio_blink_set(led_dat->gpiod, level,
						 NULL, NULL);
		led_dat->blinking = 0;
	} else if (led_dat->cdev.flags & SET_GPIO_INPUT) {
		gpiod_direction_input(led_dat->gpiod);
		led_dat->cdev.flags &= ~SET_GPIO_INPUT;
	} else if (led_dat->cdev.flags & SET_GPIO_OUTPUT) {
		gpiod_direction_output(led_dat->gpiod, level);
		led_dat->cdev.flags &= ~SET_GPIO_OUTPUT;
	} else {
		if (led_dat->can_sleep ||
			(led_dat->cdev.flags & (SET_GPIO_INPUT | SET_GPIO_OUTPUT) ))
			gpiod_set_value_cansleep(led_dat->gpiod, level);
		else
			gpiod_set_value(led_dat->gpiod, level);
	}
}

static int gpio_led_set_blocking(struct led_classdev *led_cdev,
	enum led_brightness value)
{
	gpio_led_set(led_cdev, value);
	return 0;
}

static enum led_brightness gpio_led_get(struct led_classdev *led_cdev)
{
	struct gpio_led_data *led_dat =
		container_of(led_cdev, struct gpio_led_data, cdev);
	return gpiod_get_value_cansleep(led_dat->gpiod) ? LED_FULL : LED_OFF;
}

static int gpio_blink_set(struct led_classdev *led_cdev,
	unsigned long *delay_on, unsigned long *delay_off)
{
	struct gpio_led_data *led_dat = cdev_to_gpio_led_data(led_cdev);

	led_dat->blinking = 1;
	return led_dat->platform_gpio_blink_set(led_dat->gpiod, GPIO_LED_BLINK,
						delay_on, delay_off);
}

static int create_gpio_led(const struct gpio_led *template,
	struct gpio_led_data *led_dat, struct device *parent,
	struct device_node *np, gpio_blink_set_t blink_set)
{
	int ret, state;

	led_dat->gpiod = template->gpiod;
	if (!led_dat->gpiod) {
		/*
		 * This is the legacy code path for platform code that
		 * still uses GPIO numbers. Ultimately we would like to get
		 * rid of this block completely.
		 */
		unsigned long flags = GPIOF_OUT_INIT_LOW;

		/* skip leds that aren't available */
		if (!gpio_is_valid(template->gpio)) {
			dev_info(parent, "Skipping unavailable LED gpio %d (%s)\n",
					template->gpio, template->name);
			return 0;
		}

		if (template->active_low)
			flags |= GPIOF_ACTIVE_LOW;

		ret = devm_gpio_request_one(parent, template->gpio, flags,
					    template->name);
		if (ret < 0)
			return ret;

		led_dat->gpiod = gpio_to_desc(template->gpio);
		if (!led_dat->gpiod)
			return -EINVAL;
	}

	led_dat->cdev.name = template->name;
	led_dat->cdev.default_trigger = template->default_trigger;
	led_dat->can_sleep = gpiod_cansleep(led_dat->gpiod);
	if (!led_dat->can_sleep)
		led_dat->cdev.brightness_set = gpio_led_set;
	else
		led_dat->cdev.brightness_set_blocking = gpio_led_set_blocking;
	led_dat->blinking = 0;
	if (blink_set) {
		led_dat->platform_gpio_blink_set = blink_set;
		led_dat->cdev.blink_set = gpio_blink_set;
	}
	led_dat->cdev.brightness_get = gpio_led_get;
	if (template->default_state == LEDS_GPIO_DEFSTATE_KEEP) {
		state = gpiod_get_value_cansleep(led_dat->gpiod);
		if (state < 0)
			return state;
	} else {
		state = (template->default_state == LEDS_GPIO_DEFSTATE_ON);
	}
	led_dat->cdev.brightness = state ? LED_FULL : LED_OFF;
	if (!template->retain_state_suspended)
		led_dat->cdev.flags |= LED_CORE_SUSPENDRESUME;
	if (template->panic_indicator)
		led_dat->cdev.flags |= LED_PANIC_INDICATOR;
	if (template->retain_state_shutdown)
		led_dat->cdev.flags |= LED_RETAIN_AT_SHUTDOWN;

	ret = gpiod_direction_output(led_dat->gpiod, state);
	if (ret < 0)
		return ret;

	return devm_of_led_classdev_register(parent, np, &led_dat->cdev);
}

struct gpio_leds_priv {
	int num_leds;
	struct gpio_led_data leds[];
};

static inline int sizeof_gpio_leds_priv(int num_leds)
{
	return sizeof(struct gpio_leds_priv) +
		(sizeof(struct gpio_led_data) * num_leds);
}

static struct gpio_leds_priv *gpio_leds_create(struct platform_device *pdev)
{
	struct device *dev = &pdev->dev;
	struct fwnode_handle *child;
	struct gpio_leds_priv *priv;
	int count, ret;

	count = device_get_child_node_count(dev);
	if (!count)
		return ERR_PTR(-ENODEV);

	priv = devm_kzalloc(dev, sizeof_gpio_leds_priv(count), GFP_KERNEL);
	if (!priv)
		return ERR_PTR(-ENOMEM);

	device_for_each_child_node(dev, child) {
		struct gpio_led_data *led_dat = &priv->leds[priv->num_leds];
		struct gpio_led led = {};
		const char *state = NULL;
		struct device_node *np = to_of_node(child);

		ret = fwnode_property_read_string(child, "label", &led.name);
		if (ret && IS_ENABLED(CONFIG_OF) && np)
			led.name = np->name;
		if (!led.name) {
			fwnode_handle_put(child);
			return ERR_PTR(-EINVAL);
		}

		led.gpiod = devm_fwnode_get_gpiod_from_child(dev, NULL, child,
							     GPIOD_ASIS,
							     led.name);
		if (IS_ERR(led.gpiod)) {
			fwnode_handle_put(child);
			return ERR_CAST(led.gpiod);
		}

		fwnode_property_read_string(child, "linux,default-trigger",
					    &led.default_trigger);

		if (!fwnode_property_read_string(child, "default-state",
						 &state)) {
			if (!strcmp(state, "keep"))
				led.default_state = LEDS_GPIO_DEFSTATE_KEEP;
			else if (!strcmp(state, "on"))
				led.default_state = LEDS_GPIO_DEFSTATE_ON;
			else
				led.default_state = LEDS_GPIO_DEFSTATE_OFF;
		}

		if (fwnode_property_present(child, "retain-state-suspended"))
			led.retain_state_suspended = 1;
		if (fwnode_property_present(child, "retain-state-shutdown"))
			led.retain_state_shutdown = 1;
		if (fwnode_property_present(child, "panic-indicator"))
			led.panic_indicator = 1;

		ret = create_gpio_led(&led, led_dat, dev, np, NULL);
		if (ret < 0) {
			fwnode_handle_put(child);
			return ERR_PTR(ret);
		}
		led_dat->cdev.dev->of_node = np;
		priv->num_leds++;
	}

	return priv;
}

static const struct of_device_id of_gpio_leds_match[] = {
	{ .compatible = "gpio-leds", },
	{},
};

MODULE_DEVICE_TABLE(of, of_gpio_leds_match);

static int gpio_led_probe(struct platform_device *pdev)
{
	struct gpio_led_platform_data *pdata = dev_get_platdata(&pdev->dev);
	struct gpio_leds_priv *priv;
	int i, ret = 0;

	if (pdata && pdata->num_leds) {
		priv = devm_kzalloc(&pdev->dev,
				sizeof_gpio_leds_priv(pdata->num_leds),
					GFP_KERNEL);
		if (!priv)
			return -ENOMEM;

		priv->num_leds = pdata->num_leds;
		for (i = 0; i < priv->num_leds; i++) {
			ret = create_gpio_led(&pdata->leds[i], &priv->leds[i],
					      &pdev->dev, NULL,
					      pdata->gpio_blink_set);
			if (ret < 0)
				return ret;
		}
	} else {
		priv = gpio_leds_create(pdev);
		if (IS_ERR(priv))
			return PTR_ERR(priv);
	}

	platform_set_drvdata(pdev, priv);

	return 0;
}

static void gpio_led_shutdown(struct platform_device *pdev)
{
	struct gpio_leds_priv *priv = platform_get_drvdata(pdev);
	int i;

	for (i = 0; i < priv->num_leds; i++) {
		struct gpio_led_data *led = &priv->leds[i];

		if (!(led->cdev.flags & LED_RETAIN_AT_SHUTDOWN))
			gpio_led_set(&led->cdev, LED_OFF);
	}
}

static struct platform_driver gpio_led_driver = {
	.probe		= gpio_led_probe,
	.shutdown	= gpio_led_shutdown,
	.driver		= {
		.name	= "leds-gpio",
		.of_match_table = of_gpio_leds_match,
	},
};

module_platform_driver(gpio_led_driver);

MODULE_AUTHOR("Raphael Assenat <raph@8d.com>, Trent Piepho <tpiepho@freescale.com>");
MODULE_DESCRIPTION("GPIO LED driver");
MODULE_LICENSE("GPL");
MODULE_ALIAS("platform:leds-gpio");

注册platform device

在树莓派的系统中是通过设备树来注册leds设备的

在bcm2708-rpi.dtsi中

	leds: leds {
		compatible = "gpio-leds";
	};

在bcm2709-rpi-2-b.dts

&leds {
	act_led: act {
		label = "led0";
		linux,default-trigger = "mmc0";
		gpios = <&gpio 47 0>;
	};

	pwr_led: pwr {
		label = "led1";
		linux,default-trigger = "input";
		gpios = <&gpio 35 0>;
	};
};

参考：

https://blog.youkuaiyun.com/zqixiao_09/article/details/50889458

https://blog.youkuaiyun.com/subfate/article/details/53494926

https://blog.youkuaiyun.com/jklinux/article/details/78645106