platform总线的简单使用

Linux从2.6起就加入了一套新的驱动管理和注册的机制platform平台总线，是一条虚拟的总线，设备用platform_device表示，驱动用platform_driver进行注册。相对于USB、PCI、I2C、SPI等物理总线来说，platform总线是一种虚拟、抽象出来的总线，实际中并不存在这样的总线。那为什么需要platform总线呢？其实是Linux设备驱动模型为了保持设备驱动的统一性而虚拟出来的总线。因为对于usb设备、i2c设备、pci设备、spi设备等等，他们与cpu的通信都是直接挂在相应的总线下面与我们的cpu进行数据交互的，但是在我们的嵌入式系统当中，并不是所有的设备都能够归属于这些常见的总线，在嵌入式系统里面，SoC系统中集成的独立的外设控制器、挂接在SoC内存空间的外设却不依附与此类总线。所以Linux驱动模型为了保持完整性，将这些设备挂在一条虚拟的总线上（platform总线），而不至于使得有些设备挂在总线上，另一些设备没有挂在总线上。

platform总线的相关代码主要在：kernel-4.9\drivers\base\platform.c

相关结构代码在：kernel-4.9/include/linux/platform_device.h

struct platform_device {
	const char	*name;
	int		id;
	bool		id_auto;
	struct device	dev;
	u32		num_resources;
	struct resource	*resource;

	const struct platform_device_id	*id_entry;
	char *driver_override; /* Driver name to force a match */

	/* MFD cell pointer */
	struct mfd_cell *mfd_cell;

	/* arch specific additions */
	struct pdev_archdata	archdata;
};

struct platform_driver {
	int (*probe)(struct platform_device *);
	int (*remove)(struct platform_device *);
	void (*shutdown)(struct platform_device *);
	int (*suspend)(struct platform_device *, pm_message_t state);
	int (*resume)(struct platform_device *);
	struct device_driver driver;
	const struct platform_device_id *id_table;
	bool prevent_deferred_probe;
};

其中，platform_device 有一个成员是struct resource，它定义在kernel-4.9\include\linux\ioport.h

/*
 * Resources are tree-like, allowing
 * nesting etc..
 */
struct resource {
	resource_size_t start;
	resource_size_t end;
	const char *name;
	unsigned long flags;
	unsigned long desc;
	struct resource *parent, *sibling, *child;
};

注册和卸载设备&设备驱动

/*
 * use a macro to avoid include chaining to get THIS_MODULE
 */
#define platform_driver_register(drv) \
	__platform_driver_register(drv, THIS_MODULE)
extern int __platform_driver_register(struct platform_driver *,
					struct module *);
extern void platform_driver_unregister(struct platform_driver *);


extern int platform_device_register(struct platform_device *);
extern void platform_device_unregister(struct platform_device *);

设备和设备驱动绑定，是通过比较名字是否一致来确认的，具体方法定义在kernel-4.9\drivers\base\platform.c

可以看到驱动部分是通过platform_driver的成员driver的名字来比较的。

/**
 * platform_match - bind platform device to platform driver.
 * @dev: device.
 * @drv: driver.
 *
 * Platform device IDs are assumed to be encoded like this:
 * "<name><instance>", where <name> is a short description of the type of
 * device, like "pci" or "floppy", and <instance> is the enumerated
 * instance of the device, like '0' or '42'.  Driver IDs are simply
 * "<name>".  So, extract the <name> from the platform_device structure,
 * and compare it against the name of the driver. Return whether they match
 * or not.
 */
static int platform_match(struct device *dev, struct device_driver *drv)
{
	struct platform_device *pdev = to_platform_device(dev);
	struct platform_driver *pdrv = to_platform_driver(drv);

	/* When driver_override is set, only bind to the matching driver */
	if (pdev->driver_override)
		return !strcmp(pdev->driver_override, drv->name);

	/* Attempt an OF style match first */
	if (of_driver_match_device(dev, drv))
		return 1;

	/* Then try ACPI style match */
	if (acpi_driver_match_device(dev, drv))
		return 1;

	/* Then try to match against the id table */
	if (pdrv->id_table)
		return platform_match_id(pdrv->id_table, pdev) != NULL;

	/* fall-back to driver name match */
	return (strcmp(pdev->name, drv->name) == 0);
}

static const struct platform_device_id *platform_match_id(
			const struct platform_device_id *id,
			struct platform_device *pdev)
{
	while (id->name[0]) {
		if (strcmp(pdev->name, id->name) == 0) {
			pdev->id_entry = id;
			return id;
		}
		id++;
	}
	return NULL;
}

就一般项目来说，device被定义在dts中可能性比较高，可以通过struct of_device_id结构将device和driver关联。

static const struct of_device_id my_led_match[] = {	
	{ .compatible = "my-led-name" },	
	{ /* Sentinel */ }
};


static struct platform_driver my_led_driver = {
	.probe = my_led_probe,	
	.driver = {		
		.name = "my-led-name",		
		.owner = THIS_MODULE,		
		.of_match_table	= my_led_match,	
	},
};

这了需要保证.compatible的名字、.driver中的.name的名字还有dts中定义的名字是一致的。

my-led-name : my_led{
    compatible = "my-led-name";
    enable-active-high;
    my_led-gpio = <&gpio2 3 GPIO_ACTIVE_HIGH>;
    status = "okay";
};