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Linux 设备驱动的软件架构思想（三）（摘录）

输入设备驱动

struct input_dev *input_allocate_device(void);void input_free_device(struct input_dev *dev);input_allocate_device ()返回的是 1 个 input_dev 的结构体,此结构体用于表征 1 个输入设备。

注册 / 注销输入设备用的接口如下:

int __must_check input_register_device(struct input_dev *);void input_unregister_device(struct input_dev *);

报告输入事件用的接口如下:

/*报告指定type 、 code 的输入事件 */void input_event(struct input_dev *dev, unsigned int type, unsigned int code, int value);/*报告键值*/void input_report_key(struct input_dev *dev, unsigned int code, int value);/*报告相对坐标*/void input_report_rel(struct input_dev *dev, unsigned int code, int value);/*报告绝对坐标*/void input_report_abs(struct input_dev *dev, unsigned int code, int value);/*报告同步事件*/void input_sync(struct input_dev *dev);

而对于所有的输入事件,内核都用统一的数据结构来描述,这个数据结构是 input_event

struct input_event {
   	struct timeval time;	__u16 type;	__u16 code;	__s32 value;};

drivers/input/keyboard/gpio_keys.c 基于 input 架构实现了一个通用的 GPIO 按键驱动。该驱动是基于 platform_driver 架构的,名为 “gpio-keys” 。它将与硬件相关的信息(如使用的 GPIO 号,按下和抬起时的电平等)屏蔽在板文件platform_device 的 platform_data 中,因此该驱动可应用于各个处理器,具有良好的跨平台性。

static int gpio_keys_probe(struct platform_device *pdev){
   	struct device *dev = &pdev->dev;	const struct gpio_keys_platform_data *pdata = dev_get_platdata(dev);	struct fwnode_handle *child = NULL;	struct gpio_keys_drvdata *ddata;	struct input_dev *input;	size_t size;	int i, error;	int wakeup = 0;	if (!pdata) {
   		pdata = gpio_keys_get_devtree_pdata(dev);		if (IS_ERR(pdata))			return PTR_ERR(pdata);	}	size = sizeof(struct gpio_keys_drvdata) +			pdata->nbuttons * sizeof(struct gpio_button_data);	ddata = devm_kzalloc(dev, size, GFP_KERNEL);	if (!ddata) {
   		dev_err(dev, "failed to allocate state\n");		return -ENOMEM;	}	ddata->keymap = devm_kcalloc(dev,				     pdata->nbuttons, sizeof(ddata->keymap[0]),				     GFP_KERNEL);	if (!ddata->keymap)		return -ENOMEM;	input = devm_input_allocate_device(dev);	if (!input) {
   		dev_err(dev, "failed to allocate input device\n");		return -ENOMEM;	}	ddata->pdata = pdata;	ddata->input = input;	mutex_init(&ddata->disable_lock);	platform_set_drvdata(pdev, ddata);	input_set_drvdata(input, ddata);	input->name = pdata->name ? : pdev->name;	input->phys = "gpio-keys/input0";	input->dev.parent = dev;	input->open = gpio_keys_open;	input->close = gpio_keys_close;	input->id.bustype = BUS_HOST;	input->id.vendor = 0x0001;	input->id.product = 0x0001;	input->id.version = 0x0100;	input->keycode = ddata->keymap;	input->keycodesize = sizeof(ddata->keymap[0]);	input->keycodemax = pdata->nbuttons;	/* Enable auto repeat feature of Linux input subsystem */	if (pdata->rep)		__set_bit(EV_REP, input->evbit);	for (i = 0; i < pdata->nbuttons; i++) {
   		const struct gpio_keys_button *button = &pdata->buttons[i];		if (!dev_get_platdata(dev)) {
   			child = device_get_next_child_node(dev, child);			if (!child) {
   				dev_err(dev,					"missing child device node for entry %d\n",					i);				return -EINVAL;			}		}		error = gpio_keys_setup_key(pdev, input, ddata,					    button, i, child);		if (error) {
   			fwnode_handle_put(child);			return error;		}		if (button->wakeup)			wakeup = 1;	}	fwnode_handle_put(child);	error = devm_device_add_group(dev, &gpio_keys_attr_group);	if (error) {
   		dev_err(dev, "Unable to export keys/switches, error: %d\n",			error);		return error;	}	error = input_register_device(input);	if (error) {
   		dev_err(dev, "Unable to register input device, error: %d\n",			error);		return error;	}	device_init_wakeup(dev, wakeup);	return 0;}

其中

input = devm_input_allocate_device(dev);分配了 1 个输入设备。

然后

ddata->pdata = pdata; ddata->input = input; mutex_init(&ddata->disable_lock); platform_set_drvdata(pdev, ddata); input_set_drvdata(input, ddata); input->name = pdata->name : pdev->name; input->phys = "gpio-keys/input0"; input->dev.parent = &pdev->dev; input->open = gpio_keys_open; input->close = gpio_keys_close; input->id.bustype = BUS_HOST; input->id.vendor = 0x0001; input->id.product = 0x0001; input->id.version = 0x0100; 初始化了该 input_dev 的一些属性

然后

for (i = 0; i < pdata->nbuttons; i++) {
    const struct gpio_keys_button *button = &pdata->buttons[i]; struct gpio_button_data *bdata = &ddata->data[i]; error = gpio_keys_setup_key(pdev, input, bdata, button); if (error)return error; if (button->wakeup) wakeup = 1; }初始化了所用到的 GPIO

最后

error = input_register_device(input);完成了这个输入设备的注册

注册输入设备后,底层输入设备驱动的核心工作只剩下在按键、触摸等人为动作发生时报告事件。

static irqreturn_t gpio_keys_irq_isr(int irq, void *dev_id){
   	struct gpio_button_data *bdata = dev_id;	struct input_dev *input = bdata->input;	unsigned long flags;	BUG_ON(irq != bdata->irq);	spin_lock_irqsave(&bdata->lock, flags);	if (!bdata->key_pressed) {
   		if (bdata->button->wakeup)			pm_wakeup_event(bdata->input->dev.parent, 0);		input_event(input, EV_KEY, *bdata->code, 1);		input_sync(input);		if (!bdata->release_delay) {
   			input_event(input, EV_KEY, *bdata->code, 0);			input_sync(input);			goto out;		}		bdata->key_pressed = true;	}	if (bdata->release_delay)		mod_timer(&bdata->release_timer,			jiffies + msecs_to_jiffies(bdata->release_delay));out:	spin_unlock_irqrestore(&bdata->lock, flags);	return IRQ_HANDLED;}

GPIO 按键驱动通过 input_event ()、 input_sync ()这样的函数来汇报按键事件以及同步事件。从底层的 GPIO 按键驱动可以看出,该驱动中没有任何 file_operations 的动作,也没有各种 I/O 模型,注册进入系统也用的是input_register_device ()这样的与 input 相关的 API 。这是由于与 Linux VFS 接口的这一部分代码全部都在drivers/input/evdev.c 中实现了

static ssize_t evdev_read(struct file *file, char __user *buffer,			  size_t count, loff_t *ppos){
   	struct evdev_client *client = file->private_data;	struct evdev *evdev = client->evdev;	struct input_event event;	size_t read = 0;	int error;	if (count != 0 && count < input_event_size())		return -EINVAL;	for (;;) {
   		if (!evdev->exist || client->revoked)			return -ENODEV;  /*检查出是非阻塞访问后,立即返回 EAGAIN 错误*/		if (client->packet_head == client->tail &&		    (file->f_flags & O_NONBLOCK))			return -EAGAIN;		/*		 * count == 0 is special - no IO is done but we check		 * for error conditions (see above).		 */		if (count == 0)			break;		while (read + input_event_size() <= count &&		       evdev_fetch_next_event(client, &event)) {
   			if (input_event_to_user(buffer + read, &event))				return -EFAULT;			read += input_event_size();		}		if (read)			break;/*处理阻塞的睡眠情况*/		if (!(file->f_flags & O_NONBLOCK)) {
   			error = wait_event_interruptible(evdev->wait,					client->packet_head != client->tail ||					!evdev->exist || client->revoked);			if (error)				return error;		}	}	return read;}static const struct file_operations evdev_fops = {
   	.owner		= THIS_MODULE,	.read		= evdev_read,	.write		= evdev_write,	.poll		= evdev_poll,	.open		= evdev_open,	.release	= evdev_release,	.unlocked_ioctl	= evdev_ioctl,#ifdef CONFIG_COMPAT	.compat_ioctl	= evdev_ioctl_compat,#endif	.fasync		= evdev_fasync,	.flush		= evdev_flush,	.llseek		= no_llseek,};

回过头来想,其实 gpio_keys 驱动里面调用的 input_event ()、 input_sync ()有间接唤醒这个等待队列 evdev->wait 的功能,只不过这些代码都隐藏在其内部实现里了。

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