12.3.2 输入设备驱动
输入设备（如按键、键盘、触摸屏、鼠标等）是典型的字符设备，其一般的工作机理是底层在按键、触摸等动作发送时产生一个中断（或驱动通过Timer定时查询），然后CPU通过SPI、I 2 C或外部存储器总线读取键值、坐标等数据，并将键值、坐标等数据放入一个缓冲区，字符设备驱动管理该缓冲区，而驱动的read（）接口让用户读取键值、坐标等数据。
在这些工作中，只是中断、读键值/坐标值是与设备相关的，而输入事件的缓冲区管理以及字符设备驱动的file_operations接口对输入设备是通用的。基于此，内核设计了输入子系统，由核心层处理公共的工作。
Linux内核输入子系统的框架如图12.6所示。
图12.6 Linux内核输入子系统的框架
输入核心提供底层输入设备驱动程序所需的API，如分配/释放一个输入设备：
linux/input.h
struct input_dev *input_allocate_device(void);
void input_free_device(struct input_dev *dev);
input_allocate_device（）返回的是1个指向input_dev的结构体的指针，此结构体用于表征1个输入设备。
drivers/input/input.c（The input core）
/**
* input_allocate_device - allocate memory for new input device
*
* Returns prepared struct input_dev or NULL.
*
* NOTE: Use input_free_device() to free devices that have not been
* registered; input_unregister_device() should be used for already
* registered devices.
*/
struct input_dev *input_allocate_device(void)
{
struct input_dev *dev;
dev = kzalloc(sizeof(struct input_dev), GFP_KERNEL);if (dev) {
dev->dev.type = &input_dev_type;
dev->dev.class = &input_class;
device_initialize(&dev->dev);
mutex_init(&dev->mutex);
spin_lock_init(&dev->event_lock);
INIT_LIST_HEAD(&dev->h_list);
INIT_LIST_HEAD(&dev->node);
__module_get(THIS_MODULE);
}
return dev;
}
EXPORT_SYMBOL(input_allocate_device);
/**
* input_free_device - free memory occupied by input_dev structure
* @dev: input device to free
*
* This function should only be used if input_register_device()
* was not called yet or if it failed. Once device was registered
* use input_unregister_device() and memory will be freed once last
* reference to the device is dropped.
*
* Device should be allocated by input_allocate_device().
*
* NOTE: If there are references to the input device then memory
* will not be freed until last reference is dropped.
*/
void input_free_device(struct input_dev *dev)
{
if (dev)
input_put_device(dev);
}
EXPORT_SYMBOL(input_free_device);
注册/注销输入设备用的接口如下：
linux/input.h
void input_unregister_device(struct input_dev *);
drivers/input/input.c（The input core）
/**
* input_register_device - register device with input core
* @dev: device to be registered
*
* This function registers device with input core. The device must be
* allocated with input_allocate_device() and all it's capabilities
* set up before registering.
* If function fails the device must be freed with input_free_device().
* Once device has been successfully registered it can be unregistered
* with input_unregister_device(); input_free_device() should not be
* called in this case.
*/
int input_register_device(struct input_dev *dev)
{
static atomic_t input_no = ATOMIC_INIT(0);
struct input_handler *handler;
const char *path;
int error;
/* Every input device generates EV_SYN/SYN_REPORT events. */
__set_bit(EV_SYN, dev->evbit);
/* KEY_RESERVED is not supposed to be transmitted to userspace. */
__clear_bit(KEY_RESERVED, dev->keybit);
/* Make sure that bitmasks not mentioned in dev->evbit are clean. */
input_cleanse_bitmasks(dev);
if (!dev->hint_events_per_packet)
dev->hint_events_per_packet =
input_estimate_events_per_packet(dev);
/*
* If delay and period are pre-set by the driver, then autorepeating
* is handled by the driver itself and we don't do it in input.c.
*/
init_timer(&dev->timer);
if (!dev->rep[REP_DELAY] && !dev->rep[REP_PERIOD]) {
dev->timer.data = (long) dev;
dev->timer.function = input_repeat_key;
dev->rep[REP_DELAY] = 250;
dev->rep[REP_PERIOD] = 33;
}
if (!dev->getkeycode)
dev->getkeycode = input_default_getkeycode;
if (!dev->setkeycode)
dev->setkeycode = input_default_setkeycode;
dev_set_name(&dev->dev, "input%ld",
(unsigned long) atomic_inc_return(&input_no) - 1);
error = device_add(&dev->dev);
if (error)
return error;
path = kobject_get_path(&dev->dev.kobj, GFP_KERNEL);
pr_info("%s as %s\n",
dev->name ? dev->name : "Unspecified device",
path ? path : "N/A");
kfree(path);
error = mutex_lock_interruptible(&input_mutex);
if (error) {
device_del(&dev->dev);
return error;
}
list_add_tail(&dev->node, &input_dev_list);
list_for_each_entry(handler, &input_handler_list, node)
input_attach_handler(dev, handler);
input_wakeup_procfs_readers();
mutex_unlock(&input_mutex);
return 0;
}
EXPORT_SYMBOL(input_register_device);
/**
* input_unregister_device - unregister previously registered device
* @dev: device to be unregistered
*
* This function unregisters an input device. Once device is unregistered
* the caller should not try to access it as it may get freed at any moment.
*/
void input_unregister_device(struct input_dev *dev)
{
struct input_handle *handle, *next;
input_disconnect_device(dev);
mutex_lock(&input_mutex);
list_for_each_entry_safe(handle, next, &dev->h_list, d_node)
handle->handler->disconnect(handle);
WARN_ON(!list_empty(&dev->h_list));
del_timer_sync(&dev->timer);
list_del_init(&dev->node);
input_wakeup_procfs_readers();
mutex_unlock(&input_mutex);
device_unregister(&dev->dev);
}
EXPORT_SYMBOL(input_unregister_device);
报告输入事件用的接口如下：
linux/input.h
/* 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);
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，如代码清单12.10所示。
linux/input.h
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）屏蔽在板级文件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。代码清单12.11列出了该驱动的probe（）函数。
static int __devinit gpio_keys_probe(struct platform_device *pdev)
{
struct gpio_keys_platform_data *pdata = pdev->dev.platform_data; // 获取平台数据
struct gpio_keys_drvdata *ddata;
struct device *dev = &pdev->dev;
struct input_dev *input;
int i, error;
int wakeup = 0;
ddata = kzalloc(sizeof(struct gpio_keys_drvdata) +
pdata->nbuttons * sizeof(struct gpio_button_data),
GFP_KERNEL);
input = input_allocate_device();//分配输入设备
if (!ddata || !input) {
dev_err(dev, "failed to allocate state\n");
error = -ENOMEM;
goto fail1;
}
/*初始化该input_dev的一些属性*/
ddata->input = input;
ddata->n_buttons = pdata->nbuttons;
ddata->enable = pdata->enable;
ddata->disable = pdata->disable;
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;
/*初始化该input_dev的一些属性*/
/* Enable auto repeat feature of Linux input subsystem */
if (pdata->rep)
__set_bit(EV_REP, input->evbit);
// 初始化所用到的GPIO
struct gpio_keys_button *button = &pdata->buttons[i];
struct gpio_button_data *bdata = &ddata->data[i];
unsigned int type = button->type ?: EV_KEY;
bdata->input = input;
bdata->button = button;
error = gpio_keys_setup_key(pdev, bdata, button);
if (error)
goto fail2;
if (button->wakeup)
wakeup = 1;
input_set_capability(input, type, button->code);
}
error = sysfs_create_group(&pdev->dev.kobj, &gpio_keys_attr_group);
if (error) {
dev_err(dev, "Unable to export keys/switches, error: %d\n",
error);
goto fail2;
}
error = input_register_device(input);//register input device
if (error) {
dev_err(dev, "Unable to register input device, error: %d\n", error);
goto fail3;
}
/* get current state of buttons */
for (i = 0; i < pdata->nbuttons; i++)
gpio_keys_report_event(&ddata->data[i]);
input_sync(input);
device_init_wakeup(&pdev->dev, wakeup);
return 0;
fail3:
sysfs_remove_group(&pdev->dev.kobj, &gpio_keys_attr_group);
fail2:
while (--i >= 0) {
free_irq(gpio_to_irq(pdata->buttons[i].gpio), &ddata->data[i]);
if (ddata->data[i].timer_debounce)
del_timer_sync(&ddata->data[i].timer);
cancel_work_sync(&ddata->data[i].work);
gpio_free(pdata->buttons[i].gpio);
}
platform_set_drvdata(pdev, NULL);
fail1:
input_free_device(input);
kfree(ddata);
return error;
}
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。
代码清单12.12 GPIO按键中断发生时的事件报告
static irqreturn_t gpio_keys_irq_isr(int irq, void *dev_id) // 中断服务程序
{
struct gpio_button_data *bdata = dev_id;
const struct gpio_keys_button *button = bdata->button;
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, button->code, 1);
input_sync(input);
if (!bdata->timer_debounce) {
input_event(input, EV_KEY, button->code, 0);
input_sync(input);
goto out;
}
bdata->key_pressed = true;
}
if (bdata->timer_debounce)
mod_timer(&bdata->timer,
jiffies + msecs_to_jiffies(bdata->timer_debounce));
out:
spin_unlock_irqrestore(&bdata->lock, flags);
return IRQ_HANDLED;
}
GPIO按键驱动通过input_event（）、input_sync（）来汇报按键事件以及同步事件。从底层的GPIO按键驱动看出，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。
代码清单12.13 input核心层的file_operations和read（）函数
static ssize_t evdev_read(struct file *file, char __user *buffer,
size_t count, loff_t *ppos)
{
struct evdev_client *client = file->private_data;// 获取私有数据指针
struct evdev *evdev = client->evdev;
struct input_event event;
int retval = 0;
if (count < input_event_size())
return -EINVAL;
if (!(file->f_flags & O_NONBLOCK)) {
retval = wait_event_interruptible(evdev->wait,
client->packet_head != client->tail || !evdev->exist);
if (retval)
return retval;
}
if (!evdev->exist)
return -ENODEV;
while (retval + input_event_size() <= count &&
evdev_fetch_next_event(client, &event)) {
if (input_event_to_user(buffer + retval, &event))
return -EFAULT;
retval += input_event_size();
}
if (retval == 0 && file->f_flags & O_NONBLOCK)// 非阻塞访问，立即返回EAGAIN错误
retval = -EAGAIN;
return retval;
}
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,
};