device_create(...) argument 'void * drvdata' - linux-kernel

Question

The function
struct device * device_create ( struct class * class,
struct device * parent,
dev_t devt,
void * drvdata,
const char * fmt,
...);
[link to documentation] takes an argument "void * drvdata". It is described as "the data to be added to the device for callbacks". What callbacks exactly are meant here? The file-operation-functions?

Answer 1

SysFS is one example of where this could be useful: whenever userspace communicates with the driver using SysFS (see "Reading/Writing Attribute Data" in the documentation), the kernel calls the corresponding callbacks. The void * drvdata pointer supplied during device_create can then be obtained by calling dev_get_drvdata(dev). The pointer could be used to refer to a structure containing driver's state for example.
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
#include <linux/init.h> /* Needed for the macros */
#include <linux/device.h>
#include <linux/err.h>
MODULE_LICENSE("GPL");
static ssize_t show_period(struct device *dev, struct device_attribute *attr, char *buf);
static ssize_t set_period(struct device* dev,
struct device_attribute* attr,
const char* buf,
size_t count);
static DEVICE_ATTR(period, S_IWUSR | S_IRUSR, show_period, set_period);
static struct device *s_pDeviceObject;
static struct class *s_pDeviceClass;
static struct pwm_device_state
{
int m_Period;
} s_DeviceState;
static int __init pwmdriver_init(void)
{
int result;
s_pDeviceClass = class_create(THIS_MODULE, "pwmdriver");
BUG_ON(IS_ERR(s_pDeviceClass));
s_pDeviceObject = device_create(s_pDeviceClass, NULL, 0, &s_DeviceState, "channel");
result = device_create_file(s_pDeviceObject, &dev_attr_period);
BUG_ON(result < 0);
return result;
}
static ssize_t show_period(struct device *dev, struct device_attribute *attr, char *buf)
{
struct pwm_device_state *pwm_device_data;
pwm_device_data = dev_get_drvdata(dev);
return scnprintf(buf, PAGE_SIZE, "%d\n", pwm_device_data->m_Period);
}
static ssize_t set_period(struct device* dev,
struct device_attribute* attr,
const char* buf,
size_t count)
{
long period_value = 0;
struct pwm_device_state *pwm_device_data;
if (kstrtol(buf, 10, &period_value) < 0)
return -EINVAL;
if (period_value < 10) //Safety check
return -EINVAL;
pwm_device_data = dev_get_drvdata(dev);
pwm_device_data->m_Period = period_value;
return count;
}
static void __exit pwmdriver_exit(void)
{
device_remove_file(s_pDeviceObject, &dev_attr_period);
device_destroy(s_pDeviceClass, 0);
class_destroy(s_pDeviceClass);
}
module_init(pwmdriver_init);
module_exit(pwmdriver_exit);