/proc. The files in Sysfs contain information about devices and drivers. Some files in Sysfs are even writable, for configuration and control of devices attached to the system. Sysfs is always mounted on /sys.Sysfs is the commonly used method to export system information from the kernel space to the user space for specific devices. The sysfs is tied to the device driver model of the kernel. The procfs is used to export the process specific information and the debugfs is used to used for exporting the debug information by the developer.
Heart of the sysfs model is the Kobject. Kobject is the glue that binds the sysfs and the kernel, which is represented by
struct kobject and defined in
#define KOBJ_NAME_LEN 20
struct kobject {
char *k_name;
char name[KOBJ_NAME_LEN];
struct kref kref;
struct list_head entry;
struct kobject *parent;
struct kset *kset;
struct kobj_type *ktype;
struct dentry *dentry;
};
struct kobject|–
name (Name of the kobject. Current kobject are created with this name in sysfs.)|–
parent (This iskobject’s parent. When we create a directory in sysfs for current kobject, it will create under this parent directory)|–
ktype ( type associated with a kobject)|–
kset (group of kobjects all of which are embedded in structures of the same type)|–
sd (points to a sysfs_dirent structure that represents this kobject in sysfs.)|–
kref (provides reference counting)SysFS in Linux
There are several steps in creating and using sysfs.
- Create directory in
/sys - Create Sysfs file
Create directory in /sys
We can use this function (
kobject_create_and_add) to create directory.
truct kobject * kobject_create_and_add ( const char * name, struct kobject * parent);
|
Where,
<
name> – the name for the kobject
<
parent> – the parent kobject of this kobject, if any.
This function creates a kobject structure dynamically and registers it with sysfs. If the kobject was not able to be created, NULL will be returned.
Create Sysfs file
Using above function we will create directory in /sys. Now we need to create sysfs file, which is used to interact user space with kernel space through sysfs. So we can create the sysfs file using sysfs attributes.
Attributes are represented as regular files in sysfs with one value per file. There are loads of helper function that can be used to create the kobject attributes
o create a single file attribute we are going to use ‘
sysfs_create_file’.
1
|
int sysfs_create_file ( struct kobject * kobj, const struct attribute * attr);
|
Where,
kobj – object we’re creating for.attr – attribute descriptor.Create attribute
Kobj_attribute is defined as,
1
2
3
4
5
|
struct kobj_attribute {
struct attribute attr;
ssize_t (*show)(struct kobject *kobj, struct kobj_attribute *attr, char *buf);
ssize_t (*store)(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count);
};
|
Where,
attr – the attribute representing the file to be created,show – the pointer to the function that will be called when the file is read in sysfs,store – the pointer to the function which will be called when the file is written in sysfs.
#include
#include
#include
#include
#include
#include
#include
#include //kmalloc()
#include //copy_to/from_user()
#include
#include
volatile int etx_value = 0;
dev_t dev = 0;
static struct class *dev_class;
static struct cdev etx_cdev;
struct kobject *kobj_ref;
static int __init etx_driver_init(void);
static void __exit etx_driver_exit(void);
/*************** Driver Fuctions **********************/
static int etx_open(struct inode *inode, struct file *file);
static int etx_release(struct inode *inode, struct file *file);
static ssize_t etx_read(struct file *filp,
char __user *buf, size_t len,loff_t * off);
static ssize_t etx_write(struct file *filp,
const char *buf, size_t len, loff_t * off);
/*************** Sysfs Fuctions **********************/
static ssize_t sysfs_show(struct kobject *kobj,
struct kobj_attribute *attr, char *buf);
static ssize_t sysfs_store(struct kobject *kobj,
struct kobj_attribute *attr, char *buf, size_t count);
struct kobj_attribute etx_attr = __ATTR(etx_value, 0660, sysfs_show, sysfs_store);
static struct file_operations fops =
{
.owner = THIS_MODULE,
.read = etx_read,
.write = etx_write,
.open = etx_open,
.release = etx_release,
};
static ssize_t sysfs_show(struct kobject *kobj,
struct kobj_attribute *attr, char *buf)
{
printk(KERN_INFO "Sysfs - Read!!!\n");
return sprintf(buf, "%d", etx_value);
}
static ssize_t sysfs_store(struct kobject *kobj,
struct kobj_attribute *attr, char *buf, size_t count)
{
printk(KERN_INFO "Sysfs - Write!!!\n");
sscanf(buf,"%d",&etx_value);
return count;
}
static int etx_open(struct inode *inode, struct file *file)
{
printk(KERN_INFO "Device File Opened...!!!\n");
return 0;
}
static int etx_release(struct inode *inode, struct file *file)
{
printk(KERN_INFO "Device File Closed...!!!\n");
return 0;
}
static ssize_t etx_read(struct file *filp,
char __user *buf, size_t len, loff_t *off)
{
printk(KERN_INFO "Read function\n");
return 0;
}
static ssize_t etx_write(struct file *filp,
const char __user *buf, size_t len, loff_t *off)
{
printk(KERN_INFO "Write Function\n");
return 0;
}
static int __init etx_driver_init(void)
{
/*Allocating Major number*/
if((alloc_chrdev_region(&dev, 0, 1, "etx_Dev")) <0){
printk(KERN_INFO "Cannot allocate major number\n");
return -1;
}
printk(KERN_INFO "Major = %d Minor = %d \n",MAJOR(dev), MINOR(dev));
/*Creating cdev structure*/
cdev_init(&etx_cdev,&fops);
etx_cdev.owner = THIS_MODULE;
etx_cdev.ops = &fops;
/*Adding character device to the system*/
if((cdev_add(&etx_cdev,dev,1)) < 0){
printk(KERN_INFO "Cannot add the device to the system\n");
goto r_class;
}
/*Creating struct class*/
if((dev_class = class_create(THIS_MODULE,"etx_class")) == NULL){
printk(KERN_INFO "Cannot create the struct class\n");
goto r_class;
}
/*Creating device*/
if((device_create(dev_class,NULL,dev,NULL,"etx_device")) == NULL){
printk(KERN_INFO "Cannot create the Device 1\n");
goto r_device;
}
/*Creating a directory in /sys/kernel/ */
kobj_ref = kobject_create_and_add("etx_sysfs",kernel_kobj);
/*Creating sysfs file for etx_value*/
if(sysfs_create_file(kobj_ref,&etx_attr.attr)){
printk(KERN_INFO"Cannot create sysfs file......\n");
goto r_sysfs;
}
printk(KERN_INFO "Device Driver Insert...Done!!!\n");
return 0;
r_sysfs:
kobject_put(kobj_ref);
sysfs_remove_file(kernel_kobj, &etx_attr.attr);
r_device:
class_destroy(dev_class);
r_class:
unregister_chrdev_region(dev,1);
cdev_del(&etx_cdev);
return -1;
}
void __exit etx_driver_exit(void)
{
kobject_put(kobj_ref);
sysfs_remove_file(kernel_kobj, &etx_attr.attr);
device_destroy(dev_class,dev);
class_destroy(dev_class);
cdev_del(&etx_cdev);
unregister_chrdev_region(dev, 1);
printk(KERN_INFO "Device Driver Remove...Done!!!\n");
}
module_init(etx_driver_init);
module_exit(etx_driver_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("EmbeTronicX " );
MODULE_DESCRIPTION("A simple device driver - SysFs");
MODULE_VERSION("1.8");
- Build the driver by using Makefile (
sudo make) - Load the driver using
sudo insmod driver.ko - Check the directory in /sys/kernel/ using
ls -l /sys/kernel
linux@embetronicx-VirtualBox: ls -l /sys/kernel/drwxr-xr-x 2 root root 0 Dec 17 14:11 boot_paramsdrwx------ 26 root root 0 Dec 17 12:19 debugdrwxr-xr-x 2 root root 0 Dec 17 16:29 etx_sysfsdrwxr-xr-x 2 root root 0 Dec 17 14:11 fscache-r--r--r-- 1 root root 4096 Dec 17 14:11 fscapsdrwxr-xr-x 2 root root 0 Dec 17 14:11 iommu_groups-r--r--r-- 1 root root 4096 Dec 17 14:11 kexec_crash_loaded-rw-r--r-- 1 root root 4096 Dec 17 14:11 kexec_crash_size-r--r--r-- 1 root root 4096 Dec 17 14:11 kexec_loadeddrwxr-xr-x 2 root root 0 Dec 17 14:11 livepatchdrwxr-xr-x 6 root root 0 Dec 17 14:11 mm-r--r--r-- 1 root root 516 Dec 17 14:11 notes-rw-r--r-- 1 root root 4096 Dec 17 14:11 profiling-rw-r--r-- 1 root root 4096 Dec 17 14:11 rcu_expediteddrwxr-xr-x 4 root root 0 Dec 17 12:19 securitydrwxr-xr-x 117 root root 0 Dec 17 12:19 slabdr-xr-xr-x 2 root root 0 Dec 17 14:11 tracing-rw-r--r-- 1 root root 4096 Dec 17 12:19 uevent_helper-r--r--r-- 1 root root 4096 Dec 17 12:19 uevent_seqnum-r--r--r-- 1 root root 4096 Dec 17 14:11 vmcoreinfo
- Now our sysfs entry is there under
/sys/kerneldirectory. - Now check sysfs file in etx_sysfs using
ls -l /sys/kernel/etx_sysfs
linux@embetronicx-VirtualBox: ls -l /sys/kernel/etx_sysfs-rw-rw---- 1 root root 4096 Dec 17 16:37 etx_value
- Our sysfs file also there. Now go under root permission using
sudo su. - Now read that file using
cat /sys/kernel/etx_sysfs/etx_value
linux@embetronicx-VirtualBox#cat /sys/kernel/etx_sysfs/etx_value0
- So Value is 0 (initial value is 0). Now modify using echo command.
linux@embetronicx-VirtualBox#echo 123 > /sys/kernel/etx_sysfs/etx_value
- Now again read that file using
cat /sys/kernel/etx_sysfs/etx_value
linux@embetronicx-VirtualBox#cat /sys/kernel/etx_sysfs/etx_value123
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