Kernel module export many device attributes to userspace - linux-kernel

Question

I'm working with a kernel driver for an I2C device and up until now I've been making simple attributes available using the sysfs DEVICE_ATTR helper. Now I need to make a long list of attributes available like /sys/bus/i2c/device/.../param0, .../param1, etc. but it seems inefficient to write a function for each one and maybe even the wrong use of the sysfs system. For example:
static DEVICE_ATTR(param0, S_IRUGO, NULL, foo_set_param0);
static DEVICE_ATTR(param1, S_IRUGO, NULL, foo_set_param1);
...
static DEVICE_ATTR(param50, S_IRUGO, NULL, foo_set_param50);
The values on the device change frequently and reading them is expensive so constantly reading them or using one function to read all of them is not really any option. I'm a bit of C newbie so maybe there is something totally obvious I'm missing, but can you use a wrapper on the sysfs show callback to take a parameter? Or is there a better system I should use for this? I looked at debugfs and it seems like I'd need to maintain the values in memory for it.

Answer 1

You can try container_of() macro. Simply fill your attribute data in a larger structure.
Here is an example for the creation of 100 attributes in a big structure big_kattr. The parameter is UNIT_NUM.
#include <linux/init.h>
#include <linux/module.h>
#include <linux/slab.h>
#define UNIT_NUM 100
typedef struct {
struct kobj_attribute k_obj;
int num;
} big_kattr;
static struct kobject *register_kobj;
// rw functions
static ssize_t __used store_value(struct kobject *kp, struct kobj_attribute *attr, const char *buf, size_t count){
big_kattr *a = container_of(attr, big_kattr, k_obj);
sscanf(buf, "%du", &a->num);
return count;
}
static ssize_t show_value(struct kobject *kp, struct kobj_attribute *attr, char *buf) {
big_kattr *a = container_of(attr, big_kattr, k_obj);
return sprintf(buf, "%d\n", a->num);
}
// put attribute to attribute group
static struct attribute * unit_attrs[UNIT_NUM + 1];
static big_kattr full_unit_attrs[UNIT_NUM];
static struct attribute_group unit_attr_group;
static int hello_init(void){
int i;
memset(full_unit_attrs, 0, sizeof(full_unit_attrs));
memset(unit_attrs, 0, sizeof(unit_attrs));
memset(&unit_attr_group, 0, sizeof(unit_attr_group));
for(i=0; i<UNIT_NUM; i++){
char * str = kmalloc(32, GFP_KERNEL);
sprintf(str, "unit-%03d",i);
full_unit_attrs[i].k_obj.attr.name = str;
full_unit_attrs[i].k_obj.attr.mode = S_IWUSR | S_IRUGO;
full_unit_attrs[i].k_obj.show = show_value;
full_unit_attrs[i].k_obj.store = store_value;
full_unit_attrs[i].num = i;
unit_attrs[i] = &(full_unit_attrs[i].k_obj.attr);
}
unit_attr_group.attrs = unit_attrs;
// create sysfs object ( /sys/kernel/many directory )
register_kobj = kobject_create_and_add("many", kernel_kobj);
if (!register_kobj)
return -ENOMEM;
//create all attributes (files)
if(sysfs_create_group(register_kobj, &unit_attr_group)){
kobject_put(register_kobj);
return -ENOMEM;
}
return 0;
}
static void hello_exit(void){
int i;
kobject_put(register_kobj);
for(i=0; i<UNIT_NUM; i++)
kfree(full_unit_attrs[i].k_obj.attr.name);
}
MODULE_LICENSE("Dual BSD/GPL");
module_init(hello_init);
module_exit(hello_exit);
Example:
cat /sys/kernel/many/unit-077
echo 12345 > /sys/kernel/many/unit-088
cat /sys/kernel/many/unit-088