In Linux, how do you use device_create within an existing class?

Question

Note: I'm listing this problem as it is today, I'm not opposed to changing the implementation (moving the creation of the class to a common area for example) if it makes things easier... I'm just not sure how to do it. :End Note

I've got two linux kernel modules and I'm trying to update the /sys entries for them. Searching around on google and other sources, I've seen lots of code along the lines of:

static dev_t MyDev;
static struct class *c1;

static int __init start_func(void)
{
    ...
    MyDev = MKDEV(nMajor, MINOR_VERSION);
    register_chrdev_region(MyDev, 1, MODULE_NAME);
    c1 = class_create(THIS_MODULE, "chardrv");
    device_create(c1, NULL, MyDev, NULL, MODULE_NAME);
    ....

And I've verified for my first module this code works, and that it correctly creates a:

/sys/class/chardrv/<MODULE_NAME>

entry. What I'd like to know is how do you create a device in an existing class. In other words, one of my modules created this new chardrv class, now I want my other module to be able to also register its devices under the same class.

I can't call class_create() again (in the second module), because that "chardrv" class already exists...

So I can run a check to see if /sys/class/chardrv exists, and this can help me decide if I need to call class_create() or not, that's not a problem. Lets put some pseudo code in here to clarify:

if ( path "/sys/class/chardrv" does not exist)
    new_class = class_create("chardrv")
else
    new_class = some how get class "chardrv" handle, or properties, or whatever
device_create(new_class, ...)

So as per this example, if my class already exists, and I just want to add my new device into it from a second module I assume I need to create a class structure and somehow populate it with the correct "chardrv class" attributes then call device_create as before, but I'm not sure how to do that.

Answer 1

To use the device_create function with the same class, just pass it a pointer to the same class.

Since you want to call device_create in a different module than the one in which you create the class, you'll need to export the symbol for the pointer to the class. You can use the EXPORT_SYMBOL macro to do this.

---

For example:

module1.c:

extern struct class *c1;    /* declare as extern */
EXPORT_SYMBOL(c1);          /* use EXPORT_SYMBOL to export c1 */

static dev_t mod1_dev;
static int __init start_func(void)
{
        ...
        /* define class here */
        c1 = class_create(THIS_MODULE, "chardrv");

        /* create first device */
        device_create(c1, NULL, mod1_dev, NULL, "mod1_dev");
        ....
}

module2.c

extern struct class *c1;    /* declare as extern */

static dev_t mod2_dev;
static int __init start_func(void)
{
        ...
        /* c1 is defined in module 1 */

        /* create second device */
        device_create(c1, NULL, mod2_dev, NULL, "mod2_dev");
        ....
}

Note: You'll need to insert module1 before module2 since the class pointer is defined and exported in module1.

That should create the directories you are expecting:

• /sys/class/chardrv/mod1_dev
• /sys/class/chardrv/mod2_dev

---

By the way, if you are getting an Invalid parameters error when you try to load the second module, you might have to add a KBUILD_EXTRA_SYMBOLS line to your Makefile.