Linux - list of registered devices

Question

I have a linux driver that registers a char device for every compatible device-node in the Flattened-Device-Tree(FDT).

On initialization, the driver allocates a major device number and a range of minor device numbers. when i look at /proc/devices, this major device number is listed.

Now, when I define 2 device-nodes, compatible with that driver, in the FDT, the driver's platform-probe-function get's called twice, and 2 char devices are registered under the same major device number, but with ascending minor device numbers.

Since I can lookup the major number, and I do know how many devices I have and that the minor numbers start at 0, I can just type mknod -m 666 /dev/mydevice1 c 246 0 and mknod -m 666 /dev/mydevice2 c 246 1 to create device nodes for the two char devices.

But I am wondering if there is a way I can lookup all the devices known to the system with their major and minor device numbers, so I do not have to know the minor number beforehand, in order to be able to create device nodes for those devices?

skrrgwasme skrrgwasme · Accepted Answer · 2014-07-21T19:47:36

It sounds like you're asking how to have the system automatically create your device nodes for you, rather than having to use the mknod command.

Try these steps:

1) A bit of initialization

#include <linux/devices.h>
#define DEVNAME "device_name" /* you will need this a few times,
                                 so make it a macro */

static dev_t my_devt; /* make this static so we can access it
                         across function calls */
static struct class* my_class; /* pointer to device class */

2) In your __init routine after cdev_add(), initialize the class and register the device with sysfs:

my_class = class_create(THIS_MODULE, DEVNAME);

if(IS_ERR(device_create(my_class, NULL, my_devt, NULL, DEVNAME))){
    printk(KERN_ERR "Node creation for %s failed.", DEVNAME);
    /* clean up after failed initialization */
}

3) Add the corresponding clean up function calls to __exit():

device_destroy(my_class, my_devt);
class_destroy(my_class);

This should result in a node being created in /dev for a single device. Your question specifically asks about multiple nodes, so you can create an array of nodes instead, and duplicate the device_create() and device_destroy() function calls for each of the individual nodes you have.

I know that your original question was asking for a way to enumerate all devices known to the system, but you also said that your driver is already registering a char device for every compatible device in the FDT. If you add this code to your existing driver, you can take advantage of the dev_t information you already have during char device registration to create the nodes and eliminate the need to enumerate or list anything later.

For more details, check out this tutorial: Character Device Files. The kernel.org docs for device_create() may also be helpful to you.

Linux - list of registered devices

2 Answers