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I'm writing a kernel module. I can easily create a character device in /dev/ but I want to have 3 devices available to my user and it would be nice to put them all under a /dev/ subdirectory.

Possible example (exists on my openSUSE box):

# ls -l /dev/bsg
total 0
crw-rw---- 1 root root 252, 0 Jul 17 14:28 0:0:0:0
crw-rw---- 1 root root 252, 1 Jul 17 14:28 0:0:1:0
crw-rw---- 1 root root 252, 2 Jul 17 14:28 0:0:2:0
crw-rw---- 1 root root 252, 3 Jul 17 14:28 0:0:3:0
crw-rw---- 1 root root 252, 4 Jul 17 14:28 0:0:4:0
crw-rw---- 1 root root 252, 5 Jul 17 14:28 1:0:0:0

(I have looked at the bsg.c code but am at a loss to what "parent" the character devices are added to).

How can I do something similar?

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I found a way that works for me, one that doesn't use udev rules (I'd rather do it programmatically). This is easier than I thought earlier.

  1. alloc_chrdev_region for number of devices.
  2. Create classes for each device (each with different names)
  3. Set the classes' devnode field with your custom function ex: return kasprintf(GFP_KERNEL, "name/%d", global++);. This is where the naming happens. Udev should honor this function.
  4. Create cdevs with cdev_init && cdev_add and set their fops.
  5. Create devices with device_create (parent is NULL)

Note that there is no device hierarchy here, just independent devices which happen to be in the same /dev/ directory. The names specified in steps 2-5 will end up in /sys/.

  • There is no need to create a class for each device. Just use one main class and prefix the device's name given the devnode function. But I will give you a +1 for pointing me towards devnode. Also, when calling create_device or one of its extensions, you can specify the name as a path directly. – Twifty Aug 28 '19 at 7:15

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