Linux: How to Know the Module That Exports a Device Node

Linux: how do i know the module that exports a device node?

Short answer :

cd /sys/dev/char/major:minor/device/driver/
ls -al | grep module

Each device is generally associated with a driver, and this is all what the "device model" is about. The sysfs filesystem contains a representation of this devices and their associated driver. Unfortuantely, it seems not all sysfs have a representation of the device nodes, so this applyd only if your /sys directory contains a /dev directory.
Let's take an example, with /dev/video0

On my board, ls -al /dev/video0 output is

crw-------    1 root     root      81,   0 Jan  1 00:00 video0

So major number is 81 and minor number is 0.
Let's dive into sysfs :

# cd /sys
# ls
block     class     devices   fs        module
bus       dev       firmware  kernel

The sys/dev directory contains entry for the char and block devices of the system :

# cd dev
# cd char
# ls
10:61  13:64  1:3    1:8    249:0  252:0  29:0   4:65   81:0   89:1
10:62  1:1    1:5    1:9    250:0  253:0  29:1   5:0    81:2
10:63  1:11   1:7    248:0  251:0  254:0  4:64   5:1    81:3

What the hell are this links with strange names ?
Remember the major and minor number, 81 and 0 ?
Let's follow this link :

#cd major:minor (ie 81:0)
#ls -al
drwxr-xr-x    2 root     root            0 Jan  1 01:56 .
drwxr-xr-x    3 root     root            0 Jan  1 01:56 ..
-r--r--r--    1 root     root         4096 Jan  1 01:56 dev
lrwxrwxrwx    1 root     root            0 Jan  1 01:56 device -> ../../../vpfe-capture
-r--r--r--    1 root     root         4096 Jan  1 01:56 index
-r--r--r--    1 root     root         4096 Jan  1 01:56 name
lrwxrwxrwx    1 root     root            0 Jan  1 01:56 subsystem -> ../../../../../class/video4linux
-rw-r--r--    1 root     root         4096 Jan  1 01:56 uevent

Now we can see that this device nod, which is how the device is presented to userspace, is associated with a kernel device. This association is made through a link. If we follow this link, we end up in a directory, with a driver link. The name of the driver is usually the name of the module :

# ls -al
drwxr-xr-x    3 root     root            0 Jan  1 01:56 .
drwxr-xr-x   25 root     root            0 Jan  1 00:00 ..
lrwxrwxrwx    1 root     root            0 Jan  1 01:56 driver -> ../../../bus/platform/drivers/vpfe-capture
-r--r--r--    1 root     root         4096 Jan  1 01:56 modalias
lrwxrwxrwx    1 root     root            0 Jan  1 01:56 subsystem -> ../../../bus/platform
-rw-r--r--    1 root     root         4096 Jan  1 01:56 uevent
drwxr-xr-x    3 root     root            0 Jan  1 01:56 video4linux

So here the name of the module is vpfe_capture

How to create a device node from the init_module code of a Linux kernel module?

To have more control over the device numbers and the device creation you could do the following steps (instead of register_chrdev()):

1. Call alloc_chrdev_region() to get a major number and a range of minor numbers to work with.
2. Create device class for your devices with class_create().
3. For each device, call cdev_init() and cdev_add() to add the character device to the system.
4. For each device, call device_create(). As a result, among other things, Udev will create device nodes for your devices. No need for mknod or the like. device_create() also allows you to control the names of the devices.

There are probably many examples of this on the Net, one of them is here.

How to call exported kernel module functions from another module?

From my research, it seems that those are the only three ways to handle this situation, and I've gotten each of them to work, so I think I'll just pick my favorite out of those.

Creation of pseudo device-node under /dev

From your question I'm guessing your talking about Linux (since you are talking about kernel modules). In that case I'd strongly recommend reading Linux Device Driver. I'd recommend looking at chapter 14 to understand better how device work.

It should also be noted that in most current desktop and server distribution of Linux, udev is responsible for creating entries in /dev. You can configure udev with rules that allow you to create your the device node with a specific name and location. In the embedded world it might be mdev with busybox that's responsible for populating /dev or even it could simply be the deprecated devfs.

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