kernel_module: move character device example doc to README

README.adoc

@@ -2875,6 +2875,83 @@ Those commits change `BR2_LINUX_KERNEL_LATEST_VERSION` in `/linux/Config.in`.
 
 You should then look up if there is a branch that supports that kernel. Staying on branches is a good idea as they will get backports, in particular ones that fix the build as newer host versions come out.
 
+=== Pseudo filesystems
+
+==== Character device
+
+....
+/character_device.sh
+echo $?
+....
+
+Outcome: the test passes:
+
+....
+0
+....
+
+Sources:
+
+* link:rootfs_overlay/character_device.sh[]
+* link:rootfs_overlay/mknoddev.sh[]
+* link:kernel_module/character_device.c[]
+
+Charcter device files are created with:
+
+....
+mknod </dev/path_to_dev> c <major> <minor>
+....
+
+Intuitively, for physical devices like keyboards, the major number maps to which driver, and the minor number maps to which device it is.
+
+A single driver can drive multiple compatible devices.
+
+The major and minor numbers can be observed with:
+
+....
+ls -l /dev/urandom
+....
+
+Output:
+
+....
+crw-rw-rw-    1 root     root        1,   9 Jun 29 05:45 /dev/urandom
+....
+
+which means:
+
+* `c` (first letter): this is a character device. Would be `b` for a block device.
+* `1,   9`: the major number is `1`, and the minor `9`
+
+To avoid device number conflicts when registring the driver we:
+
+* ask the kernel to allocate a free major number for us with: `register_chrdev(0`
+* find ouf which number was assigned by grepping `/proc/devices` for the kernel module name
+
+Bibliography: https://unix.stackexchange.com/questions/37829/understanding-character-device-or-character-special-files/371758#371758
+
+===== Automatically create character device file on insmod
+
+And also destroy it on `rmmod`:
+
+....
+/character_device_create.sh
+echo $?
+....
+
+Outcome: the test passes:
+
+....
+0
+....
+
+Sources:
+
+* link:kernel_module/character_device_create.c[]
+* link:rootfs_overlay/character_device_create.sh[]
+
+Bibliography: https://stackoverflow.com/questions/5970595/how-to-create-a-device-node-from-the-init-module-code-of-a-linux-kernel-module/45531867#45531867
+
 === Kernel panic and oops
 
 To test out kernel panics and oops in controlled circumstances, try out the modules: