Hello,

I am a newbie to the Linux kernel development and have a question regarding major number assignment.

I came across some information that in 2.6.x kernels , during boot up , the kernel creates the sysfs file hierarchy for every device that it finds attached. It then sends hotplug events for each of these new devices to udev which then reads the sysfs filesystem to create corresponding device nodes in /dev.

However , I also noticed that there are drivers which still use a static major number (defined in linux/major.h) e.g: in a call to , the input driver uses INPUT_MAJOR ( ).

So , if the driver registers its own major number whereas udev creates a dev node with another major number , how is a driver associated with a particular device node ?

Read this section from the Linux Device Driver book version 3, LDD3 for short.

http://www.makelinux.net/ldd3/chp-3-sect-2.shtml

This describes, dynamic and static allocation of major numbers.

Should you have questions afterwards don't hasitate to ask.

Best regards,

Thanks for pointing me to that link !

Ok....So I understood from the script how dynamic allocation of major numbers takes place and how a device node is created based on this newly allocated major number.

I also read somewhere(probably in udev docs) that the sysfs information is created by the drivers in 2 ways :

1) If the driver is statically built into the kernel image , then the information is available immediately at boot time. This enables udevd to see the sysfs event immediately and thus create the device node.

2) If the driver is a module , then the information in sysfs is visible only when the driver module is loaded. Thus , udevd will react accordingly only when the driver is loaded.

So , my next questions are :
1) If a driver is responsible for getting a major number as well as creating the device node , then how does a device get associated with a particular device node ?

2) If I connect a device to my PC , after it has booted into linux , then does the kernel automatically load/register the driver (if it is a module) and create it's device node ?


Thanks !

There is a daemon called hotplug that is responsible for responding to "USB device inserted" events. When properly configured, it will handle the task of recognizing that the device has been installed, figuring out what device it is, and loading the appropriate drivers for it.

Distros are normally set up these days to do all of this correctly.

thanks sundialsvcs........
I assume this would not be true for the non-usb devices then I guess ?
I mean if a non-usb device is connected to the PC after it has booted up , would I need to load it's driver manullay in order to have it's device node created ?
I mean how does one particular device get associated with a device node ?
I can understand that the association of a driver with a device node takes place by way of the major number.