Thanks.

I also found it @ http://lwn.net/Articles/330985/
On Wed, Mar 28, 2012 at 4:53 AM, Greg KH [off-list ref] wrote:

On Tue, Mar 27, 2012 at 02:39:37PM +0800, Li Haifeng wrote:

quoted

As you know, the node under /dev are managed by udevd daemon.

Not always, usually the node is created by devtmpfs on most modern
distros.

quoted

But to the built-in driver module, when the module is probed in kernel
space, udevd daemon doesn't running. So, to ttyS node, when the
built-in driver module was probed, when was the /dev/ttyS  created?

devtmpfs does it.

Also, look at your distro startup code, it sets things up properly when
userspace starts up and has udev go through things and set up the nodes
then.

good luck,

greg k-h