devtmpfs question

[wcg]

What does devtmpfs give us that tmpfs does not?

(I see that it is required by udev beyond a certain version
and that the kernel has support for it, but why would
not the tmpfs that already existed before devtmpfs
was introduced in the kernel work for the same
purpose, an in-memory filesystem to store user
accessible device nodes?)

edit:
(I guess should have posted this question in Kernel and Hardware.
I do not know how to move it there, feel free to do so if you do.)
_________________
TIA

[fpemud]

devtmpfs = tmpfs + kernel puts standard dev files in it

Before devtmpfs, udev creates all the dev files in /dev which is a pure tmpfs.
But it's slow, because udev must iterate /sys to do it.

Since dev files have standard names, like /dev/hda /dev/sr0, we let kernel create them. It's much faster.
udev only creates symlinks which act as dev file alias, like /dev/cdrom.
udev can be removed if the system doesn't need any alias.

[Gusar]

It's very simple - devtmpfs doesn't need udev, it's standalone. This is great for rescue scenarios (when you need init=/bin/sh), or embedded systems. There's also a speed consideration, you know, the eternal quest for faster boot times. Having the kernel create device nodes is faster than creating them with udev.

devtmpfs creates device nodes, then udev works on top of that, changing permissions and creating symlinks.