linux-kernel - Re: Basic rundown of udev?

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Message-ID: <ac3eb2510904161407v242ada41tcd079e5a639a638d@mail.gmail.com>
Date:	Thu, 16 Apr 2009 23:07:40 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	John Moser <john.r.moser@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Basic rundown of udev?

On Thu, Apr 16, 2009 at 22:39, John Moser <john.r.moser@...il.com> wrote:
> I'm currently reading through umm... udevd.c for udev-141.  Trying to
> figure out how this thing works.

Udev stuff is usually handled at linux-hotplug@...r.kernel.org.

> Anyone care to give me a brief map?  It would help.  I'm just trying
> to figure out how udev talks to the kernel

Udev just listens to broadcasted events from the kernel.

Just run:
  udevadm monitor --kernel --env
and
  strace udevadm monitor --kernel
and you see what's going on.

> -- i.e. will what facilities
> would I have to implement to re-implement the kernel side of udev,
> let's say if I was to run udevd as-is on Darwin or NetBSD or
> something.

Udev uses the Linux kernel device tree exported by sysfs. It offers
simplified access to the internal device structures in the kernel. Not
sure what other platforms do here.

> How does it talk to the kernel?  And if you know off the
> top of your head, which source files (and functions?) am I looking at
> here?

The kernel sends the needed events from: lib/kobject_uevent.c, but
that does not include the driver core, which is in drivers/base/*, and
the exported device tree filesystem at fs/sysfs/*.

Good luck,
Kay
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