Re: udev and hotplug

[Kay Sievers <kay.sievers@vrfy.org>]

On Sun, Mar 19, 2006 at 09:12:14PM +0100, Andreas Jellinghaus wrote:
> as far as I know the current recommendation is to
> disable /proc/sys/kernel/hotplug and let udevd
> handle everything via user space events. right?

Yes, especially if you want to be able to handle a large number of
devices (1000's) which does not work with /sbin/hotplug, cause you
run into OOM situations by the kernel initiated fork bomb /sbin/hotplug.
But there is no hard requirement on this, you can still use it, even
when it does not make sense anymore.

> and is the hotplug package still needed, required, used?
> or does udev and hotplug even conflict?

The hotplug package's stuff can be called from udev if asked for, but it
is pretty useless in most cases cause udev rules and kernel provided
$MODALIAS passed to modprobe and the "uevent" trigger in sysfs can replace
it entirely.

> here are some sources that might need an update:
> 
> udev README:
>   - From kernel version 2.6.15 on, the hotplug helper /sbin/hotplug should
>     be disabled with an init script before actions like loading kernel
>     modules are taken, which may cause a lot of events.
> 
> http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html

This is recommendend, yes.

> Requirements
>  It requires a 2.6 Linux kernel with CONFIG_HOTPLUG enabled to run. It is
>   recommended that you also have the Linux Hotplug scripts installed, but it
>   is not necessary for it to work properly.
> 
> http://www.gentoo.org/doc/en/udev-guide.xml

I can't speak for Gentoo, but I can't see where the scripts would be useful
when a recent kernel is used. Older kernels will need it, sure.

>   udev
>   Every time the kernel notices an update in the device structure, it calls 
>   the /sbin/hotplug program. Hotplug runs the applications linked in the
>   /etc/hotplug.d/default directory where you will also find a symlink to the
>   udev application. Hotplug directs the information given by the kernel to
>    the udev application which performs the necessary actions on the /dev
>    structure (creating or deleting device files).

Most of the recent distro releases don't even have /etc/hotplug.d/. You
can still plug it into the udev event handling with a special program
provided by the udev package, but better get rid of that stuff entirely.

> http://fedora.redhat.com/docs/udev/
>   udev gets called by hotplug, if a module is loaded, and a device is added
>   or removed. udev looks in /sys/, if the driver provides a "dev" file,
>   which contains the major and minor number for a device node to communicate
>   with the driver. After looking in the udev rules (in
>   the /etc/udev/rules.d/ directory), which specify the device node filename
>    and symlinks, a device node is created in /dev/ with the permissions,
>    which are specified in /etc/udev/permissions.d/.

That is almost correct besides that the udev daemon connecs directly to the
kernel and there is no "hotplug" which could "call" anymore these days, and
udevd relies on the event environment and not the "dev" file to get the
major/minor for the node.

> I use none of those distributions, but maybe these people are on the list?

Kay


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