udev / hotplug / hald / dbus

udev / hotplug / hald / dbus

[Lindsay Haisley]

I'd like to know how the udev / hotplug mechanism works with the hald / dbus
hardware abstraction/detection/notification system which is apparently
becoming the defacto standard in Gnome.  It looks as if there may be some
duplication of effort here, at some level.  I had to disable my auto device
mounts in /etc/dev.d after I recently updated gnome and had to install hald
and the dbus-daemon to get features in Gnome to work, such as auto-detection
of CD-RW disks.  It looks as if auto-detection and mounting of USB storage
devices is also supported and enabled by hald / dbus, at least in Gnome.

I'm certainly no expert in these matters, and the answer may be obvioius to
anyone with more experience at a hardware level, but it looks as if there
may be some functional overlap and possibly potential for conflict with
these two systems.

-- 
Lindsay Haisley       | "Fighting against human |     PGP public key
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                      |      (Pamela Jones)     |


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Re: udev / hotplug / hald / dbus

[Kay Sievers]

On Thu, 2004-12-09 at 11:15 -0600, Lindsay Haisley wrote:
> I'd like to know how the udev / hotplug mechanism works with the hald / dbus
> hardware abstraction/detection/notification system which is apparently
> becoming the defacto standard in Gnome.  It looks as if there may be some
> duplication of effort here, at some level.  I had to disable my auto device
> mounts in /etc/dev.d after I recently updated gnome and had to install hald
> and the dbus-daemon to get features in Gnome to work, such as auto-detection
> of CD-RW disks.  It looks as if auto-detection and mounting of USB storage
> devices is also supported and enabled by hald / dbus, at least in Gnome.
> 
> I'm certainly no expert in these matters, and the answer may be obvioius to
> anyone with more experience at a hardware level, but it looks as if there
> may be some functional overlap and possibly potential for conflict with
> these two systems.

The kernel creates/removes devices. The information about that is
exposed in sysfs. Changes in sysfs are notified from the kernel to
userspace by executing /sbin/hotplug.

udev is a devfs implementation. It uses the events and sysfs to maintain
the /dev directory. On node creation/removal the dev.d/ scripts are
called.

HAL also listens to the hotplug events and gets the name of the created
node from udev. HAL is a hardware abstraction that keeps it's own device
list inside a daemon, based on the events hotplug sends.
HAL's string based interface is easy to use from any application, cause
the D-BUS IPC can inform any application interested in device changes.
An application can subscribe to certain device events or a whole class
of device events.

One example of a HAL subscriber is the gnome-volume-manager, which can
mount your devices automatically. I don't see any overlap, besides your
own dev.d/ script. :)

Kay



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