Do I want to use udev?

Do I want to use udev?

[Brian Sammon]

I'm trying to figure out if I should be using udev, and if so, how.
I'm upgrading to a recent 2.6 kernel, and various documentation says that udev 
is required, but I'm not sure.

I'm not interested in having a dynamic /dev filesystem (for the moment); a 
static /dev filesystem has served me just fine up to this point, and I think 
it will continue to work for a little while longer.

So that leaves me with the other hotplug-related functionality, automatic 
driver loading.  I can't figure out if, or how, udev does that.  In 
particular, 99% of the hotplugging I do is USB devices, and I can't find any 
file in my (Debian) udev package that configures which USB drivers get loaded 
for which USB devices.

This leads me to wonder-- can I disable/uninstall udev altogether, or just 
disable the dynamic /dev filesystem and still have automatic USB driver 
loading?

If part/all of my problem is debian specific, I'd still appreciate general 
information, such as how it should be done, or information about how this 
works on your system.




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Re: Do I want to use udev?

[Greg KH]

On Wed, Jan 04, 2006 at 03:47:06PM -0500, Brian Sammon wrote:
> I'm trying to figure out if I should be using udev, and if so, how.
> I'm upgrading to a recent 2.6 kernel, and various documentation says that udev 
> is required, but I'm not sure.
> 
> I'm not interested in having a dynamic /dev filesystem (for the moment); a 
> static /dev filesystem has served me just fine up to this point, and I think 
> it will continue to work for a little while longer.
> 
> So that leaves me with the other hotplug-related functionality, automatic 
> driver loading.  I can't figure out if, or how, udev does that.

The single rule:
	ENV{MODALIAS}="?*",    RUN+="/sbin/modprobe $env{MODALIAS}"
will cause udev do that.

> In particular, 99% of the hotplugging I do is USB devices, and I can't
> find any file in my (Debian) udev package that configures which USB
> drivers get loaded for which USB devices.

That's the "magic" of the kernel driver providing a
MODULE_DEVICE_TABLE() logic with the modprobe program matching up module
aliases.  See my very old OLS 2001 paper on the basic ideas behind the
whole thing (but it's simpler now, as modprobe does the matching for us,
we don't have to scan the alias tables anymore by hand.)

> This leads me to wonder-- can I disable/uninstall udev altogether, or just 
> disable the dynamic /dev filesystem and still have automatic USB driver 
> loading?

You can, if you only use the one rule above :)

But if that's all that you want to have happen, stick with the old
hotplug package, it still works just fine.  Why would you want to use
udev if you don't care about persistant names and other good stuff it
provides?

> If part/all of my problem is debian specific, I'd still appreciate general 
> information, such as how it should be done, or information about how this 
> works on your system.

I suggest you ask debian udev specific questions on a debian specific
mailing list, you might have better luck that way :)

Hope this helps,

greg k-h


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Re: Do I want to use udev?

[Brian Sammon]

> But if that's all that you want to have happen, stick with the old
> hotplug package, it still works just fine.  Why would you want to use
> udev if you don't care about persistant names and other good stuff it
> provides?

That's a good question.  Debian says that recent 2.6 kernel packages "require" 
udev.  Based on that, and the fact that recent kernels have a udev line in the 
"Current Minimal Requirements" section of Documentation/Changes,
I (wrongly, it seems) inferred that something had changed in recent 
(kernel.org) kernel sources that made them incompatible with hotplug.

Since I often compile my own kernel, I suppose I don't have to obey debian 
kernel policies too closely.

Considering my lack of interest in dynamic /dev filesystem support, which of 
  hotplug
  hotplug-ng
  udev
would people here recommend I use?

I notice that there hasn't been a new hotplug release in over a year.  Is it 
unmaintained, or has it simply not needed updating?




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Re: Do I want to use udev?

[Aras Vaichas]

> Considering my lack of interest in dynamic /dev filesystem support, which of 
>   hotplug
>   hotplug-ng
>   udev
> would people here recommend I use?

udev because it has up-to-date support, keeps my /dev minimal and clean, is 
simple to install, easy to modify rules, does all my old hotplug and coldplug, 
and is fast.

regards,

Aras Vaichas


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Re: Do I want to use udev?

[Greg KH]

On Wed, Jan 04, 2006 at 05:56:37PM -0500, Brian Sammon wrote:
> > But if that's all that you want to have happen, stick with the old
> > hotplug package, it still works just fine.  Why would you want to use
> > udev if you don't care about persistant names and other good stuff it
> > provides?
> 
> That's a good question.  Debian says that recent 2.6 kernel packages "require" 
> udev.

That's a Debian issue, 2.6 kernels do not require this.

> Based on that, and the fact that recent kernels have a udev line in
> the "Current Minimal Requirements" section of Documentation/Changes, I
> (wrongly, it seems) inferred that something had changed in recent
> (kernel.org) kernel sources that made them incompatible with hotplug.

No, not at all, that's there to show the version of udev that you need
to use, if you want to use it.  Older versions will not work with that
specific kernel version.

> Since I often compile my own kernel, I suppose I don't have to obey
> debian kernel policies too closely.

Heh, if at all :)

> Considering my lack of interest in dynamic /dev filesystem support, which of 
>   hotplug
>   hotplug-ng
>   udev
> would people here recommend I use?

Just plain hotplug will probably work for you, if all you want is the
automatic module loading.

hotplug-ng is a dead project, it was replaced with a single udev rule
(that one previously mentioned), so don't use it.

> I notice that there hasn't been a new hotplug release in over a year.  Is it 
> unmaintained, or has it simply not needed updating?

Pretty much both.  I have a bunch of patches floating around that the
last release needs, but as distros are moving away from it, I'm loath to
do a new release.

But maybe I need to flush out the pending stuff, and mark it
"depreciated"...

thanks,

greg k-h


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