Re: [PLUG] cdrom mounting

[Will Dyson <will_dyson@pobox.com>]

Arthur S. Alexion wrote:

> Wait, Will.  I'm new at this.  What do you mean do a modprobe?

Modprobe is a command-line program that finds linux kernel modules (device 
drivers) by name and loads them into your kernel.

In the most basic usage, it takes one argment (the name of the module to 
load). So in order to load the ide-cd module, you would run this command 
as root:

	modprobe ide-cd

And if the ide-cd module is available (under 
/lib/modules/<kernel-name>/kernel/drivers/), it will be loaded and its 
functionality made available to the rest of the system. To check what 
modules are currently loaded into your kernel, use the command 'lsmod' 
(which does not need root privalge).

In order to make sure that the ide-cd module is automaticly loaded for you 
on every boot, you should add a line consisting soley of:

ide-cd

to your /etc/modules file.


> what is devfs?  If it solves my problem, how do I use it?
>

Bit of an advanced topic, really. I wouldn't actually recommend setting it 
up until you feel really comfortable with linux (and feel confident that 
you could repair your system by booting from a rescue disk if you screwed 
it up). But thats ok. I'll tell you about it anyway.

In the "traditional" way of doing things, device files (the special files 
that represent your various peices of hardware, eg: /dev/hda and /dev/dsp) 
are actuall files that reside on your disk in /dev, with no data and with 
a flag set on them so that the system knows they are not regular files.

In addition to this "I'm a device file" flag, the device files have 
information telling the system what device they represent (which device 
driver should be use used to handle reads and writes performed on the 
file). This information takes the form of the Major and Minor device 
numbers. The Major number actually tells the kernel what device driver to 
use, while the Minor number distingishes between multiple devices that use 
the same driver (such as 2 hard drives, or /dev/dsp and /dev/mixer).

This has worked well for a long time. However, as linux has grown, the 
traditional /dev has started to show its age. One problem is that 
distributions must put device files in /dev for every possible device that 
linux can support (unless they want their users to wrestle with creating 
their own device files when they install hardware, ugh). As linux has 
grown as an OS, this has lead directly to /dev having 400+ files in it, 
most of them representing devices the user does not actually have.

Devfs is a different way of handling device files. Rather than having 
actuall files on disk, devfs is a virtual filesystem (mounted on /dev) 
that only exists in ram. Rather than having device files for every 
possible device, devfs creates device files dynamicly, when the device 
driver that needs them is loaded into the kernel and finds its hardware.

To go along with the devfs filesystem in the kernel, there is a daemon 
program called devfsd that helps manage /dev. Devfs gives the device files 
different, more verbose names (such as 
/dev/ide/host0/bus0/target0/lun0/part1 rather than /dev/hda1) than they 
had under the old system. Devfsd moniters /dev and when new device files 
are created by the loading of a driver, creates symlinks from the old 
names to the new names, so as old programs don't break.

Now I can finally answer your question about how this can help in your 
situation.

As long as we have this devfsd program cooperating with the devfs 
filesystem, we might as well use it for more than just creating symlinks. 
One cool thing we can do is to have the filesystem automaticly load the 
device driver that coresponds to a given device file when you try to 
access that device file.

For instance, say your cd-rom driver is not loaded and therefor 
/dev/cdroms/cdrom0 does not exist. When you try to access that noexistant 
file, rather than just give up and return an error, the filesystem tells 
the daemon what file you tried to access. The daemon then looks up that 
filename in a list, which (hopfully) tells the daemon what driver provides 
that device file. The daemon then loads the driver (which creates the 
device file) and then the filesystem lets you access it, just as if it had 
been there all along.

Thus, no more confusion about what devices are available or what driver 
you must load to make a device file go "live". Many other operating 
systems (freebsd, for example) have done it this way for a long time. The 
most recent release of Mandrake linux uses devfs by default. There is a 
lot of inertia behind using regular device files (and a history of bugs in 
devfs), but I expect that more and more distributions will start to use 
devfs as time goes by.

> Thanks for the help and patience.

You're welcome. And welcome to linux.

--
Will Dyson
"Back off man, I'm a scientist!" -Dr. Peter Venkman


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