Re: Help wanted with parallel port rule

[Barrie Stott <G.B.Stott@bolton.ac.uk>]

Many thanks, Kay, for the reply.

On Thu, Nov 29, 2007 at 12:09:18PM +0100, Kay Sievers wrote:
> On Nov 29, 2007 9:48 AM, Barrie Stott <G.B.Stott@bolton.ac.uk> wrote:
> > I have a Laserjet 4l printer on my parallel port and am having trouble
> > creating a udev rule that will deal with it. Below I give my reasons
> > for what I have done in the hope that someone can tell me where I am
> > going wrong. My main source of information is `Writing udev rules' by
> > Daniel Drake and am using a GNU/Linux system running Debian Lenny.
> >
> > 1. I knew that I had modules parport and parport_pc by looking as the
> > output from lsmod so I ran:
> >
> >         ls -lR /sys | grep 'parport:\|parport_pc:'
> >
> > The complicated grep parameter merely restricts the output to desired
> > directories and gave as result:
> >
> > /sys/bus/pci/drivers/parport_pc:
> > /sys/bus/platform/drivers/parport_pc:
> > /sys/bus/pnp/drivers/parport_pc:
> > /sys/module/parport:
> > /sys/module/parport_pc:
> >
> > 2. I chose the smallest of these as simplest to type and ran:
> >
> >         udevinfo -a -p /sys/module/parport
> >
> > `Writing udev rules' says that there should be no initial `/sys' but I
> > get the same result with and without it. The essential result from
> > running udevinfo was:
> >
> >   looking at device '/module/parport':
> >     KERNEL="parport"
> >     SUBSYSTEM="module"
> >     DRIVER=""
> >     ATTR{refcnt}="1"
> >     ATTR{initstate}="live"
> 
> This is just the kernel module, not a device you can access from userspace.

I've reread the Writing Udev Rules document and you're right. His
emphasis like yours is on `device' and he searches sysfs only for
directories containing a dev file. Unfortunately, I took that to be an
example rather than a rule.

> > 3. I created the following udev rule from this:
> >
> > KERNEL="parport", SUBSYSTEM="module", DRIVER="", ATTR{refcnt}="1", \
> > ATTR{initstate}="live", SYMLINK+="ljet4l"
> 
> But again, where should the symlink you request point to? There is no
> device node.

I was hoping that udev would give me the default name as the WUR paper
says under Basic Rules: `Note that we did not specify a device node
name, so udev uses the default.'.

> Udevtrigger handles only devices, modules will not be coldplugged.

I'd not realised that. Would switching on the printer trigger udev? Is
there any way to mimic/achieve coldplugging apart from a reboot? Could
a udev restart or reload?

> I guess, you are looking for a device like /dev/lp0?

Yes. My initial attempts were to get udev to create it but without any
success. I did find a file, links.conf, that I modified to allow
/dev/lp0 to be made but I found no way to get /dev/lp0 using rules. My
main problem here is that I don't know what, in /sys corresponds to
lp0 in /dev and udev would need that information.

> You probably don't
> need any rule or custom symlink for it.

I don't *need* a rule; I can use links.conf. However, with an initial
comment in links.conf saying `This file doesn't exist', I get the
feeling it's a back door: something we'd rather not talk about. I
would love to know how to create /dev/lp0 without using links.conf.

Thanks again, Kay.

Barrie.


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