Re: SCSI Tapes and devfs

["brett holcomb" <brettholcomb@xxxxxxxxxxx>]

Okay.  Thank you for the explanations.  Things are a 
little clearer now.  More comments inline.

On Thu, 16 Jan 2003 19:33:47 +0300
 Borzenkov Andrey <Andrey.Borzenkov@xxxxxxxxxxx> wrote:
> > At that point I decided to build st support as a module 
> > and autoload it.
> >  Did that and there was /dev/st with my tape devices!
>
> /dev/st are just compatibility names. Do you have
> /dev/scsi/hostX/busY/targetZ/lunW/mt* for your device?

Not that I remember.

> > At this point I'm confused.  My understanding of devfs 
> > is that upon
> > startup (Gentoo boots with devfs running) devfs will 
> > find the devices
> > and create the necessary directories under /dev/. 
>  
> No. Devfs does not find any device. Devfs is just a 
> repository where drivers
> can register devices they have found or created.

Okay - the light is starting to dawn.  Something has to 
trigger devfs into seeing the device.  I assume that can 
be a module loading (such as I did with st in the current 
system) or by some other means - whatever that is? 

> will cause devfs to
> > see a device added and create the directories.  But why 
> > didn't devfs see
> > the devices when st support was built into the kernel??
>
> because something went differently and driver did not 
> tell devfs about
> existence of this tape.

That's what I thought.

> Besides the process has more than one step. Drivers 
> usually register
> canonical names that are selected to be hierarchical to 
> avoid possible
> confusion. The names you are talking about (specifically, 
> /dev/st directory)
> are created by devfsd program. So the reason why you did 
> not see /dev/st may
> be also devfsd malfunction.
>
> > Do I have to manually edit /etc/devfsd.conf to tell it 
> > to find my tape
> > and autochanger? 
>
> Usually not.
>
>                           I thought that devfs would 
> find them by itself.
> >
>
> Devfs does not find anything. It is very dumb. It sits 
> there and waits until
> somebody tells it "hey, I have SCSI tape, please show it 
> to user now".

Okay, then what is supposed to tell devfs that there is 
now a scsi tape drive/changer ready to use.  In the case 
of st being a module and me loading it at startup time 
causes devfs to see it.  But what SHOULD trigger this if 
the st support is built into a kernel - what is the 
mechanism there for Linux?

> > I've read man pages, the FAQ at the devfs author's site 
> > and asked on the
> > Gentoo and other Linux newsgroups but am still confused. 
> > In fact the
> > deafing silence on the mailing lists and newsgroups 
> > shows me not many
> > people understand devfs or so it appears.  
>  
> I guess silence means Devfs works as expected :)))

As long as you don't try and do what I'm doing <G>.  I 
think devfs is a neat concept and from what I've read has 
a lot of advantages although my knowledge of devices under 
Linux has been limited to createing entries in /dev under 
the old system and making that work.  I don't know what 
distros use it (not Caldera's old WS 3.1 or RH 7.3) so 
maybe most of the people I see aren't running distros with 
it.



>                                               Any help 
> in unconfusing me
> > would be appreciated.
>
>
> Please show /etc/devfsd.conf, /etc/modules.devfs, explain 
> how and when
> devfsd is loaded in Gentoo and show dmesg output for the 
> case /dev/st does
> not appear.

I'll get them when I get back to the machine.  I'll create 
a kernel with st in the kernel and get that data.

Thank you.