Subject: Re: Another changer, another changer problem
To: David Maxwell <>
From: Brett Lymn <>
List: current-users
Date: 10/07/1998 11:20:19
According to David Maxwell:
>Currently, the requirement to build a kernel for each change makes the
>whole wiring down suggestion echo around here a bit troublesome.

Ummm you don't _have_ to just wire what you have.  You can build in
support for some devices that you may reasonably expect to add.  On
the SunOS machines I run you can reasonably expect that scsi id's 4 &
5 are tapes and 6 is the cdrom - regardless of whether they are there
or not (these are the "normal" id's for these devices on a Sun).  I do
the same on my NetBSD machines.  Alternatively, you can wire down what
you _do_ have and leave a catch-all auto-probe line in to support an
emergency device add, this means that you can keep your working
devices where they are and add other devices on the fly.

> It
>would be more practical when we have a boot time method for changing IRQ,
>IO, DMA, dev, etc....

With the rider that this can make the system slightly more frail.  A
build on the fly system is convenient when you are adding/removing
devices but there is a large scope for screwing up.  It can also,
paradoxically, tie the kernel to the hardware more tightly than we
already have.  For an educational exercise, try installing Solaris2 on
a SS5 and then try booting that disk on a SS20.  The machines have the
same architecture but Solaris2 will not boot - all the dev links are
stuffed and boot -r will not fix it because it does not have enough
information to find the disks to reconfigure.  You can fix this, the
process is involved and requires you booting from a CD to do it.  Sun
says doing this is "not supported" and suggest a re-install.  OK, so
you may say that this is a design flaw in Solaris which is an easy
thing to say but it does highlight a technical challenge with going
the path of building your device tree dynamically - you need to keep
state, you need a device to keep this on, you need to have this device
available on boot to get the state to config the devices it's at this
point things get interesting ;-)

Brett Lymn, Computer Systems Administrator, British Aerospace Australia
  And the monks would cry unto them, "Keep the bloody noise down!"
  - Mort, Terry Pratchett.