[Linus Torvalds <torvalds@transmeta.com>] Re: LANANA: To Pending

[Linus Torvalds <torvalds@transmeta.com>] Re: LANANA: To Pending

[Miles Lane]

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I presume everyone on this list has been following this discussion on
LKML.  If not, I highly recommend you check out the discussion thread in
the LKML archive.  This is an important architecture discussion that has
very important implications for the Hotplug system.

Here's the first message in the thread:

http://www.uwsg.indiana.edu/hypermail/linux/kernel/0105.1/1042.html

There are currently ~30 messages in the thread.  I think many of the
most recent messages are probably the most informative and interesting.

Cheers,
    Miles

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From: Linus Torvalds <torvalds@transmeta.com>
To: James Simmons <jsimmons@transvirtual.com>
Cc: Jeff Garzik <jgarzik@mandrakesoft.com>, Alan Cox <alan@lxorguk.ukuu.org.uk>, Neil Brown <neilb@cse.unsw.edu.au>, "H. Peter Anvin" <hpa@transmeta.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, viro@math.psu.edu
Subject: Re: LANANA: To Pending Device Number Registrants
Date: Tue, 15 May 2001 10:43:18 -0700 (PDT)
Message-ID: <Pine.LNX.4.21.0105151031320.2112-100000@penguin.transmeta.com>


On Tue, 15 May 2001, James Simmons wrote:
> > 
> > Static devices like /dev/fbN are no different. They were just plugged in
> > before the OS booted.
> 
> Actually their are hotplug video cards. High end servers have hot swapable 
> graphcis cards. Would you want to take down a very important server
> because the graphics card went dead. You pull it out and you plug a new
> one in. Also their are PCMCIA video cards. I have seen them for the hand
> held ipaqs. It is only a matter of time before all devices are hot
> swappable. 

True, but not really necessarily important.

The thing is, even if the device happens to be soldered down, inside a
computer that is locked in a safe, the question boils down to a fairly
simple one: "how do we approach devices?".

Do we approach devices as something static, or do we approach them as more
dynamic entities? Do we consider soldered-down devices to be fundamentally
different from the ones that can be hot-plugged?

And my opinion is that the "hot-plugged" approach works for devices even
if they are soldered down - the "plugging" event just always happens
before the OS is booted, and people just don't unplug it. So we might as
well consider devices to always be hot-pluggable, whether that is actually
physically true or not. Because that will always work, and that way we
don't create any artificial distinctions (and they often really _are_
artifical: historically soldered-down devices tend to eventually move in a
more hot-pluggable direction, as you point out).

Now, if we just fundamentally try to think about any device as being
hot-pluggable, you realize that things like "which PCI slot is this device
in" are completely _worthless_ as device identification, because they
fundamentally take the wrong approach, and they don't fit the generic
approach at all.

But this is also why I don't think static device numbers make any
sense. It's silly to have the same disk show up as different devices just
because it is connected to a different kind of controller. And it is
_really_ silly to statically pre-allocate device numbers based on the
"location" of a device. 

We should strive for a setup where device plugin causes that device to
show up in /dev, and everywhere else it is needed. And the logical
extension of such a setup is to consider built-in devices to be plugged in
at bootup.

This is true to the point that I would not actually think that it is a bad
idea to call /sbin/hotplug when we enumerate the motherboard devices. In
fact, if you look at the current network drivers, this is exactly what
will happen: when we auto-detect the motherboard devices, we _will_
actually call /sbin/hotplug to tell that we've "inserted" a network
device.

It's just that we haven't really mounted the root filesystem yet, so
user-land never actually "sees" this fact. But I think it's the right
approach to take, and realizing that even static devices are just a
sub-case of the problem of dynamic allocation means that you tend to
automatically also see that static device number allocation is just
broken.

[ The biggest silliness is this "let's try to make the disks appear in the
  same order that the BIOS probes them". Now THAT is really stupid, and it
  goes on a lot more than I'd ever like to see. ]

		Linus

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