LANANA: To Pending

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

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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