Re: (Semi-random) thoughts on device tree structure and devfs

[Iain Hibbert <plunky%rya-online.net@localhost>]

On Mon, 8 Mar 2010, David Young wrote:

> On Mon, Mar 08, 2010 at 12:34:23PM -0700, Ted Lemon wrote:
> > On Mar 8, 2010, at 12:47 AM, Masao Uebayashi wrote:
> > > How do you write a kernel config which can always identify my USB disk as
> > > sd0a, even if I plug random devices?
> >
> > You'd need to put the UUID in the kernel config.
>
> I'd go further and say that we should be able to supply a set of device
> properties (such as drvctl -p prints) to the kernel.  Let us match a
> device by its intrinsic properties (MAC address, serial number, and/or
> GUID), and set the unit number according to the device property.
>
> Quentin is right that this *only* helps us to fix the unit number, but I
> think that in itself is an important, *feasible* step forward.

One thing that I think is problematic about trying to do that, is that you
might sometimes need to attach a device (allocate the unit number) in
order to discover its intrinsic properties. It can't always be done in the
attach routine because you might have to wait for a query (or several) to
return. For that reason, we should consider that the dv_xname is not
necessarily a useful tag.

(I say "device" rather than disk because I know that Bluetooth controllers
work this way - you can't get the BDADDR until it is up and running)

I have never used wedges but, for the disk case, would it not be better to
make a method of configuring a dk in advance, so that whenever a disk
appears with the correct parameters it will already be mapped to the dk
you expect? (perhaps a daemon could handle it) Then you know that /dev/dk3
is your USB stick and will never be anything else..

iain