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

[David Young <dyoung%pobox.com@localhost>]

On Wed, Mar 10, 2010 at 01:49:02PM +0900, Masao Uebayashi wrote:
> > The code providing DKWEDGE_METHOD_GPT already has the knowledge.  I
> > don't think that the knowledge has to move from there.  All that dk(4)
> > has to do is to match device-properties lists, and for that it can use
> > the same library function as every other match routine will use.
> 
> What if you want to mount a NIC as /?  You'll fix all drivers?

Of course you have to fix drivers.  Drivers don't extract the device
properties and store them in a standard form, today.

> All of you say that lookup-by-ID works in your way.  It's possible,
> because ID is unique.  What I'm talking is the best design how to do
> it.  Now raidframe(4) alreadys does it itself, why do you have same
> logic in raidframe(4) and dk(4)?

What do you mean, the same logic?

> I think dk(4) does too many things.  That means you have to
> re-implement same logic in many places.  That also means users have to
> learn all devices' behavior.

Drivers have to know how to extract properties such as MAC address from
their devices.  I don't think that we can avoid that.  If drivers record
the properties that they extract under standard keys, then we can match
them using a library function.

> If you say all drivers behave same way, why do you have multiple
> copies of code?

I said that there would be a library function. :-)

> > The point is to make the device node, /dev/dk3, a reliable handle for
> > the volume.
> 
> The point is you can't rely on device unit numbers of pseudo devices.

We can discard the pseudo-devices concept, if need be.

We cannot rely on any device's unit numbers, now, if it can change
slot/port/chassis.  If we extend the set of "locators" to include
intrinsic device properties such as MAC address, volume GUID, and serial
number, then we can establish a permanent correspondence between a
device unit and a physical device.

Dave

-- 
David Young             OJC Technologies
dyoung%ojctech.com@localhost      Urbana, IL * (217) 278-3933