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SubjectRe: LANANA: To Pending Device Number Registrants

On Tue, 15 May 2001, Neil Brown wrote:
> I want to create a new block device - it is a different interface to
> the software-raid code that allows the arrays to be partitioned using
> normal partition tables.

See the other posts about creating a "disk" layer. Think of it as just a
simple "lvm" thing, except on a higher level (ie not on the request level,
but on the level _before_ we get to queuing the thing at all).

Plug the thing in at "__blk_get_queue()", and you're done.

> So I need a major number - to give to devfs_register_blkdev at least.
> You don't want me to have a hardcoded one (which is fine) so I need a
> dynamically allocated one - yes?

If you are willing to use devfs, you can just use a major nr of zero, and
devfs will allocate a device for you.

Not everybody likes devfs, and there are bootstrap issues with this
approach, but it is the simple "get things working quickly" approach that
needs _zero_ changes or infrastructure.

> This means that we need some analogue to {get,put}_unnamed_dev that
> manages a range of dynamically allocated majors.

We already do have that. And have had it for a long time. It's pretty much
been part of "register_blkdev()" since day one (not quite true, but I bet
that code has been there since the days of Linux-1.0.x).

You just pass in a major number of zero to "register_blkdev()", and it
will make one up for you.

devfs inherited this behaviour from the first version, I think.

> Am I missing something obvious here?

The fact that it already exists, and has existed for 5+ years, but that
nobody really uses it?

Nobody really uses it because it would require you to add a line or two to
your init scripts to pick up the major number from /proc/devices, and
that's obviously too hard. Much better to just hardcode randome numbers,


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