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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,
    right?

    Linus

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