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    SubjectRe: [PATCH] driver-core: devtmpfs - driver core maintained /dev tmpfs
    On 5/1/09, Kay Sievers <> wrote:
    > On Fri, May 1, 2009 at 12:19, Alan Jenkins
    > <> wrote:
    >> On 4/30/09, Kay Sievers <> wrote:
    >>> With the kernel populated /dev, existing initramfs or kernel-mount
    >>> bootup logic can be optimized to be more efficient, and not to require a
    >>> full coldplug run, which is currently needed to bootstrap the inital
    >>> /dev directory content, before continuing bringing up the rest of
    >>> the system. There will be no missed events to replay, because /dev is
    >>> available before the first kernel device is registered with the core.
    >>> A coldplug run can take, depending on the speed of the system and the
    >>> amount of devices which need to be handled, from one to several seconds.
    >> Aren't you overeaching in your claims here? I'm sure you can't avoid
    >> at least one coldplug run on a contemporary general purpose system,
    > You still need coldplug, sure. But distros have 2 full coldplugs
    > today, one in initramfs, one in the rootfs.
    >> because you lose so much of the functionality provided by udev.
    > No, udev will do the same thing as it does today, it will not be
    > different, it's just that the colplug can run in parallel with other
    > stuff, and the initramfs coldplug can be much simplified, or even
    > avoided.

    Ok, I can see there's no problem there, given that coldplug still
    happens eventually.

    >> I'm
    >> sure of that, but it would be nice if you could address it in the
    >> changelog. And modern initramfs' require udev RUN rules to read UUIDs
    >> and set up LVM.
    > A simple uuid/label lookup will not need a full coldplug run in
    > initramfs. You just need to start udev, and wait for the links to show
    > up.Possibly running triggering block events, but not the hundreds of
    > other devices.

    Assuming you have all the necessary modules loaded! I guess that's
    easy enough though

    find /sys -name modalias -exec cat \{\} \; | xargs -n 1 modprobe -Qa

    bonus: increase "-n" for reduced modprobe fork overhead.

    >> I'm loving this for embedded, init=/bin/sh, and rescue floppies :-).
    >> But I can't understand how you plan to use this as an optimisation.
    >> And - I'm sure you must have considered this in a moment of madness -
    >> do you know why we couldn't just start _udev_ "before the first kernel
    >> device is registered with the core"?
    > I think it's pretty obvious, that this is the fastest you can get for
    > /dev. And it's not about the time when udev is started, its about
    > doing the stuff when the devices are created, and not to need to redo
    > it at bootup, or do it even twice.

    Ok, yes. I can see that cutting initramfs coldplug down to size is a win.

    I thought it was a useful comparison. Start udev early enough, and
    you could avoid re-doing absolutely anything. Thinking about, the
    reasons it doesn't work are

    a) running before /dev/null and /dev/console requires hacks
    b) it requires an initramfs
    c) it pulls everything that hooks into or otherwise affects udev into
    the initramfs; that's much more than we have at present, and a bigger
    initramfs' can only make bootup _slower_.


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