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    SubjectRe: Device enumeration (was Re: CD writing in future Linux (stirring up a hornets' nest))
    Date
    On Tuesday 14 February 2006 5:40 am, Olivier Galibert wrote:

    > > > 4- sysfs has all the information you need, just read it

    There's no ownership or permissions information in sysfs. Even busybox's
    eight kilobyte micro-udev replacement has the option for an /etc/mdev.conf to
    specify permissions and ownership on device nodes.

    > > That mapping should not live in sysfs,
    > > /dev is none of the kernel's business and sysfs is the kernel's
    > > playground.
    >
    > Why not have udev and whatever comes after tell the kernel so that a
    > symlink is done in sysfs? The kernel not deciding policy do not
    > prevent it from storing and giving back userland-provided information.

    That wouldn't help us. If userspace generates the info, then userspace can
    drop a note in /dev or something to keep it there.

    > I guess you didn't bother to read the "answer 3" paragraph of my
    > email. Do you trust udev to still exist two years from now, given
    > that hotplug died in less than that? Do you trust udevinfo to have
    > the same interface two years from now given that the current interface
    > is already incompatible with a not even two-years old one (udev 039,
    > 15-Oct-2004 according to kernel.org) which is widely deployed as part
    > of fedora core 3?

    You want something simple and stable?

    Busybox's mdev should still be there, and have the same interface, two
    years from now. (We may have to fix it between now and then if the kernel
    keeps moving out from under us, but that shouldn't change how you set up and
    use it.)

    When you call "mdev -s", we iterate through /sys/class and sys/block looking
    for "dev" nodes containing a "major:minor" string, and take the name of the
    directory we find each /dev node in as the name of the device to mknod
    in /dev. (As an option, it can check /etc/mdev.conf which has three
    space-separated fields: a regex, a numeric colon-separated uid:gid pair, and
    octal permissions. "tty[0-9]* 0:42 770". Stops at the first match and uses
    that ownership and permission info for the new node. If there's no mdev.conf
    or it doesn't match any regex against the name it's creating, it defaults to
    0:0 and 660.)

    If you call it without -s, it assumes it was called from /sbin/hotplug and
    looks at its environment variables to figure out what device node to
    create/delete.

    That's it. That's all we do. No persistent naming, no device renaming, /dev
    is a flat namespace with no subdirectories, mounting tmpfs on it before
    calling us is your problem, as is putting /dev/pts and /dev/shm in there...

    Changes to the kernel have already managed to break us twice
    (switching /sys/class from real subdirectories to symlinks means we can't
    just ignore symlinks when recursing down through directories anymore, which
    is a problem because the existing symlinks form loops. And
    deprecating /sbin/hotplug means we've got to bloat the code with netlink
    stuff.) But we'll cope, and the user interface isn't changing.

    We can extend the mdev.conf file to specify other stuff. (Such as append a
    command line as an optional argument #4 to execute when one of these suckers
    is created/destroyed. But so far, it's really really simple and our target
    audience hasn't needed more than that.)

    If you want to try mdev, grab the most recent snapshot from
    busybox.net/downloads/snapshots and build it this way:

    make allnoconfig
    echo "CONFIG_MDEV=y" >> .config
    echo "CONFIG_FEATURE_MDEV_CONF=y" >> .config
    make
    mv busybox mdev

    There you go, standalone 8k binary. It'll come standard in the busybox 1.1.1
    release. (It was in 1.1.0, but had a bug.)

    Rob
    --
    Never bet against the cheap plastic solution.
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