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    SubjectRe: [PATCH v2 3/3] nvme: fire discovery log page change events to userspace
    On Tue, Sep 03, 2019 at 06:35:30PM -0700, Sagi Grimberg wrote:
    >
    > > > Still don't understand how this is ok...
    > > >
    > > > I have /dev/nvme0 represents a network endpoint that I would discover
    > > > from, it is raising me an event to do a discovery operation (namely to
    > > > issue an ioctl to it) so my udev code calls a systemd script.
    > > >
    > > > By the time I actually get to do that, /dev/nvme0 represents now a new
    > > > network endpoint (where the event is no longer relevant to). I would
    > > > rather the discovery to explicitly fail than to give me something
    > > > different, so we pass some arguments that we verify in the operation.
    > > >
    > > > Its a stretch case, but it was raised by people as a potential issue.
    > >
    > > Ok, and how do you handle this same thing for something like /dev/sda ?
    > > (hint, it isn't new, and is something we solved over a decade ago)
    > >
    > > If you worry about stuff like this, use a persistant device naming
    > > scheme and have your device node be pointed to by a symlink. Create
    > > that symlink by using the information in the initial 'ADD' uevent.
    > >
    > > That way, when userspace opens the device node, it "knows" exactly which
    > > one it opens. It sounds like you have a bunch of metadata to describe
    > > these uniquely, so pass that in the ADD event, not in some other crazy
    > > non-standard manner.
    >
    > We could send these variables when adding the device and then validating
    > them using a rich-text-explanatory symlink. Seems slightly backwards to
    > me, but that would work too.

    That's the way the driver model is expected to work, instead of having
    to do crazy device-specific stuff.

    > We create the char device using device_add (in nvme_init_subsystem),
    > I didn't find any way to append env variables to that ADD uevent.

    You do that in your uevent or dev_uevent callback like all other
    subsystems. Nothing "new" to do here, again, it's been working fine for
    everyone else for well over a decade now :)

    thanks,

    greg k-h

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