Re: [PATCH v2 3/3] nvme: fire discovery log page change events to userspace

[Sagi Grimberg]

> > 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.

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.

Did you mean that we should add another flavor of device_add that
accepts char *envp_ext[]?

What exactly is the "standard manner" to pass these variables to
the chardev KOBJ_ADD uevent? All other examples I could find use
KOBJ_CHANGE to pass private stuff..