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Date:   Wed, 4 Sep 2019 07:25:38 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Sagi Grimberg <sagi@grimberg.me>
Cc:     Christoph Hellwig <hch@lst.de>, linux-nvme@lists.infradead.org,
        Keith Busch <keith.busch@intel.com>,
        James Smart <james.smart@broadcom.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/3] nvme: fire discovery log page change events to
 userspace
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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