Re: Hotplug-- who notifies whom of what?

[Greg KH <greg@kroah.com>]

On Mon, Feb 16, 2004 at 06:54:16PM -0600, Bryan W. Headley wrote:
> Greg,
> 
> Regards to Aiptek USB tablets...
> 
> I'm trying to figure out a strategy for hotplugging.
> 
> 1) The tablet is mapped to the input device system.
>    a) Because the order in which drivers complete loading/registering
>       is not deterministic (e.g., mouse driver spawns thread), I do not
>       know with certainty that the tablet will always be mapped to
>       /dev/input/event27. So, configuration files of certain (XFree)
>       applications that require that the path be known and static enough
>       to put in a text file.

Use udev to always create your "/dev/input/tablet_mine_all_mine" node so
that XFree does not have to be reconfigured all the time.

>    b) No problem. So I whip up a startup process that finds the tablet
>       and creates a softlink that XF86Config-4 can refer to. That is
>       relying on the sysvinit system, and is wrong insofar as it knows
>       there's a tablet owned by the user (I'm thinking of RH, Mandrake,
>       et al, quality scripts, here. Not little one-liners we all write.)

Sure, create a symlink in udev instead.  It can do that just fine if you
don't want to use a "real" node.

> 2) In case of a hotplug event, e.g., the tablet being disconnected, I
>    really should get rid of the softlink...
>    a) But is that the domain of a script in hotplug?

Nope, that's udev's job.

> 3) In case of hotplug-connect, I should re-create the softlink.

udev does that for you just fine.

>    a) Again, in the hotplug domain? I ask because I have an rc script
>       that does the same thing on boot-up, and I'd rather not have
>       things cross-referenced between two packages.
>    b) Plus, what I do to find the aiptek tablet COULD BE scriptable on
>       2.6, would be hard to script on 2.4. Anyway, it's not a script
>       now, it makes ioctl calls.
> 
> 4) Moving upstream... X needs to be notified that device 'x', that it's
>    using for input, has been plugged/unplugged. No facility for that,
>    except in my xf86aiptek device driver for X, which DOES have a
>    bidirectional communications channel, which could have a "Unwind
>    driver" API call
>    a) Right now, the only thing that knows my API is my user-friendly
>       front-end. It's not something you'd run as a daemon...
>    b) But a subset of it certainly could. Question is, what does it
>       listen for? Hotplug messages forwarded off of dbus? Does it
>       monitor Hal? Is it really something that's fired off directly
>       by hotplug? Or owned by hotplug?

How about listening to dbus messages?  That is the way that both GNOME
and KDE are moving to using.

> 5) Does udev fit into this conversation anywhere? It can be hotplug,
>    dbus, or hal-aware, or do it's thing by itself...

Yes it does, see above.

Hope this helps,

greg k-h


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