Re: gnome 2.19 schedule.

[David Zeuthen <david fubar dk>]

On Mon, 2007-03-19 at 10:30 -0400, Donald Straney wrote:
> > In fact, imcapd is going to be a HAL add-on, but registering dbus call
> > in order to reclaim/release device. Donald, can you confirm that ?
> 
> Right, it could definitely do that as a HAL addon.  Each scanner could
> have ReleaseDevice and ReclaimDevice methods which would make it
> temporarily close the device so another program could use it, but that
> still seems like a hack.  I guess the real solution here would be to
> make sure all SANE drivers can take multiple connections, but I don't
> know if that's possible (maybe some hardware limits it to one
> connection?).  Does anyone know more about this?
> 
> So using libusb directly would get around locking problems?  Sounds
> good, but it seems like we'd end up with a bunch of duplicated driver
> code, and what if a SCSI scanner or parallel-port scanner has buttons?
>  It's not likely, but it seems better to keep it flexible instead of
> tying it into USB.

No, device access via libusb is exclusive (as it needs!) so this is not
possible. One way around this, though, would be to modify libsane to
send a D-Bus message to the HAL add-on so it releases the device (unless
you configured libsane to not do this). That's probably not a lot of
work, I'd just patch backend/dll.c:sane_open() in sane-backends and hey
presto this should work. Getting such a (compile-time) option past the
SANE developers may be harder though, but I guess if you make a good
case for it, then it should be feasible.

Btw, looking at how sanebuttonsd does button detection, it's pretty ugly

 http://alioth.debian.org/plugins/scmcvs/cvsweb.php/experimental/button-daemon/sanebuttonsd.c?rev=1.3;cvsroot=sane

e.g. it's polling. I don't suppose there's any way to avoid this, it
only will happen when the scanner is plugged in and powered on.

So here's what I would do

 1. Start writing the buttons addon for HAL - this would just link
    with libsane and do sane_open (device_file) where you get
    device_file from HAL, e.g. "/dev/bus/usb/001/002", when the
    addon is launched. Make sure it emit D-Bus events when you
    press buttons and this should work if the underlying sane driver
    supports button presses.

 2. Make the addon export the o.fd.Hal.Device.ScannerButtonMonitor
    interface with the InhibitMonitoring and AllowMonitoring that
    tells the add-on to release resp. acquire the device file for
    monitoring. Need to track when the caller disconnects from the
    bus, see e.g. gnome-power-manager and gnome-screensaver for how
    to do this. Now you can disable this from the command line and
    things actually work

 3. Patch libsane's backend/dll.c sane_open() (and sane_close()!) to use
    InhibitMonitoring() / AllowMonitoring() on the addon. This probably
    includes things like using

     libhal_manager_find_device_string_match(hal_ctx, "scanner.device",
                                             "/dev/device_file");
     
    to find the device object and then call InhibitMonitoring() (and
    "/dev/device_file" is what the caller passed to sane_open()).

What do you think? Just ask on the HAL list if you need any help!

Hope this helps,
David