[sane-devel] Sane and HAL

[Johannes Meixner]

Hello,

On Jan 1 23:16 abel deuring wrote (shortened):
> a happy new year to you all!

A happy new year to you too!



We use HAL in Suse Linux 10.1 and in openSUSE 10.2 to set
access permissions for normal users.
In Suse Linux 10.1 only for USB scanners and in
openSUSE 10.2 for USB and SCSI scanners.
Therefore I have a tiny experience in taming the udev->HAL beast.

What happens is:
When a device becomes detected by the kernel and when there
is a udev event (which doesn't happen in any case), then HAL
is notified by udev and then HAL adds it to its device list
so that it will be listed in the "lshal" output.
Furthermore HAL looks up its fdi files if something special
is to be done for this device.
If an entry in a fdi file matches to how it is listed in the
"lshal" output, HAL would do the action specified in the
fdi file but again there are some (unexpected) constraints:
For example HAL may kill a program which was started as the
action specified in the fdi file after a short timeout
(I guess to avoid too long delays).


> The minimum we can (and IMO should) do is to provide an XML file (or
> ".fdi file" in HAL terminology) that describes the scanners
> supported by Sane, and optionally the unsupported scanners.

Have a look at
https://bugzilla.novell.com/show_bug.cgi?id=160899
in particular my bash script in
https://bugzilla.novell.com/show_bug.cgi?id=160899#c20


For some already known problems have a look at
https://bugzilla.novell.com/show_bug.cgi?id=218393#c19
starting at comment #19
and
https://bugzilla.novell.com/show_bug.cgi?id=226044
The latter may be in particular useful to show an example
how to debug a problematic case.


> ... "interested" applications
> can use this data to find a scanner and a suitable Sane backend and
> launch for example xsane or access the scanner directly.

This does not work if a special setup for firmware upload
is required.
Therefore a firmware related entry would also be needed in
the desc files and the in the fdi file.

My personal opinion is that I do not like it when my computer
launches whatever program it thinks to be useful for me
automatically (perhaps I don't want to use any graphical
frontend at all or I prefer xscanimage).

Furthermore it becomes tricky if more than one backend works
for a particular model (e.g. epson versus epkowa backend).
Perhaps it is o.k. to simply activate all matching backends
but more than one backend might confuse an unexperienced user?


> Another option would be another meta-backend named "udi", but I
> believe that this would be overkill.

I think SANE should not depend on udev/HAL (i.e. it should
still work even without any udev/HAL running or installed).
Perhaps therefore another meta-backend named "udi" might be
not a bad idea to keep the udev/HAL stuff strictly seperated?


> HAL might be a way to integrate the permission handling
> for parport scanners into the general "permission management" of a
> Linux distribution.

Yes!
For example we use "resmgr" for our permission management
and since openSUSE 10.2 the udev->HAL-resmgr chain works
without any patches in SANE, see our RPM changelog:
------------------------------------------------------------------
- disable-resmgr-support.patch disables the resmgr support in SANE
  which is no longer needed in SANE because resmgr works now
  outside of SANE via ACLs for the scanner device nodes.
------------------------------------------------------------------
Since openSUSE 10.2 I can recommend resmgr for such kind of
permission management. Before I was not happy about how it works
because it required that the software was patched so that
third-party packages would have worked different than our
patched packages.

Note that in our scanner related fdi files there is no "resmgr"
mentioned. All what is done via the scanner related fdi files
is to mark a HAL device udi to be a "scanner" and then via well
seperated resmgr related fdi files (with fixed content) the
permissions are set for those HAL devices which are a "scanner".

I.e. it is independent to tell HAL which device is a scanner
and what a distribution or an application may like to do with
this information.


Kind Regards
Johannes Meixner
-- 
SUSE LINUX Products GmbH, Maxfeldstrasse 5      Mail: jsmeix at suse.de
90409 Nuernberg, Germany                    WWW: http://www.suse.de/