HAL and CD monitoring

[David Zeuthen]

On Tue, 2003-12-09 at 17:49, Bastien Nocera wrote:
> Hello David, and all,
> 
> As Owen was overly busy already, and I was hacking a tidbit on magicdev,
> I'm now the new maintainer for it.
> 
> And I was looking at the current monitoring code in HAL, and it looks
> very much like the functionality it implements is something that I was
> looking into for the future of magicdev (see my dead "magicplug" code,
> and things like that ;)
> 
> 2 questions:
> - Are there any plans to implement the monitoring code for 2.4 kernels?

Hi,

It's certainly *possible* to support 2.4 as one of the ideas of HAL is
to be able to support many operating systems and indeed 2.4 was the
first target. 

However, I quickly got turned off by the difficulties in 2.4 in reliably
obtaining e.g. the /dev/sda1 node for e.g. usb-storage and started using
2.6 and udev. Having said that, it should be possible to write 2.4 code
for HAL as well. It may difficult to do and it may be distro-specific
though. I'm not sure... 

My 2.6 code does not appear to be distro-specific at all; I've got good
feedback from people using other distros than myself (And as soon as I
got 2.6 working on my Powerbook I will test on debian as well).

> - Would changes to implement disc-change monitoring be accepted (once
> written and reviewed) in HAL itself?
> 

Disc-change monitoring is clearly something that should just work! 

I see it closely related to mounting removable storage, e.g. a volume
manager as Carlos suggested in September on this list.

With the new (almost 100% rewritten) HAL it should be possible to write
such a volume manager. I actually wrote a small python example for doing
most of this here

http://freedesktop.org/cgi-bin/viewcvs.cgi/hal/examples/volumed/volumed.py?rev=1.1&cvsroot=hal&content-type=text/vnd.viewcvs-markup

It's quite simple, yes? If you look at

http://freedesktop.org/~david/hal-cdrom.png

you can see that HAL also detects optical and floppy drives; optical
drives got the capability removableMedia.cdrom; this is all derived from
/sys and /proc in the 2.6 kernel (there are still issues with handling
SCSI hardware devices as I've got none, but this can be resolved once
someone tests this on a system with SCSI hardware).

So, yeah, it's indeed possible for a volume manager to find these
devices using HAL and do the disc-change monitoring on them. When a disc
is detected this volume manager should create and maintain one or more
(multi-session discs may have several volumes) Volume device objects as
seen here for usb-storage

http://freedesktop.org/~david/hal-usb-mounted.png

Now, desktop file managers can simply just listen for hal device objects
with the capability volume. 

(it should be noted that hal can interoperate with existing volume
managers / automounters since the hal monitor parts looks for changes in
/etc/mtab).

> If so, then we might be looking at the future for magicdev in HAL :)
> 

Yeah, that would be great! magicdev is also a more catchy name than
volume manager :-)

I must admit that I need to update the hal spec to mention all this and
make the 0.2 release; I promise to do this real soon now when I've
shuffled internals around enough. Until then, it's still in CVS.

Cheers,
David