Re: [gentoo-user] How the HAL are you supposed to use these files?

[Mike Edenfield]

On 2/8/2010 5:20 PM, Alan Mackenzie wrote:

> .  Am I the only person that finds this semantic gibberish?  Is there
> any explanation somewhere of what a "policy" aka "device rule" is?  What
> is the semantic significance of a "device rule"?  What does it mean, to
> "rule a device", or what sort of restrictions are being placed on this
> device?

> Given that one might desire a "basic working keyboard/mouse
> combination", what is the chain of reasoning that ends up selecting the
> file called "10-input-policy.fdi" from all the other ones?

> This file is an inpenetrable stanza of uncommented XML.  Are its verbs
> documented somewhere?  What do "<match ...>" and"<append ....>" mean,
> for example?

The way HAL works, in a nutshell, is to scan your system for 
every known piece of hardware it can find, and stores the 
information in a tree-like database of key/value pairs. 
Software can then query this database for information about 
whatever hardware you have.  The information includes things 
like the bus location of the hardware, the manufacturer 
information, state information, and a lists of known 
capabilities like "keyboard", "mouse", "disk", etc.

Device Rules are simply ways for the user to change values 
in the database after a device has been detected.  The XML 
files work in two steps:

1. <match> an existing node in the database,
2. <append> or <merge new values into those nodes.

For example, the hplip printer/scanner drivers include a set 
of HAL rules that match HP devices by their PCI device 
information, then append "scanner" to their list of 
capabilities.  Other software can then scan the HAL database 
for "all scanners" and find them.  The synaptics touchpad 
driver (if you build it +hal) includes a set of HAL rules 
that overrides the standard 'mouse' rules to make your 
touchpad more useful.

That bit about you having to do anything to make a "basic 
working" setup function, though, is wrong.  On Gentoo, at 
least, everything you need for a working keyboard and mouse 
in X should be installed properly for you by default.  You'd 
only need to mess with the rules if something didn't work. 
But, see below.

> Can this new-style fragmented XML configuration do anything that a good
> old-fashioned, human-readable and compact xorg.conf can't?  If so, what?
> What am I missing here?

HAL manages a *lot* more than just your X configuration. 
It's intended to be a complete hardware management layer, 
one that was able to keep pace with new hardware more 
quickly than the kernel could.  If you run the HAL database 
dump utility "lshal" you'll see more information about your 
hardware than you could ever possible care to know.

> Please, somebody, tell me all this HAL stuff is straightforwardly
> explained in an easily accessible Gentoo document, so that I can hang my
> head in shame and apologise for the noise!  ;-)

Oddly enough, the most complete explanation of HAL I've ever 
found was on the Gentoo wiki and I think the page may be 
lost.  It was never really documented that well, though 
there are a number of places you can find specific ways to 
do specific things (like using a touchpad) with HAL.

The key point here, though, is that HAL is going away.  Not 
because it was hard to configure, though -- because the code 
is an "unmaintainable mess" and because other software, like 
udev, duplicated much of its purpose.

At this point, if it's not working for you out of the box, 
turn it back off and revert to the old style configuration file.