[sane-devel] As instructed by /etc/udev/rules.d/libsane.rules

[Richard Ryniker]

The syscall interface is how applications request kernel services.  It is
not a general mechanism for the kernel to start userspace applications.

HAL and udev are mechanisms intended to give the kernel a flexible way to
invoke userspace applications when events such as "hardware device
connection" occur.  They certainly are less than perfect, but they are a
lot better than a new kernel module for every device in the world.  As I
understand it, HAL and udev use syscalls to tell the kernel "I'm
listening; tell me when something of interest happens" and then try to be
helpful when notified of events.  This is a sensible strategy, but we
still have much to learn about how to use it most effectively.

I was not aware of the "global label glitch" for udev rules, but it seems
a good example of where improvement is needed.  The hardware target moves
rapidly (device manufacturers seem obsessed with a need to make esoteric,
incompatible products - SANE authors may have unsurpassed experience with
this) and until it stops, no scheme is likely to be completely
satisfactory.  More general message passing schemes (like D-bus) may
offer more sophisticated event-handling frameworks that supplant (or
augment) udev and HAL in the future.  Clearly, we are short of any
consensus about best practice in this area.