[Libdlo] Equivalent for $DISPLAY

[Phil Endecott]

Dear All,

libdlo provides a convenient function, dlo_claim_first_device(), to get 
the first attached device.  This is great if you only have one device 
connected.  But there's a danger that we'll end up with utilities that 
call this and so cannot be made to work on systems with more than one 
device attached.

I'd like to propose that we remove dlo_claim_first_device() and instead 
provide something like dlo_claim_default_device().  This will behave 
like X applications and check for an environment variable or 
command-line option that specifies the device to use, and will fall 
back to the first unclaimed device if nothing else is specified.  
Suggestions for environment variable and command line option names are 
welcome; my initial idea is $DLDISPLAY and --dldisplay.

This leaves the question of how to name the displays.  We can see 
serial numbers in the dlo_devinfo_t; is this really a unique number?  
I.e. do the devices have PROMs in them?  I've dismantled my dongle and 
it does indeed have an I2C PROM, but is this essential to the design or 
is it the first thing that el cheapo OEM will not implement?  (I have 
seen other USB devices that present a default set of IDs if no PROM is attached.)

I'm trying to think of precedents, but I can only think of things where 
the kernel and udev choose names; that doesn't apply to libusb stuff.  
What else uses libusb?  How does SANE name devices?


Regards,  Phil.