Logitech HID confusion

[Christoph Böhmwalder]

Kind of off topic for this list, but maybe somebody on here knows
something about this.

Looking through some the HID drivers, I've noticed that the Logitech
Unifying Receiver HID driver is actually split up into
hid-logitech-dj.c and hid-logitech-hidpp.c.

Now I've heard about HID++, and from my research it seems that it's
Logitech's custom HID protocol which it uses to communicate through the
Unifying Receiver.  What's odd to me is the "dj" though.  Looking
through the HID documentation really gave no clues.

Skimming through the source code it seems like there's some sort of "DJ
Mode" for certain Logitech devices?  I also found an old patch [1] that
talks about "DJ devices [...] (T400, M325,...)".  Googling these product
names reveals that there seems to be nothing special about them.

I'm really confused, as there seems to be no public documentation on
what this DJ mode actually is or does (or maybe I'm just overlooking
something glaringly obvious).  I'd be grateful if someone took the time
to point me in the right direction here.

Thanks!

--
Regards,
Christoph