Input: atkbd - add 'terminal' parameter for IBM Terminal keyboards

Input: atkbd - add 'terminal' parameter for IBM Terminal keyboards

Many of the IBM Terminal keyboards from the 1980s and early 1990s communicate
using a protocol similar, but not identical to the AT keyboard protocol.
(Models known to be like this include 6110344, 6110668, 1390876, 1386887, and
possibly others.)

When the connector is rewired or adapter to an AT-DIN or PS/2 connector, they
can be connected to a standard PC, with three caveats:

a) They can only use scancode set 3; requests to use anything else are
quietly ignored.
b) The AT Command to request Make, Break and Repeat codes is not properly
interpreted.
c) The top function keys on a 122 key keyboard, and the arrow/edit keys in
the middle of the board send non-standard scancodes.

C) is easily taken care of in userspace, by use of setkeycodes
B) can be taken care of by a userspace hack (that makes the kernel complain
in dmesg)
A) is fixable in theory, but on the keyboard i tested on (6110668), it seems
to be detected unoverridably as Set 2, causing userspace oddities that make
it harder to fix C).

Enclosed is a small patch to the kernel that fixes A) and B) in the kernel,
making it much easier to fix C) in userspace. It adds a single kernel
command line parameter that overrides the detection that sets these boards
as set 2, and instead of sending the Make-break-repeat command to the
keyboard, it sends the make-break command, which is properly recognized by
these keyboards. Software level key repeating seems to make up for the lack
of hardware repeat codes perfectly.

Without manually setting the command line parameter (tentatively named
atkbd.terminal), this code has no effect, and the driver works exactly as
before.

See also:

http://www.seasip.info/VintagePC/ibm_1390876.html
http://www.seasip.info/VintagePC/ibm_6110344.html
http://geekhack.org/showwiki.php?title=Island:7306

Signed-off-by: Erika Quinn <erikas.aubade@gmail.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>