pckbd: PS/2 keyboard throttle

pckbd: PS/2 keyboard throttle

Limit the keyboard data rate to the serial link speed. Some old
DOS software relies on being able to read an incoming scan-code
more than once. After reading keyboard data from the i8042
controller, the guest software has 1ms to read the same data
again.

Use -global i8042.kbd-throttle=on to enable this feature.

To see how this patch works, start a FreeDOS 1.2 guest with the
qemu option -global i8042.kbd-throttle=on and open a text file
with the vim 7.3 32 bit for DOS executable. Then use the cursor
keys (not the cursor keys on the numeric keypad) to move through
the text. Without the kbd-throttle option enabled each keystroke
will move the cursor two positions.

Buglink: https://bugs.launchpad.net/bugs/1895363
Buglink: https://bugs.launchpad.net/bugs/1897568
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20210525181441.27768-6-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>