Comment 11 for bug 815914

In case somebody would debug this problem again, here is answer to my question from Jean-Louis back in 2015:

AT Translated Set 2 keyboard reports these events:

left button:

EV_MSC MSC_SCAN 219
EV_KEY KEY_LEFTMETA (0x7d) pressed
EV_SYN code=0 value=0
EV_MSC MSC_SCAN 45
EV_KEY KEY_X (0x2d) pressed
EV_SYN code=0 value=0
EV_MSC MSC_SCAN 45
EV_KEY KEY_X (0x2d) released
EV_SYN code=0 value=0
EV_MSC MSC_SCAN 219
EV_KEY KEY_LEFTMETA (0x7d) released
EV_SYN code=0 value=0

middle button:

EV_MSC MSC_SCAN 133
EV_KEY KEY_BRIGHTNESSDOWN (0xe0) pressed
EV_SYN code=0 value=0
EV_MSC MSC_SCAN 133
EV_KEY KEY_BRIGHTNESSDOWN (0xe0) released
EV_SYN code=0 value=0

right button:

EV_MSC MSC_SCAN 132
EV_KEY KEY_NEXTSONG (0xa3) pressed
EV_SYN code=0 value=0
EV_MSC MSC_SCAN 132
EV_KEY KEY_NEXTSONG (0xa3) released
EV_SYN code=0 value=0

So left button generates combination META+X, middle button is reported as brightness down and right button as next song.

So keys are reported via PS/2 keyboard and also by WMI. Dell WMI events are ignored by kernel to prevent duplication.