Is the wakeup source relevant only for suspend/resume , or also for
runtime PM ? for example, is the keybaord example , which you linked,
relevant for runtime PM too ?

Refer to my reply in the other thread.

If yes - is the keybaord interrupt wakeup only the keyboard driver,
and nothing else, including the cpu ?

In your case, the keyboard interrupt being the wakeup source, wakes up the
entire system