Where are the documents that describe Linux system's sleep/wake process (note, not the states but the process) and where is the code that implements entering and exiting them?
Wake-On-Keypress is probably the most common/standard action pair on GUIs. Windows is better at Wake-On-Mouse movement. Servers should be smooth with Wake-On-Lan.
Asking here to learn/get a handle on the process as it is done currently by a common Linux GUI - Gnome. Can't find much mention of it via search engining.
Most of that is implemented in the kernel (see e.g. here). Gnome suspends a system by making a request to the kernel -- either via a system call or via the interface in
/sys/power/state(see docs here).The ability to wake on a keypress is implemented primarily by your hardware -- which ultimately triggers an event that is processed by the kernel, which takes care of restoring system state and then Gnome picks up where it left off.