1========================
2How to get s2ram working
3========================
4
52006 Linus Torvalds
62006 Pavel Machek
7
81) Check suspend.sf.net, program s2ram there has long whitelist of
9   "known ok" machines, along with tricks to use on each one.
10
112) If that does not help, try reading tricks.txt and
12   video.txt. Perhaps problem is as simple as broken module, and
13   simple module unload can fix it.
14
153) You can use Linus' TRACE_RESUME infrastructure, described below.
16
17Using TRACE_RESUME
18~~~~~~~~~~~~~~~~~~
19
20I've been working at making the machines I have able to STR, and almost
21always it's a driver that is buggy. Thank God for the suspend/resume
22debugging - the thing that Chuck tried to disable. That's often the _only_
23way to debug these things, and it's actually pretty powerful (but
24time-consuming - having to insert TRACE_RESUME() markers into the device
25driver that doesn't resume and recompile and reboot).
26
27Anyway, the way to debug this for people who are interested (have a
28machine that doesn't boot) is:
29
30 - enable PM_DEBUG, and PM_TRACE
31
32 - use a script like this::
33
34	#!/bin/sh
35	sync
36	echo 1 > /sys/power/pm_trace
37	echo mem > /sys/power/state
38
39   to suspend
40
41 - if it doesn't come back up (which is usually the problem), reboot by
42   holding the power button down, and look at the dmesg output for things
43   like::
44
45	Magic number: 4:156:725
46	hash matches drivers/base/power/resume.c:28
47	hash matches device 0000:01:00.0
48
49   which means that the last trace event was just before trying to resume
50   device 0000:01:00.0. Then figure out what driver is controlling that
51   device (lspci and /sys/devices/pci* is your friend), and see if you can
52   fix it, disable it, or trace into its resume function.
53
54   If no device matches the hash (or any matches appear to be false positives),
55   the culprit may be a device from a loadable kernel module that is not loaded
56   until after the hash is checked. You can check the hash against the current
57   devices again after more modules are loaded using sysfs::
58
59	cat /sys/power/pm_trace_dev_match
60
61For example, the above happens to be the VGA device on my EVO, which I
62used to run with "radeonfb" (it's an ATI Radeon mobility). It turns out
63that "radeonfb" simply cannot resume that device - it tries to set the
64PLL's, and it just _hangs_. Using the regular VGA console and letting X
65resume it instead works fine.
66
67NOTE
68====
69pm_trace uses the system's Real Time Clock (RTC) to save the magic number.
70Reason for this is that the RTC is the only reliably available piece of
71hardware during resume operations where a value can be set that will
72survive a reboot.
73
74pm_trace is not compatible with asynchronous suspend, so it turns
75asynchronous suspend off (which may work around timing or
76ordering-sensitive bugs).
77
78Consequence is that after a resume (even if it is successful) your system
79clock will have a value corresponding to the magic number instead of the
80correct date/time! It is therefore advisable to use a program like ntp-date
81or rdate to reset the correct date/time from an external time source when
82using this trace option.
83
84As the clock keeps ticking it is also essential that the reboot is done
85quickly after the resume failure. The trace option does not use the seconds
86or the low order bits of the minutes of the RTC, but a too long delay will
87corrupt the magic value.
88