linux/Documentation/power/s2ram.txt
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   1                        How to get s2ram working
   2                        ~~~~~~~~~~~~~~~~~~~~~~~~
   3                        2006 Linus Torvalds
   4                        2006 Pavel Machek
   5
   61) Check suspend.sf.net, program s2ram there has long whitelist of
   7   "known ok" machines, along with tricks to use on each one.
   8
   92) If that does not help, try reading tricks.txt and
  10   video.txt. Perhaps problem is as simple as broken module, and
  11   simple module unload can fix it.
  12
  133) You can use Linus' TRACE_RESUME infrastructure, described below.
  14
  15                      Using TRACE_RESUME
  16                      ~~~~~~~~~~~~~~~~~~
  17
  18I've been working at making the machines I have able to STR, and almost
  19always it's a driver that is buggy. Thank God for the suspend/resume
  20debugging - the thing that Chuck tried to disable. That's often the _only_
  21way to debug these things, and it's actually pretty powerful (but
  22time-consuming - having to insert TRACE_RESUME() markers into the device
  23driver that doesn't resume and recompile and reboot).
  24
  25Anyway, the way to debug this for people who are interested (have a
  26machine that doesn't boot) is:
  27
  28 - enable PM_DEBUG, and PM_TRACE
  29
  30 - use a script like this:
  31
  32        #!/bin/sh
  33        sync
  34        echo 1 > /sys/power/pm_trace
  35        echo mem > /sys/power/state
  36
  37   to suspend
  38
  39 - if it doesn't come back up (which is usually the problem), reboot by
  40   holding the power button down, and look at the dmesg output for things
  41   like
  42
  43        Magic number: 4:156:725
  44        hash matches drivers/base/power/resume.c:28
  45        hash matches device 0000:01:00.0
  46
  47   which means that the last trace event was just before trying to resume
  48   device 0000:01:00.0. Then figure out what driver is controlling that
  49   device (lspci and /sys/devices/pci* is your friend), and see if you can
  50   fix it, disable it, or trace into its resume function.
  51
  52   If no device matches the hash (or any matches appear to be false positives),
  53   the culprit may be a device from a loadable kernel module that is not loaded
  54   until after the hash is checked. You can check the hash against the current
  55   devices again after more modules are loaded using sysfs:
  56
  57        cat /sys/power/pm_trace_dev_match
  58
  59For example, the above happens to be the VGA device on my EVO, which I
  60used to run with "radeonfb" (it's an ATI Radeon mobility). It turns out
  61that "radeonfb" simply cannot resume that device - it tries to set the
  62PLL's, and it just _hangs_. Using the regular VGA console and letting X
  63resume it instead works fine.
  64
  65NOTE
  66====
  67pm_trace uses the system's Real Time Clock (RTC) to save the magic number.
  68Reason for this is that the RTC is the only reliably available piece of
  69hardware during resume operations where a value can be set that will
  70survive a reboot.
  71
  72Consequence is that after a resume (even if it is successful) your system
  73clock will have a value corresponding to the magic number instead of the
  74correct date/time! It is therefore advisable to use a program like ntp-date
  75or rdate to reset the correct date/time from an external time source when
  76using this trace option.
  77
  78As the clock keeps ticking it is also essential that the reboot is done
  79quickly after the resume failure. The trace option does not use the seconds
  80or the low order bits of the minutes of the RTC, but a too long delay will
  81corrupt the magic value.
  82
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