Merge tag 'docs-6.4-2' of git://git.lwn.net/linux

[linux-block.git] / Documentation / power / s2ram.rst

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How to get s2ram working
========================

2006 Linus Torvalds
2006 Pavel Machek

1) Check suspend.sf.net, program s2ram there has long whitelist of
   "known ok" machines, along with tricks to use on each one.

2) If that does not help, try reading tricks.txt and
   video.txt. Perhaps problem is as simple as broken module, and
   simple module unload can fix it.

3) You can use Linus' TRACE_RESUME infrastructure, described below.

Using TRACE_RESUME
~~~~~~~~~~~~~~~~~~

I've been working at making the machines I have able to STR, and almost
always it's a driver that is buggy. Thank God for the suspend/resume
debugging - the thing that Chuck tried to disable. That's often the _only_
way to debug these things, and it's actually pretty powerful (but
time-consuming - having to insert TRACE_RESUME() markers into the device
driver that doesn't resume and recompile and reboot).

Anyway, the way to debug this for people who are interested (have a
machine that doesn't boot) is:

 - enable PM_DEBUG, and PM_TRACE

 - use a script like this::

	#!/bin/sh
	sync
	echo 1 > /sys/power/pm_trace
	echo mem > /sys/power/state

   to suspend

 - if it doesn't come back up (which is usually the problem), reboot by
   holding the power button down, and look at the dmesg output for things
   like::

	Magic number: 4:156:725
	hash matches drivers/base/power/resume.c:28
	hash matches device 0000:01:00.0

   which means that the last trace event was just before trying to resume
   device 0000:01:00.0. Then figure out what driver is controlling that
   device (lspci and /sys/devices/pci* is your friend), and see if you can
   fix it, disable it, or trace into its resume function.

   If no device matches the hash (or any matches appear to be false positives),
   the culprit may be a device from a loadable kernel module that is not loaded
   until after the hash is checked. You can check the hash against the current
   devices again after more modules are loaded using sysfs::

	cat /sys/power/pm_trace_dev_match

For example, the above happens to be the VGA device on my EVO, which I
used to run with "radeonfb" (it's an ATI Radeon mobility). It turns out
that "radeonfb" simply cannot resume that device - it tries to set the
PLL's, and it just _hangs_. Using the regular VGA console and letting X
resume it instead works fine.

NOTE
====
pm_trace uses the system's Real Time Clock (RTC) to save the magic number.
Reason for this is that the RTC is the only reliably available piece of
hardware during resume operations where a value can be set that will
survive a reboot.

pm_trace is not compatible with asynchronous suspend, so it turns
asynchronous suspend off (which may work around timing or
ordering-sensitive bugs).

Consequence is that after a resume (even if it is successful) your system
clock will have a value corresponding to the magic number instead of the
correct date/time! It is therefore advisable to use a program like ntp-date
or rdate to reset the correct date/time from an external time source when
using this trace option.

As the clock keeps ticking it is also essential that the reboot is done
quickly after the resume failure. The trace option does not use the seconds
or the low order bits of the minutes of the RTC, but a too long delay will
corrupt the magic value.