1 files changed, 87 insertions, 0 deletions
diff --git a/Documentation/power/s2ram.rst b/Documentation/power/s2ram.rst
new file mode 100644
index 000000000..d739aa7c7
--- /dev/null
+++ b/Documentation/power/s2ram.rst
@@ -0,0 +1,87 @@
+========================
+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.