[RFC][PATCH] PM: Document requirements for basic PM support in drivers

["Rafael J. Wysocki" <rjw@sisk.pl>]

Hi,

Here's my attempt to document the requirements with respect to the basic PM
support in drivers and the testing of that.  Comments welcome.

Greetings,
Rafael

---
 Documentation/SubmittingDrivers         |   10 ++
 Documentation/power/drivers-testing.txt |  119 ++++++++++++++++++++++++++++++++
 2 files changed, 129 insertions(+)

Index: linux-2.6.20-git4/Documentation/SubmittingDrivers
===================================================================
--- linux-2.6.20-git4.orig/Documentation/SubmittingDrivers
+++ linux-2.6.20-git4/Documentation/SubmittingDrivers
@@ -87,6 +87,16 @@ Clarity:	It helps if anyone can see how 
 		driver that intentionally obfuscates how the hardware works
 		it will go in the bitbucket.
 
+PM support:	Since Linux is used on many portable and desktop systems, your
+		driver is likely to be used on such a system and therefore it
+		should support basic power management by implementing, if
+		necessary, the .suspend and .resume methods used during the
+		system-wide suspend and resume transitions.  You should verify
+		that your driver correctly handles the suspend and resume, but
+		if you are unable to ensure that, please at least define the
+		.suspend method returning the -ENOSYS ("Function not
+		implemented") error.
+
 Control:	In general if there is active maintainance of a driver by
 		the author then patches will be redirected to them unless
 		they are totally obvious and without need of checking.
Index: linux-2.6.20-git4/Documentation/power/drivers-testing.txt
===================================================================
--- /dev/null
+++ linux-2.6.20-git4/Documentation/power/drivers-testing.txt
@@ -0,0 +1,119 @@
+Testing suspend and resume support in drivers
+	(C) 2007 Rafael J. Wysocki <rjw@sisk.pl>
+
+Unfortunately, to effectively test the support for the system-wide suspend and
+resume transitions in a driver, it is necessary to suspend and resume a fully
+functional system with this driver loaded.  Moreover, that should be done many
+times, preferably many times in a row, and separately for the suspend to disk
+(STD) and the suspend to RAM (STR) transitions, because each of these cases
+involves different ordering of operations and different interactions with the
+machine's BIOS.
+
+Of course, for this purpose the test system has to be known to suspend and
+resume without the driver being tested.  Thus, if possible, you should first
+resolve all suspend/resume-related problems in the test system before you start
+testing the new driver.
+
+I. Preparing the test system
+
+1. To verify that the STD works, you can try to suspend in the "reboot" mode:
+
+# echo reboot > /sys/power/disk
+# echo disk > /sys/power/state
+
+and the system should suspend, reboot, resume and get back to the command prompt
+where you have started the transition.  If that happens, the STD is most likely
+to work correctly, but you can repeat the test a couple of times in a row for
+confidence.  You should also test the "platform" and "shutdown" modes of
+suspend:
+
+# echo platform > /sys/power/disk
+# echo disk > /sys/power/state
+
+or
+
+# echo shutdown > /sys/power/disk
+# echo disk > /sys/power/state
+
+in which cases you will have to press the power button to make the system
+resume.  If that works, you are ready to test the STD with the new driver
+loaded.  Otherwise, you have to identify what is wrong.
+
+a) To verify if there are any drivers that cause problems you can run the STD
+in the test mode:
+
+# echo test > /sys/power/disk
+# echo disk > /sys/power/state
+
+in which case the system should freeze tasks, suspend devices, disable nonboot
+CPUs (if any), wait for 5 seconds, enable nonboot CPUs, resume devices, thaw
+tasks and return to your command prompt.  If that fails, most likely there is
+a driver that fails to either suspend or resume (in the latter case the system
+may hang or be unstable after the test, so please take that into consideration).
+To find this driver, you can carry out a binary search according to the rules:
+- if the test fails, unload a half of the drivers currently loaded and repeat
+(that would probably involve rebooting the system, so always note what drivers
+have been loaded before the test),
+- if the test succeeds, load a half of the drivers you have unloaded most
+recently and repeat.
+
+Once you have found the failing driver (there can be more than just one of
+them), you have to unload it every time before the STD transition.  In that case
+please make sure to report the problem with the driver.
+
+b) If the test mode of STD works, you can boot the system with "init=/bin/bash"
+and attempt to suspend in the "reboot", "shutdown" and "platform" modes.  If
+that does not work, there probably is a problem with one of the low level
+drivers and you generally cannot do much about it except for reporting it
+(fortunately, that does not happen very often these days).  Otherwise, there is
+a problem with a modular driver and you can find it by loading a half of the
+modules you normally use and binary searching in accordance with the algorithm:
+- if there are n modules loaded and the attempt to suspend and resume fails,
+unload n/2 of the modules and try again (that would probably involve rebooting
+the system),
+- if there are n modules loaded and the attempt to suspend and resume succeeds,
+load n/2 modules more and try again.
+
+Again, if you find the offending module(s), it(they) must be unloaded every time
+before the STD transition, and please report the problem with it(them).
+
+2. To verify that the STR works, it is generally more convenient to use the
+s2ram tool available from http://suspend.sf.net and documented at
+http://en.opensuse.org/s2ram .  However, before doing that it is recommended to
+carry out the procedure described in section 1.
+
+Assume you have resolved the problems with the STD and you have found some
+failing drivers.  These drivers are also likely to fail during the STR or
+during the resume, so it is better to unload them every time before the STR
+transition.  Now, you can follow the instructions at
+http://en.opensuse.org/s2ram to test the system, but if it does not work
+"out of the box", you may need to boot it with "init=/bin/bash" and test
+s2ram in the minimal configuration.  In that case, you may be able to search
+for failing drivers by following the procedure analogous to the one described in
+1b).  If you find some failing drivers, you will have to unload them every time
+before the STR transition (ie. before you run s2ram), and please report the
+problem with them.
+
+II. Testing the driver
+
+Once you have resolved the suspend/resume-related problems with your test system
+without the new driver, you are ready to test it:
+
+1. Build the driver as a module, load it and try the STD in the test mode
+(cf. 1a)).
+
+2. Compile the driver directly into the kernel and try the STD in the test mode
+(cf. 1a)).
+
+3. Build the driver as a module, load it and attempt to suspend to disk in the
+"reboot", "shutdown" and "platform" modes (cf. 1).
+
+4. Compile the driver directly into the kernel and attempt to suspend to disk in
+the "reboot", "shutdown" and "platform" modes (cf. 1).
+
+5. Build the driver as a module, load it and attempt to run s2ram (cf. 2).
+
+6. Compile the driver directly into the kernel and attempt to run s2ram (cf. 2).
+
+Each of the above tests should be repeated several times and if any of them
+fails, the driver cannot be regarded as suspend/resume-safe.
-
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