[RFC][PATCH] Documentation: Ask driver writers to provide suspend/resume support

From: Rafael J. Wysocki
Date: Sat Feb 17 2007 - 05:34:29 EST

Add a paragraph in Documentation/SubmittingDrivers requesting that the
susped/resume support be provided by new device drivers.

Add the document Documentation/power/drivers-testing.txt giving general
instructions for the testing of suspend/resume support in device drivers.

Documentation/SubmittingDrivers | 10 ++
Documentation/power/drivers-testing.txt | 146 ++++++++++++++++++++++++++++++++
2 files changed, 156 insertions(+)

Index: linux-2.6.20-git13/Documentation/SubmittingDrivers
--- linux-2.6.20-git13.orig/Documentation/SubmittingDrivers
+++ linux-2.6.20-git13/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-git13/Documentation/power/drivers-testing.txt
--- /dev/null
+++ linux-2.6.20-git13/Documentation/power/drivers-testing.txt
@@ -0,0 +1,146 @@
+Testing suspend and resume support in drivers
+ (C) 2007 Rafael J. Wysocki <rjw@xxxxxxx>
+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. Testing suspend to disk (STD)
+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 need to repeat the test at least a couple of times in
+a row for confidence. This is necessary because some problems only show up on a
+second attempt at suspending and resuming a driver. You should also test the
+"platform" and "shutdown" modes of suspend:
+# echo platform > /sys/power/disk
+# echo disk > /sys/power/state
+# 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) Test mode of STD
+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.
+It is also possible that a cycle can still fail after you have unloaded
+all modules. In that case, you would want to look in your kernel configuration
+for the drivers that can be compiled as modules (testing again with them as
+modules), and possibly also try boot time options such as "noapic" or "noacpi".
+b) Testing minimal configuration
+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 a driver statically
+compiled into the kernel and you can try to compile more drivers as modules,
+so that they can be tested individually. 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).
+c) Advanced debugging
+In case the STD does not work on your system even in the minimal configuration
+and compiling more drivers as modules is not practical or some modules cannot
+be unloaded, you can use one of the more advanced debugging techniques to find
+the problem. First, if there is a serial port in your box, you can set the
+CONFIG_DISABLE_CONSOLE_SUSPEND kernel configuration option and try to log kernel
+messages using the serial console. This may provide you with some information
+about the reasons of the suspend (resume) failure. Alternatively, it may be
+possible to use a FireWire port for debugging with firescope
+(ftp://ftp.firstfloor.org/pub/ak/firescope/). On i386 it is also possible to
+use the PM_TRACE mechanism documented in Documentation/s2ram.txt .
+2. Testing suspend to RAM (STR)
+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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