Re: [linux-pm] [RFC][PATCH] PM: Document requirements for basic PM support in drivers

[Igor Stoppa <igor.stoppa@nokia.com>]

On Tue, 2007-02-13 at 10:47 +1100, ext Nigel Cunningham wrote:
> Hi.
> 
> On Tue, 2007-02-13 at 00:23 +0100, Rafael J. Wysocki wrote:
> > 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
> 
> I would say "you need to repeat the test at least a couple of times...",
> perhaps adding something along the lines of "This is necessary because
> some problems only show up on a second attempt at suspending and
> resuming a driver. You can think of it as the driver coming back 'dazed
> and confused' after the first cycle, and only being properly killed by
> the second attempt."

"at least a couple of times" sounds very optimistic :-D

> 
> > +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.
> 
> 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 possibilities that can be modularised (testing again
> with them as modules), and possibly also try boot time options such as
> noapic or noacpi.

The first step, imho, would be to identify all the peripherals requiired
for a barebone configuration to run (i.e. serial console) and verify
that at least those can reliably go through several suspend/resume
cycles. Then the dicotomic approach can be used.
> 
> > +
> > +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
> 
> Oh. Perhaps some of the suggestions from above belong here?
> 
> > +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:
Why not first get the driver working against the barebone configuration?

> > +
> > +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.
> 
> Apart from the minor comments above, looks good to me.
> 
> Regards,
> 
> Nigel

-- 
Cheers, Igor

Igor Stoppa <igor.stoppa@nokia.com>
(Nokia M - OSSO /Helsinki Finland)