Re: Change behaviour when tracing ... nasty trap (was Re: [PATCH] PM/Trace: get rid of synchronous resume limit during PM trace)

Pavel Machek
2015-01-26T13:43:10Z

Document pm_tracing actually affecting suspend in non-trivial way.

Signed-off-by: Pavel Machek <pavel@ucw.cz>

---

On Mon 2015-01-26 14:41:02, Rafael J. Wysocki wrote:
> On Monday, January 26, 2015 12:05:16 PM Pavel Machek wrote:
> > On Mon 2015-01-26 10:39:04, Liu, Chuansheng wrote:

> > > > > @@ -517,8 +517,7 @@ static int device_resume_noirq(struct device *dev,
> > > > pm_message_t state, bool asyn
> > > > >
> > > > > static bool is_async(struct device *dev)
> > > > > {
> > > > > - return dev->power.async_suspend && pm_async_enabled
> > > > > - && !pm_trace_is_enabled();
> > > > > + return dev->power.async_suspend && pm_async_enabled;
> > > > > }
> > > > >
> > > >
> > > > Actually... whoever did the original patch was evil person. Changing
> > > > behaviour when tracing is requested is evil, evil, evil. Git blame
> > > > tells me
> > > >
> > > > Signed-off-by: Chuansheng Liu <chuansheng.liu@intel.com>
> > > >
> > > > went to the dark side.
> > >
> > > Although I didn't get where is something wrong, but the is_async() is not created by my commit,
> > > it is from commit (PM: Start asynchronous resume threads upfront), I just moved it ahead.
> > >
> > > And like other phases, I added it into resum/suspend_noirq()...
> >
> > I see, blame blamed wrong person. It looks like Rafael is evil:
> >
> > commit 97df8c12995c5bac73e3bfeea4c5be155c1f4401
> > Author: Rafael J. Wysocki <rjw@sisk.pl>
> > Date: Sat Jan 23 22:25:31 2010 +0100
> >
> > PM: Start asynchronous resume threads upfront
>
> This only means we won't use asyc suspend/resume at all when the RTC-based
> resume debug is enabled, because it wouldn't make sense (the RTC-based
> debug requires strict ordering of callbacks between devices or we may find
> that device A hanged the resume while actually device B that was running in
> parallel with A did that).
>
> And I shouldn't even need to explain this ... Sad.

Well, I forgot that pm_trace_is_enabled() is the simple, RTC based
one, and believe it would be worth a comment...

diff --git a/Documentation/power/s2ram.txt b/Documentation/power/s2ram.txt
index 1bdfa04..4685aee 100644
--- a/Documentation/power/s2ram.txt
+++ b/Documentation/power/s2ram.txt
@@ -69,6 +69,10 @@ 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

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