Date:	Mon, 9 Aug 2010 21:28:04 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Matthew Garrett <mjg59@...f.ucam.org>, david@...g.hm,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	pavel@....cz, florian@...kler.org, swetland@...gle.com,
	peterz@...radead.org, tglx@...utronix.de, alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

2010/8/9 Rafael J. Wysocki <rjw@...k.pl>:
> On Monday, August 09, 2010, Arve Hjønnevåg wrote:
>> 2010/8/8 Rafael J. Wysocki <rjw@...k.pl>:
>> > On Saturday, August 07, 2010, Arve Hjønnevåg wrote:
>> >> 2010/8/7 Arve Hjønnevåg <arve@...roid.com>:
>> >> > 2010/8/7 Rafael J. Wysocki <rjw@...k.pl>:
>> >> >> On Saturday, August 07, 2010, Arve Hjønnevåg wrote:
>> >> >>> 2010/8/6 Alan Stern <stern@...land.harvard.edu>:
>> >> >>> > On Thu, 5 Aug 2010, Arve Hjønnevåg wrote:
>> >> ...
>> >> >>> >> total_time, total time the wake lock has been active. This one should
>> >> >>> >> be obvious.
>> >> >>> >
>> >> >>> > Also easily added.
>> >> >>> >
>> >> >>> Only with a handle passed to all the calls.
>> >> >>
>> >> >> Well, I'm kind of tired of this "my solution is the only acceptable one"
>> >> >> mindset.  IMHO, it's totally counter productive.
>> >> >>
>> >> >
>> >> > How do you propose to track how long a driver has blocked suspend when
>> >> > you have an unblock call that takes no arguments.
>> >> >
>> >>
>> >> Also, I did not not see a response to my question about why you don't
>> >> want to pass a handle.
>> >
>> > It doesn't really matter what I personally want.  In fact, I'm not totally
>> > opposed to that idea, although there are disadvantages (eg. a "handle"
>> > would really mean a pointer to an object with certain life cycle that needs to
>> > be managed by the caller and it's not that clear to me who should manage the
>> > objects that the PCI wakeup code would pass to pm_wakeup_event(), for one
>>
>> Wouldn't a single global handle work for the way you are handling pci
>> wakeup events?
>
> Not really, because I'd like to know the number of wakeups associated with
> the given device.
>

For debugging purposes right? I'm not sure automatically counting
wakeup events from pm_stay_awake and pm_wakeup_event is the best way
to do this. To avoid race conditions these calls have to be made for
every event that could also be a wakeup event, which means the actual
wakeup event easily gets lost in the noise. This is why I had a
separate wakeup count that only increments on the fist call to
wake_lock after each suspend, but this did not work that well either.

>> It looked like you just reset a global timeout every time a pci wakeup event
>> occurs.
>
> We bump up the per-device counter of wakeup events in addition to that.
>
>> > example).  I sent a pull request for your original patchset to Linus after all. :-)
>> >
>> > I said I didn't think "it would fly", meaning that I was afraid the other kernel
>> > developers wouldn't like that change.
>> >
>> > The reason why I think so is that you'd like to add a whole new infrastructure
>> > whose only purpose would be debugging that would only be useful to systems
>> > using opportunistic suspend.  That, however, is only Android right now and it
>> > cannot use the mainline kernel for other reasons, so basically we would add
>> > infrastructure that's useful to no one.
>> >
>>
>> I'm not sure what you mean by this. The debugging is useful for anyone
>> using the api, not just Android, and a handle is also needed to mix
>> timeouts and pm_relax.
>
> The purpose of the debugging would be to be able to figure out why the system
> is staying in the working state, which is only relevant for systems that use
> opportunistic suspend.
>

Only if the drivers do not have bugs. A driver calling pm_say_awake
without a matching pm_relax call will prevent any race free suspend
from succeeding.

> If opportunistic suspend isn't used, it makes sense to ask which
> device caused suspend (initiated by the user) to be aborted and for this
> purpose it is sufficient to count wakeup events associated with each
> device (you need to preserve the pre-suspend values of these counters, but
> that can be done by a user space power manager just fine).
>
>> The handle can be the device, but some drivers need several handles per
>> device.
>
> That depends on how precise the collected debug information should be and
> that, in turn, depends on what it's going to be used for.
>

It is not just debug information. Drivers that mix wake_lock_timeout
and wake_unlock do not map to the current api.

> Anyway, as I said I'm not opposed to the idea of using a special type of
> objects for collecting debug information on wakeup events, so please free to
> submit patches modifying the current mainline kernel code in that direction.
>

How do you prefer to handle your pci wakeup events? Add a handle to
every device or pci device? Or use a global handle to avoid the race
and report wakeup events for debugging separately?

-- 
Arve Hjønnevåg
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