Re: [linux-pm] [PATCH 1/8] PM: Add suspend block api.

[Brian Swetland]

On Wed, May 5, 2010 at 4:47 PM, Tony Lindgren <tony@xxxxxxxxxxx> wrote:
> * Brian Swetland <swetland@xxxxxxxxxx> [100505 14:34]:
>> On Wed, May 5, 2010 at 2:12 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>> >>
>> >> Oh, like tell the modem that user mode has handled the ring event and
>> >> its ok to un-block?
>> >
>> > No, that's not how it works. ÂIt would go like this:
>> >
>> > Â Â Â ÂThe modem IRQ handler queues its event to the input subsystem.
>> > Â Â Â ÂAs it does so the input subsystem enables a suspend blocker,
>> > Â Â Â Âcausing the system to stay awake after the IRQ is done.
>
> How about instead the modem driver fails to suspend until it's done?
>
> Each driver could have a suspend_policy sysfs entry with options such
> as [ forced | safe ]. The default would be forced. Forced would
> be the current behaviour, while safe would refuse suspend until the
> driver is done processing.
>
>> > Â Â Â ÂThe user program enables its own suspend blocker before reading
>> > Â Â Â Âthe input queue. ÂWhen the queue is empty, the input subsystem
>> > Â Â Â Âreleases its suspend blocker.
>
> And also the input layer could refuse to suspend until it's done.
>
>> > Â Â Â ÂWhen the user program finishes processing the event, it
>> > Â Â Â Âreleases its suspend blocker. ÂNow the system can go back to
>> > Â Â Â Âsleep.
>
> And here the user space just tries to suspend again when it's done?
> It's not like you're trying to suspend all the time, so it should be
> OK to retry a few times.

We actually are trying to suspend all the time -- that's our basic
model -- suspend whenever we can when something doesn't prevent it.

>> > At no point does the user program have to communicate anything to the
>> > modem driver, and at no point does it have to do anything out of the
>> > ordinary except to enable and disable a suspend blocker.
>>
>> Exactly -- and you can use the same style of overlapping suspend
>> blockers with other drivers than input, if the input interface is not
>> suitable for the particular interaction.
>
> Would the suspend blockers still be needed somewhere in the example
> above?

How often would we retry suspending?

If we fail to suspend, don't we have to resume all the drivers that
suspended before the one that failed?  (Maybe I'm mistaken here)

With the suspend block model we know the moment we're capable of
suspending and then can suspend at that moment.  Continually trying to
suspend seems like it'd be inefficient power-wise (we're going to be
doing a lot more work as we try to suspend over and over, or we're
going to retry after a timeout and spend extra time not suspended).

We can often spend minutes (possibly many) at a time preventing
suspend when the system is doing work that would be interrupted by a
full suspend.

Brian
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