Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

[Alan Stern]

On Tue, 1 Jun 2010, Neil Brown wrote:

> > With wakeup events the problem isn't so bad.  Wakeup events are always
> > noticed, and if the system is designed properly they will either abort
> > a suspend-in-progress or else cause the system to resume as soon as the
> > suspend is complete.  (Linux is not yet properly designed in this
> > sense.)
> 
> This is exactly the issue.  Linux doesn't get this right.  A wakeup that is
> processed by the driver just before suspend may not get all the way out to
> it's final destination (possibly in meat-space) until after some other event
> wakes the device.  It is not possible to guarantee full delivery of a wakeup
> event with Linux in its current state.  This is a bug.

That wasn't what I meant.  An important requirement is that a wakeup 
event received after the device has been suspended, but before the 
system suspend is complete, should cause the system suspend to abort.  
This is one of the things suspend blockers were meant to help with.  In 
theory it should already work correctly -- but it doesn't.  That's a 
real bug.

The other things you're talking about aren't really bugs.  That is,
they are things which the kernel _ought_ to do but it wasn't _intended_
to do.  They are misfeatures: failures of design, not failures of 
execution.

> I don't think I'm missing that.  Wakeup events are the only events of
> interest.  But wakeup events are more than just low-level hardware events.
> They are also the virtual events that are a direct consequence of the
> original hardware event.

This is a matter of semantics and it is highly debatable.

> A UART asserts 'data ready' which is routed to an IO interrupt line that
> wakes the device.  This is a wakeup event.

Yes.

> So are the bits that appear on the data line which represent the data
> So are the characters that appear on the serial port
> So is the "here is a message from the GSM processor" event
> So is the "there is a new TXT message" event
> So is the "'beep beep beep' out the speaker" event

None of those things can cause the system to wake up after it is
suspended.  Indeed, they can't happen at all if the CPU isn't running
(unless the GSM processor runs autonomously -- let's ignore such
details).  Hence it makes little sense to call them "wakeup" events.

> All these events are wakeup events, and should abort or instant-resume the
> system.  Currently only the first can be guaranteed to do this (... and maybe
> the second, depending on the details of the implementation).
> suspend-blocks effectively pass a lock from one event to the next to ensure
> that all of these event block the suspend.  There are various other ways to
> achieve the same thing, and I think it can be done with no significant change
> to the API.  But some how we need to allow all of these events to be
> linked wake-up events, not just the first one.

That's not entirely clear.  Yes, for Android's use case that's what you 
want.  But in other situations maybe you don't.  Consider the example 
of someone who closes the lid of their laptop and throws it in a 
backpack.  The computer should suspend immediately; it shouldn't be 
delayed by these "virtual wakeup" events.

Alan Stern

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