Message-Id: <201110172327.18954.rjw@sisk.pl>
Date:	Mon, 17 Oct 2011 23:27:18 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	NeilBrown <neilb@...e.de>,
	Linux PM list <linux-pm@...r.kernel.org>,
	mark gross <markgross@...gnar.org>,
	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [RFC][PATCH 0/2] PM / Sleep: Extended control of suspend/hibernate interfaces

On Monday, October 17, 2011, Alan Stern wrote:
> On Sun, 16 Oct 2011, Rafael J. Wysocki wrote:
...
> > > The only important requirement is that processes can use poll system 
> > > calls to wait for wakeup events.  This may not always be true (consider 
> > > timer expirations, for example), but we ought to be able to make some 
> > > sort of accomodation.
> 
> This requirement remains somewhat tricky.  Can we guarantee it?  It 
> comes down to two things.  When an event occurs that will cause a 
> program to want to keep the system awake:
> 
>      A. The event must be capable of interrupting a poll system
> 	call.  I don't think it matters whether this interruption
> 	takes the form of a signal or of completing the system call.
> 
>      B. The program must be able to detect, in a non-blocking way, 
> 	whether the event has occurred.
> 
> Of course, any event that adds data to an input queue will be okay.  
> But I don't know what other sorts of things we will have to handle.

Well, wakealarms don't do that, for one exaple.  Similarly for WoL through
a magic packet AFAICS.  Similarly for "a cable has been plugged in"
type of events.

> > > The PM daemon will communicate with its clients over a Unix-domain
> > > socket.  The protocol can be extremely simple: The daemon sends a byte
> > > to the client when it wants to sleep, and the client sends the byte
> > > back when it is ready to allow the system to go to sleep.  There's
> > > never more than one byte outstanding at any time in either direction.
> > > 
> > > The clients would be structured like this:
> > > 
> > > 	Open a socket connection to the PM daemon.
> > > 
> > > 	Loop:
> > > 
> > > 		Poll on possible events and the PM socket.
> > > 
> > > 		If any events occurred, handle them.
> > > 
> > > 		Otherwise if a byte was received from the PM daemon,
> > > 		send it back.
> > > 
> > > In non-legacy mode, the PM daemon's main loop is also quite simple:
> > > 
> > > 	1. Read /sys/power/wakeup_count.
> > > 
> > > 	2. For each client socket:
> > > 
> > > 		If a response to the previous transmission is still
> > > 		pending, wait for it.
> > > 
> > > 		Send a byte (the data can be just a sequence number).
> > > 
> > > 		Wait for the byte to be echoed back.
> > > 
> > > 	3. Write /sys/power/wakeup_count.
> > > 
> > > 	4. Write a sleep command to /sys/power/manage.
> > > 
> > > A timeout can be added to step 2 if desired, but in this mode it isn't
> > > needed.
> > > 
> > > With legacy support enabled, we probably will want something like a 
> > > 1-second timeout for step 2.  We'll also need an extra step at the 
> > > beginning and one at the end:
> > > 
> > > 	0. Wait for somebody to write "standy" or "mem" to 
> > > 	   /sys/power/state (received via the /sys/power/manage file).
> 
> This would be replaced by: Wait for a sleep request to be received over 
> the legacy interface.
> 
> > > 	5. Send the final status of the suspend command back to the
> > > 	   /sys/power/state writer.
> 
> I haven't received any comments on these designs so far.  They seem
> quite simple and adequate for what we want.  We may want to make the PM
> daemon also responsible for keeping track of RTC wakeup alarm requests,
> as Neil pointed out; that shouldn't be hard to add on.

Well, it's not a bad idea in principle and I think it will work, so long
as we can ensure that the PM daemon will be the only process using
suspend/hibernate interfaces.

Apart from this, steps 1.-3. represent a loop with quite a bit of socket
traffic if wakeup events occur relatively often (think someone typing on
a keyboard being a wakeup device or moving a mouse being a wakeup device).

> > > Equivalent support for hibernation is left as an exercise for the 
> > > reader.
> > 
> > Hehe.  Quite a difficult one for that matter. :-)
> 
> That's another thing we need to think about more carefully.  How 
> extravagant do we want to make the wakeup/hibernation interaction?  My 
> own feeling is: as little as possible (whatever that amounts to).

I don't agree with that.  In my opinion all system sleep interfaces should
be handled.

> > > This really seems like it could work, and it wouldn't be tremendously 
> > > complicated.  The only changes needed in the kernel would be the 
> > > "virtualization" (or forwarding) mechanism for legacy support.
> > 
> > Yes, it could be made work, just as the hibernate user space interface,
> > but would it be really convenient to use?  I have some doubts.
> 
> In terms of integration with current systems (and without the
> virtualization), it should be very easy.  There will be a new daemon to
> run when the system starts up, and a new program that will communicate
> with that daemon (or will write to /sys/power/state if the daemon isn't
> available).  That's all.
> 
> In terms of writing wakeup-aware clients, it's a little hard to say in 
> the absence of any examples.  The client protocol described above 
> shouldn't be too hard to use, especially if a wakeup library can be 
> provided.
> 
> For something like a firmware update program, all the program has to do
> is open a connection to the PM daemon before writing the new firmware.  
> Nothing more -- if the program does not send any data over the socket
> then the PM daemon will not allow sleep requests to go through.
> 
> Of course, the Android people have the most experience with this sort
> of thing.  In an earlier discussion with Arve, he expressed some
> concerns about getting the PM daemon started early enough (obviously it
> needs to be running before any of its clients) and the fact that the
> daemon would have to be multi-threaded.  I got the feeling that he was
> complaining just for the sake of complaining, not because these things
> would present any serious problems.
> 
> Converting the programs that currently use Android's userspace
> wakelocks might be somewhat more difficult.  Simply releasing a
> wakelock would no longer be sufficient; a program would need to respond
> to polls from the PM daemon whenever it was willing to let the system
> go to sleep.

I honestly don't think it will be very practical to expect all of the
existing Androig applications to be reworked this way ...

Thanks,
Rafael
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