Re: [RFC][PATCH 0/2] PM / Sleep: Extended control of suspend/hibernate interfaces

[Rafael J. Wysocki]

On Sunday, October 16, 2011, Alan Stern wrote:
> On Sat, 15 Oct 2011, Alan Stern wrote:
> 
> > Basically, what we need is a reliable way to intercept the existing
> > mechanisms for suspend/hibernate and to redirect the requests to the PM
> > daemon.  When the daemon is started up in "legacy" mode, it assumes
> > there is a legacy client (representing the entire set of
> > non-wakeup-aware programs) that always forbids suspend _except_ when
> > one of the old mechanisms is invoked.
> 
> The more I think about this, the better it seems.  In essence, it 
> amounts to "virtualizing" the existing PM interface.
> 
> Let's add /sys/power/manage, and make it single-open.

I'm not sure how to do that in sysfs.

Also I'm not sure what the real difference between /sys/power/manage
and my /sys/power/sleep_mode is (I could make /sys/power/sleep_mode
single-open too, if I knew how to do that).

> Whenever that file is open, writes to /sys/power/state and /dev/snapshot
> don't work normally; instead they get forwarded over /sys/power/manage (and 
> results get sent back).  Suspend is easy; hibernation (because of its 
> multi-step nature) will be more difficult.
> 
> 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.
> 
> 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).
> 
> 	5. Send the final status of the suspend command back to the
> 	   /sys/power/state writer.
> 
> Equivalent support for hibernation is left as an exercise for the 
> reader.

Hehe.  Quite a difficult one for that matter. :-)

> Obviously the PM daemon will need a secondary thread to accept new 
> incoming socket connections, and these connections will have to be 
> synchronized with the end of the iteration in step 2 (i.e., don't 
> accept new connections between the end of step 2 and the end of step 
> 4).
> 
> Initial startup of the daemon will be a little tricky, because it
> shouldn't start carrying out suspends until some clients have had a
> chance to connect.  For that matter, in non-legacy mode the daemon
> might not want to initiate suspends when there are no clients -- the
> system would never get anything done because it would go back to sleep
> as soon as the kernel finished processing each wakeup event.
> 
> 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.

Thanks,
Rafael
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