Re: [Update][PATCH 7/8] PM / Domains: System-wide transitions support for generic domains (v3)

On Thursday, June 23, 2011, Alan Stern wrote:

On Thu, 23 Jun 2011, Rafael J. Wysocki wrote:

quoted

Well, let's say this part of the documentation is slightly outdated.

It basically refers to the model in which system suspend is a separate global
hardware or firmware operation, so the state of devices may be changed by the
BIOS or whatever takes over control in the meantime.  In that case the kernel
has to ensure that the states of devices are consistent with what it thinks
about them and the simplest way to achieve that is to put the devices to
full power during resume (and back to low power if that's desirable).

However, in the case of the systems this patchset is intended for system
suspend is achieved by putting various hardware components into low-power
states directly in a coordinated way and the system sleep state effectively
follows from the low-power states the hardware components end up in.  The
system is woken up from this state by an interrupt or another mechanism under
the kernel's control.  As a result, the kernel never gives control away, so
the state of devices after the resume is precisely known to it.
In consequence, it need not ensure that the state of devices is consistent with
its view, because it knows that this is the case. :-)

That's true for system suspend, but it's probably not true for
hibernation, even in embedded systems.  Of course, many embedded
systems don't use hibernation at all -- but those that do should be
aware of this issue.

quoted

So the documentation should be updated to say what hardware model it is
referring to.

It might be worthwhile to include a little warning about the difference 
between suspend and hibernate.

Well, there already is one:

"* The driver's idea of the device state may not agree with the device's
    physical state.  This can happen during resume from hibernation."

(in Documentation/power/runtime_pm.txt).  Also, the new text in the patch
https://lkml.org/lkml/2011/6/23/200 I've just sent says literally:

"If that is the case and none of the situations listed above takes place, it
 may be more efficient to leave the devices that had been suspended before
 the system sleep began in the suspended state."

where the "situations listed above" include it.

Do you think that's not sufficient?

Rafael