Message-Id: <201106260055.09150.rjw@sisk.pl>
Date:	Sun, 26 Jun 2011 00:55:08 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, Alan Stern <stern@...land.harvard.edu>,
	Greg KH <greg@...ah.com>, LKML <linux-kernel@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>,
	Kevin Hilman <khilman@...com>, linux-scsi@...r.kernel.org
Subject: [PATCH 1/3] PM / Runtime: Update documentation of interactions with system sleep

From: Rafael J. Wysocki <rjw@...k.pl>

The documents describing the interactions between runtime PM and
system sleep generally refer to the model in which the system sleep
state is entered through a global firmware or hardware operation.
As a result, some recommendations given in there are not entirely
suitable for systems in which this is not the case.  Update the
documentation take the existence of those systems into accout.

Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
Reviewed-by: Kevin Hilman <khilman@...com>
---
 Documentation/power/devices.txt    |    6 +++---
 Documentation/power/runtime_pm.txt |   27 +++++++++++++++++++--------
 2 files changed, 22 insertions(+), 11 deletions(-)

Index: linux-2.6/Documentation/power/runtime_pm.txt
===================================================================
--- linux-2.6.orig/Documentation/power/runtime_pm.txt
+++ linux-2.6/Documentation/power/runtime_pm.txt
@@ -537,9 +537,9 @@ suspend routine).  It may be necessary t
 in order to do so.  The same is true if the driver uses different power levels
 or other settings for run-time suspend and system sleep.
 
-During system resume, devices generally should be brought back to full power,
-even if they were suspended before the system sleep began.  There are several
-reasons for this, including:
+During system resume, the simplest approach is to bring all devices back to full
+power, even if they had been suspended before the system suspend began.  There
+are several reasons for this, including:
 
   * The device might need to switch power levels, wake-up settings, etc.
 
@@ -556,16 +556,27 @@ reasons for this, including:
   * Even though the device was suspended, if its usage counter was > 0 then most
     likely it would need a run-time resume in the near future anyway.
 
-  * Always going back to full power is simplest.
-
-If the device was suspended before the sleep began, then its run-time PM status
-will have to be updated to reflect the actual post-system sleep status.  The way
-to do this is:
+If the device had been suspended before the system suspend began and it's
+brought back to full power during resume, then its run-time PM status will have
+to be updated to reflect the actual post-system sleep status.  The way to do
+this is:
 
 	pm_runtime_disable(dev);
 	pm_runtime_set_active(dev);
 	pm_runtime_enable(dev);
 
+On some systems, however, system sleep is not entered through a global firmware
+or hardware operation.  Instead, all hardware components are put into low-power
+states directly by the kernel in a coordinated way.  Then, the system sleep
+state effectively follows from the states the hardware components end up in
+and the system is woken up from that state by a hardware interrupt or a similar
+mechanism entirely under the kernel's control.  As a result, the kernel never
+gives control away and the states of all devices during resume are precisely
+known to it.  If that is the case and none of the situations listed above takes
+place (in particular, if the system is not waking up from hibernation), it may
+be more efficient to leave the devices that had been suspended before the system
+suspend began in the suspended state.
+
 7. Generic subsystem callbacks
 
 Subsystems may wish to conserve code space by using the set of generic power
Index: linux-2.6/Documentation/power/devices.txt
===================================================================
--- linux-2.6.orig/Documentation/power/devices.txt
+++ linux-2.6/Documentation/power/devices.txt
@@ -604,7 +604,7 @@ state temporarily, for example so that i
 disabled.  This all depends on the hardware and the design of the subsystem and
 device driver in question.
 
-During system-wide resume from a sleep state it's best to put devices into the
-full-power state, as explained in Documentation/power/runtime_pm.txt.  Refer to
-that document for more information regarding this particular issue as well as
+During system-wide resume from a sleep state it's easiest to put devices into
+the full-power state, as explained in Documentation/power/runtime_pm.txt.  Refer
+to that document for more information regarding this particular issue as well as
 for information on the device runtime power management framework in general.

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