PM / sleep: Update device PM documentation to cover direct_complete

Update the device PM documentation in devices.txt and runtime_pm.txt
to reflect the changes in the system suspend and resume handling
related to the introduction of the new power.direct_complete flag.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>

2 files changed, 47 insertions, 4 deletions

diff --git a/Documentation/power/devices.txt b/Documentation/power/devices.txt
index 47d46dff70f7..d172bce0fd49 100644
--- a/Documentation/power/devices.txt
+++ b/Documentation/power/devices.txt
@@ -2,6 +2,7 @@ Device Power Management
 
 Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
 Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
+Copyright (c) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
 
 
 Most of the code in Linux is device drivers, so most of the Linux power
@@ -326,6 +327,20 @@ the phases are:
         driver in some way for the upcoming system power transition, but it
         should not put the device into a low-power state.
 
+        For devices supporting runtime power management, the return value of the
+        prepare callback can be used to indicate to the PM core that it may
+        safely leave the device in runtime suspend (if runtime-suspended
+        already), provided that all of the device's descendants are also left in
+        runtime suspend.  Namely, if the prepare callback returns a positive
+        number and that happens for all of the descendants of the device too,
+        and all of them (including the device itself) are runtime-suspended, the
+        PM core will skip the suspend, suspend_late and suspend_noirq suspend
+        phases as well as the resume_noirq, resume_early and resume phases of
+        the following system resume for all of these devices.   In that case,
+        the complete callback will be called directly after the prepare callback
+        and is entirely responsible for bringing the device back to the
+        functional state as appropriate.
+
     2.  The suspend methods should quiesce the device to stop it from performing
         I/O.  They also may save the device registers and put it into the
         appropriate low-power state, depending on the bus type the device is on,
@@ -400,12 +415,23 @@ When resuming from freeze, standby or memory sleep, the phases are:
         the resume callbacks occur; it's not necessary to wait until the
         complete phase.
 
+        Moreover, if the preceding prepare callback returned a positive number,
+        the device may have been left in runtime suspend throughout the whole
+        system suspend and resume (the suspend, suspend_late, suspend_noirq
+        phases of system suspend and the resume_noirq, resume_early, resume
+        phases of system resume may have been skipped for it).  In that case,
+        the complete callback is entirely responsible for bringing the device
+        back to the functional state after system suspend if necessary.  [For
+        example, it may need to queue up a runtime resume request for the device
+        for this purpose.]  To check if that is the case, the complete callback
+        can consult the device's power.direct_complete flag.  Namely, if that
+        flag is set when the complete callback is being run, it has been called
+        directly after the preceding prepare and special action may be required
+        to make the device work correctly afterward.
+
 At the end of these phases, drivers should be as functional as they were before
 suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
-gated on.  Even if the device was in a low-power state before the system sleep
-because of runtime power management, afterwards it should be back in its
-full-power state.  There are multiple reasons why it's best to do this; they are
-discussed in more detail in Documentation/power/runtime_pm.txt.
+gated on.
 
 However, the details here may again be platform-specific.  For example,
 some systems support multiple "run" states, and the mode in effect at
diff --git a/Documentation/power/runtime_pm.txt b/Documentation/power/runtime_pm.txt
index 5f96daf8566a..e1bee8a4aaac 100644
--- a/Documentation/power/runtime_pm.txt
+++ b/Documentation/power/runtime_pm.txt
@@ -2,6 +2,7 @@ Runtime Power Management Framework for I/O Devices
 
 (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
 (C) 2010 Alan Stern <stern@rowland.harvard.edu>
+(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
 
 1. Introduction
 
@@ -444,6 +445,10 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
   bool pm_runtime_status_suspended(struct device *dev);
     - return true if the device's runtime PM status is 'suspended'
 
+  bool pm_runtime_suspended_if_enabled(struct device *dev);
+    - return true if the device's runtime PM status is 'suspended' and its
+      'power.disable_depth' field is equal to 1
+
   void pm_runtime_allow(struct device *dev);
     - set the power.runtime_auto flag for the device and decrease its usage
       counter (used by the /sys/devices/.../power/control interface to
@@ -644,6 +649,18 @@ 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.
 
+To this end, the PM core provides a mechanism allowing some coordination between
+different levels of device hierarchy.  Namely, if a system suspend .prepare()
+callback returns a positive number for a device, that indicates to the PM core
+that the device appears to be runtime-suspended and its state is fine, so it
+may be left in runtime suspend provided that all of its descendants are also
+left in runtime suspend.  If that happens, the PM core will not execute any
+system suspend and resume callbacks for all of those devices, except for the
+complete callback, which is then entirely responsible for handling the device
+as appropriate.  This only applies to system suspend transitions that are not
+related to hibernation (see Documentation/power/devices.txt for more
+information).
+
 The PM core does its best to reduce the probability of race conditions between
 the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
 out the following operations: