[PATCH 2/3] PM / sleep: Update device PM documentation to cover direct_complete

Rafael J. Wysocki
2014-05-16T00:47:37Z

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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>
---
Documentation/power/devices.txt | 34 ++++++++++++++++++++++++++++++----
Documentation/power/runtime_pm.txt | 17 +++++++++++++++++
2 files changed, 47 insertions(+), 4 deletions(-)

Index: linux-pm/Documentation/power/devices.txt
===================================================================
--- linux-pm.orig/Documentation/power/devices.txt
+++ linux-pm/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 me
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
Index: linux-pm/Documentation/power/runtime_pm.txt
===================================================================
--- linux-pm.orig/Documentation/power/runtime_pm.txt
+++ linux-pm/Documentation/power/runtime_pm.txt
@@ -2,6 +2,7 @@ Runtime Power Management Framework for I

(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
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 n
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