[RFC][PATCH 2/3] PM / sleep: Mechanism to avoid resuming runtime-suspended devices unnecessarily

Rafael J. Wysocki
2014-05-13T01:10:19Z

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

Currently, some subsystems (e.g. PCI and the ACPI PM domain) have to
resume all runtime-suspended devices during system suspend, mostly
because those devices may need to be reprogrammed due to different
wakeup settings for system sleep and for runtime PM.

For some devices, though, it's OK to remain in runtime suspend
throughout a complete system suspend/resume cycle (if the device was in
runtime suspend at the start of the cycle). We would like to do this
whenever possible, to avoid the overhead of extra power-up and power-down
events.

However, problems may arise because the device's descendants may require
it to be at full power at various points during the cycle. Therefore the
most straightforward way to do this safely is if the device and all its
descendants can remain runtime suspended until the complete stage of
system resume.

To this end, introduce a new device PM flag, power.direct_complete
and modify the PM core to use that flag as follows.

If the ->prepare() callback of a device returns a positive number,
the PM core will regard that as an indication that it may leave the
device runtime-suspended. It will then check if the system power
transition in progress is a suspend (and not hibernation in particular)
and if the device is, indeed, runtime-suspended. In that case, the PM
core will set the device's power.direct_complete flag. Otherwise it
will clear power.direct_complete for the device and it also will later
clear it for the device's parent (if there's one).

Next, the PM core will not invoke the ->suspend() ->suspend_late(),
->suspend_irq(), ->resume_irq(), ->resume_early(), or ->resume()
callbacks for all devices having power.direct_complete set. It
will invoke their ->complete() callbacks, however, and those
callbacks are then responsible for resuming the devices as
appropriate, if necessary.

Changelog partly based on an Alan Stern's description of the idea
(http://marc.info/?l=linux-pm&m=139940466625569&w=2).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
drivers/base/power/main.c | 45 ++++++++++++++++++++++++++++-----------------
include/linux/pm.h | 1 +
2 files changed, 29 insertions(+), 17 deletions(-)

Index: linux-pm/include/linux/pm.h
===================================================================
--- linux-pm.orig/include/linux/pm.h
+++ linux-pm/include/linux/pm.h
@@ -546,6 +546,7 @@ struct dev_pm_info {
bool is_late_suspended:1;
bool ignore_children:1;
bool early_init:1; /* Owned by the PM core */
+ bool direct_complete:1; /* Owned by the PM core */
spinlock_t lock;
#ifdef CONFIG_PM_SLEEP
struct list_head entry;
Index: linux-pm/drivers/base/power/main.c
===================================================================
--- linux-pm.orig/drivers/base/power/main.c
+++ linux-pm/drivers/base/power/main.c
@@ -479,7 +479,7 @@ static int device_resume_noirq(struct de
TRACE_DEVICE(dev);
TRACE_RESUME(0);

- if (dev->power.syscore)
+ if (dev->power.syscore || dev->power.direct_complete)
goto Out;

if (!dev->power.is_noirq_suspended)
@@ -605,7 +605,7 @@ static int device_resume_early(struct de
TRACE_DEVICE(dev);
TRACE_RESUME(0);

- if (dev->power.syscore)
+ if (dev->power.syscore || dev->power.direct_complete)
goto Out;

if (!dev->power.is_late_suspended)
@@ -732,7 +732,7 @@ static int device_resume(struct device *
TRACE_DEVICE(dev);
TRACE_RESUME(0);

- if (dev->power.syscore)
+ if (dev->power.syscore || dev->power.direct_complete)
goto Complete;

dpm_wait(dev->parent, async);
@@ -1007,7 +1007,7 @@ static int __device_suspend_noirq(struct
goto Complete;
}

- if (dev->power.syscore)
+ if (dev->power.syscore || dev->power.direct_complete)
goto Complete;

dpm_wait_for_children(dev, async);
@@ -1146,7 +1146,7 @@ static int __device_suspend_late(struct
goto Complete;
}

- if (dev->power.syscore)
+ if (dev->power.syscore || dev->power.direct_complete)
goto Complete;

dpm_wait_for_children(dev, async);
@@ -1312,7 +1312,7 @@ static int __device_suspend(struct devic

dpm_wait_for_children(dev, async);

- if (async_error || dev->power.syscore)
+ if (async_error || dev->power.syscore || dev->power.direct_complete)
goto Complete;

dpm_watchdog_set(&wd, dev);
@@ -1365,10 +1365,19 @@ static int __device_suspend(struct devic

End:
if (!error) {
+ struct device *parent = dev->parent;
+
dev->power.is_suspended = true;
- if (dev->power.wakeup_path
- && dev->parent && !dev->parent->power.ignore_children)
- dev->parent->power.wakeup_path = true;
+ if (parent) {
+ spin_lock_irq(&parent->power.lock);
+
+ dev->parent->power.direct_complete = false;
+ if (dev->power.wakeup_path
+ && !dev->parent->power.ignore_children)
+ dev->parent->power.wakeup_path = true;
+
+ spin_unlock_irq(&parent->power.lock);
+ }
}

device_unlock(dev);
@@ -1470,7 +1479,7 @@ static int device_prepare(struct device
{
int (*callback)(struct device *) = NULL;
char *info = NULL;
- int error = 0;
+ int ret = 0;

if (dev->power.syscore)
return 0;
@@ -1518,17 +1527,19 @@ static int device_prepare(struct device
callback = dev->driver->pm->prepare;
}

- if (callback) {
- error = callback(dev);
- suspend_report_result(callback, error);
- }
+ if (callback)
+ ret = callback(dev);

device_unlock(dev);

- if (error)
+ if (ret < 0) {
+ suspend_report_result(callback, ret);
pm_runtime_put(dev);
-
- return error;
+ return ret;
+ }
+ dev->power.direct_complete = ret > 0 && state.event == PM_EVENT_SUSPEND
+ && pm_runtime_suspended(dev);
+ return 0;
}

/**