
Device Power Management


Device power management encompasses two areas - the ability to save
state and transition a device to a low-power state when the system is
entering a low-power state; and the ability to transition a device to
a low-power state while the system is running (and independently of
any other power management activity). 


Methods

The methods to suspend and resume devices reside in struct bus_type: 

struct bus_type {
       ...
       int             (*suspend)(struct device * dev, u32 state);
       int             (*resume)(struct device * dev);
};

Each bus driver is responsible implementing these methods, translating
the call into a bus-specific request and forwarding the call to the
bus-specific drivers. For example, PCI drivers implement suspend() and
resume() methods in struct pci_driver. The PCI core is simply
responsible for translating the pointers to PCI-specific ones and
calling the low-level driver.

This is done to a) ease transition to the new power management methods
and leverage the existing PM code in various bus drivers; b) allow
buses to implement generic and default PM routines for devices, and c)
make the flow of execution obvious to the reader. 


System Power Management

When the system enters a low-power state, the device tree is walked in
a depth-first fashion to transition each device into a low-power
state. The ordering of the device tree is guaranteed by the order in
which devices get registered - children are never registered before
their ancestors, and devices are placed at the back of the list when
registered. By walking the list in reverse order, we are guaranteed to
suspend devices in the proper order. 

Devices are suspended once with interrupts enabled. Drivers are
expected to stop I/O transactions, save device state, and place the
device into a low-power state. Drivers may sleep, allocate memory,
etc. at will. 

Some devices are broken and will inevitably have problems powering
down or disabling themselves with interrupts enabled. For these
special cases, they may return -EAGAIN. This will put the device on a
list to be taken care of later. When interrupts are disabled, before
we enter the low-power state, their drivers are called again to put
their device to sleep. 

On resume, the devices that returned -EAGAIN will be called to power
themselves back on with interrupts disabled. Once interrupts have been
re-enabled, the rest of the drivers will be called to resume their
devices. On resume, a driver is responsible for powering back on each
device, restoring state, and re-enabling I/O transactions for that
device. 

System devices follow a slightly different API, which can be found in

	include/linux/sysdev.h
	drivers/base/sys.c

System devices will only be suspended with interrupts disabled, and
after all other devices have been suspended. On resume, they will be
resumed before any other devices, and also with interrupts disabled.


Runtime Power Management

Many devices are able to dynamically power down while the system is
still running. This feature is useful for devices that are not being
used, and can offer significant power savings on a running system. 

In each device's directory, there is a 'power' directory, which
contains at least a 'state' file. Reading from this file displays what
power state the device is currently in. Writing to this file initiates
a transition to the specified power state, which must be a decimal in
the range 1-3, inclusive; or 0 for 'On'.

The PM core will call the ->suspend() method in the bus_type object
that the device belongs to if the specified state is not 0, or
->resume() if it is. 

Nothing will happen if the specified state is the same state the
device is currently in. 

If the device is already in a low-power state, and the specified state
is another, but different, low-power state, the ->resume() method will
first be called to power the device back on, then ->suspend() will be
called again with the new state. 

The driver is responsible for saving the working state of the device
and putting it into the low-power state specified. If this was
successful, it returns 0, and the device's power_state field is
updated. 

The driver must take care to know whether or not it is able to
properly resume the device, including all step of reinitialization
necessary. (This is the hardest part, and the one most protected by
NDA'd documents). 

The driver must also take care not to suspend a device that is
currently in use. It is their responsibility to provide their own
exclusion mechanisms.

The runtime power transition happens with interrupts enabled. If a
device cannot support being powered down with interrupts, it may
return -EAGAIN (as it would during a system power management
transition),  but it will _not_ be called again, and the transaction
will fail.

There is currently no way to know what states a device or driver
supports a priori. This will change in the future. 


Driver Detach Power Management

The kernel now supports the ability to place a device in a low-power
state when it is detached from its driver, which happens when its
module is removed. 

Each device contains a 'detach_state' file in its sysfs directory
which can be used to control this state. Reading from this file
displays what the current detach state is set to. This is 0 (On) by
default. A user may write a positive integer value to this file in the
range of 1-4 inclusive. 

A value of 1-3 will indicate the device should be placed in that
low-power state, which will cause ->suspend() to be called for that
device. A value of 4 indicates that the device should be shutdown, so
->shutdown() will be called for that device. 

The driver is responsible for reinitializing the device when the
module is re-inserted during it's ->probe() (or equivalent) method. 
The driver core will not call any extra functions when binding the
device to the driver. 

