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PCI Power Management

~~~~~~~~~~~~~~~~~~~~

An overview of the concepts and the related functions in the Linux kernel

Patrick Mochel 

---------------------------------------------------------------------------

1. Overview

2. How the PCI Subsystem Does Power Management

3. PCI Utility Functions

4. PCI Device Drivers

5. Resources

1. Overview

~~~~~~~~~~~

The PCI Power Management Specification was introduced between the PCI 2.1 and

PCI 2.2 Specifications. It a standard interface for controlling various

power management operations.

Implementation of the PCI PM Spec is optional, as are several sub-components of

it. If a device supports the PCI PM Spec, the device will have an 8 byte

capability field in its PCI configuration space. This field is used to describe

and control the standard PCI power management features.

The PCI PM spec defines 4 operating states for devices (D0 - D3) and for buses

(B0 - B3). The higher the number, the less power the device consumes. However,

the higher the number, the longer the latency is for the device to return to

an operational state (D0).

Bus power management is not covered in this version of this document.

Note that all PCI devices support D0 and D3 by default, regardless of whether or

not they implement any of the PCI PM spec.

The possible state transitions that a device can undergo are:

+---------------------------+

| Current State | New State |

+---------------------------+

| D0            | D1, D2, D3|

+---------------------------+

| D1            | D2, D3    |

+---------------------------+

| D2            | D3        |

+---------------------------+

| D1, D2, D3    | D0        |

+---------------------------+

Note that when the system is entering a global suspend state, all devices will

be placed into D3 and when resuming, all devices will be placed into D0.

However, when the system is running, other state transitions are possible.

2. How The PCI Subsystem Handles Power Management

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The PCI suspend/resume functionality is accessed indirectly via the Power

Management subsystem. At boot, the PCI driver registers a power management

callback with that layer. Upon entering a suspend state, the PM layer iterates

through all of its registered callbacks. This currently takes place only during

APM state transitions.

Upon going to sleep, the PCI subsystem walks its device tree twice. Both times,

it does a depth first walk of the device tree. The first walk saves each of the

device's state and checks for devices that will prevent the system from entering

a global power state. The next walk then places the devices in a low power

state.

The first walk allows a graceful recovery in the event of a failure, since none

of the devices have actually been powered down.

In both walks, in particular the second, all children of a bridge are touched

before the actual bridge itself. This allows the bridge to retain power while

its children are being accessed.

Upon resuming from sleep, just the opposite must be true: all bridges must be

powered on and restored before their children are powered on. This is easily

accomplished with a breadth-first walk of the PCI device tree.

3. PCI Utility Functions

~~~~~~~~~~~~~~~~~~~~~~~~

These are helper functions designed to be called by individual device drivers.

Assuming that a device behaves as advertised, these should be applicable in most

cases. However, results may vary.

Note that these functions are never implicitly called for the driver. The driver

is always responsible for deciding when and if to call these.

pci_save_state

--------------

Usage:

        pci_save_state(dev, buffer);

Description:

        Save first 64 bytes of PCI config space. Buffer must be allocated by

        caller.

pci_restore_state

-----------------

Usage:

        pci_restore_state(dev, buffer);

Description:

        Restore previously saved config space. (First 64 bytes only);

        If buffer is NULL, then restore what information we know about the

        device from bootup: BARs and interrupt line.

pci_set_power_state

-------------------

Usage:

        pci_set_power_state(dev, state);

Description:

        Transition device to low power state using PCI PM Capabilities

        registers.

        Will fail under one of the following conditions:

        - If state is less than current state, but not D0 (illegal transition)

        - Device doesn't support PM Capabilities

        - Device does not support requested state

pci_enable_wake

---------------

Usage:

        pci_enable_wake(dev, state, enable);

Description:

        Enable device to generate PME# during low power state using PCI PM

        Capabilities.

        Checks whether if device supports generating PME# from requested state

        and fail if it does not, unless enable == 0 (request is to disable wake

        events, which is implicit if it doesn't even support it in the first

        place).

        Note that the PMC Register in the device's PM Capabilties has a bitmask

        of the states it supports generating PME# from. D3hot is bit 3 and

        D3cold is bit 4. So, while a value of 4 as the state may not seem

        semantically correct, it is.

4. PCI Device Drivers

~~~~~~~~~~~~~~~~~~~~~

These functions are intended for use by individual drivers, and are defined in

struct pci_driver:

        int  (*save_state) (struct pci_dev *dev, u32 state);

        int  (*suspend) (struct pci_dev *dev, u32 state);

        int  (*resume) (struct pci_dev *dev);

        int  (*enable_wake) (struct pci_dev *dev, u32 state, int enable);

save_state

----------

Usage:

if (dev->driver && dev->driver->save_state)

        dev->driver->save_state(dev,state);

The driver should use this callback to save device state. It should take into

account the current state of the device and the requested state in order to

avoid any unnecessary operations.

For example, a video card that supports all 4 states (D0-D3), all controller

context is preserved when entering D1, but the screen is placed into a low power

state (blanked).

The driver can also interpret this function as a notification that it may be

entering a sleep state in the near future. If it knows that the device cannot

enter the requested state, either because of lack of support for it, or because

the device is middle of some critical operation, then it should fail.

This function should not be used to set any state in the device or the driver

because the device may not actually enter the sleep state (e.g. another driver

later causes causes a global state transition to fail).

Note that in intermediate low power states, a device's I/O and memory spaces may

be disabled and may not be available in subsequent transitions to lower power

states.

suspend

-------

Usage:

if (dev->driver && dev->driver->suspend)

        dev->driver->suspend(dev,state);

A driver uses this function to actually transition the device into a low power

state. This may include disabling I/O, memory and bus-mastering, as well as

physically transitioning the device to a lower power state.

Bus mastering may be disabled by doing:

pci_disable_device(dev);

For devices that support the PCI PM Spec, this may be used to set the device's

power state:

pci_set_power_state(dev,state);

The driver is also responsible for disabling any other device-specific features

(e.g blanking screen, turning off on-card memory, etc).

The driver should be sure to track the current state of the device, as it may

obviate the need for some operations.

The driver should update the current_state field in its pci_dev structure in

this function.

resume

------

Usage:

if (dev->driver && dev->driver->suspend)

        dev->driver->resume(dev)

The resume callback may be called from any power state, and is always meant to

transition the device to the D0 state.

The driver is responsible for reenabling any features of the device that had

been disabled during previous suspend calls and restoring all state that was

saved in previous save_state calls.

If the device is currently in D3, it must be completely reinitialized, as it

must be assumed that the device has lost all of its context (even that of its

PCI config space). For almost all current drivers, this means that the

initialization code that the driver does at boot must be separated out and

called again from the resume callback. Note that some values for the device may

not have to be probed for this time around if they are saved before entering the

low power state.

If the device supports the PCI PM Spec, it can use this to physically transition

the device to D0:

pci_set_power_state(dev,0);

Note that if the entire system is transitioning out of a global sleep state, all

devices will be placed in the D0 state, so this is not necessary. However, in

the event that the device is placed in the D3 state during normal operation,

this call is necessary. It is impossible to determine which of the two events is

taking place in the driver, so it is always a good idea to make that call.

The driver should take note of the state that it is resuming from in order to

ensure correct (and speedy) operation.

The driver should update the current_state field in its pci_dev structure in

this function.

enable_wake

-----------

Usage:

if (dev->driver && dev->driver->enable_wake)

        dev->driver->enable_wake(dev,state,enable);

This callback is generally only relevant for devices that support the PCI PM

spec and have the ability to generate a PME# (Power Management Event Signal)

to wake the system up. (However, it is possible that a device may support

some non-standard way of generating a wake event on sleep.)

Bits 15:11 of the PMC (Power Mgmt Capabilities) Register in a device's

PM Capabilties describe what power states the device supports generating a

wake event from:

+------------------+

|  Bit  |  State   |

+------------------+

|  15   |   D0     |

|  14   |   D1     |

|  13   |   D2     |

|  12   |   D3hot  |

|  11   |   D3cold |

+------------------+

A device can use this to enable wake events:

         pci_enable_wake(dev,state,enable);

Note that to enable PME# from D3cold, a value of 4 should be passed to

pci_enable_wake (since it uses an index into a bitmask). If a driver gets

a request to enable wake events from D3, two calls should be made to

pci_enable_wake (one for both D3hot and D3cold).

5. Resources

~~~~~~~~~~~~

PCI Local Bus Specification

PCI Bus Power Management Interface Specification

  http://pcisig.org