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diff --git a/Documentation/power/pci.txt b/Documentation/power/pci.txt new file mode 100644 index 0000000..c85428e --- /dev/null +++ b/Documentation/power/pci.txt @@ -0,0 +1,332 @@ + +PCI Power Management +~~~~~~~~~~~~~~~~~~~~ + +An overview of the concepts and the related functions in the Linux kernel + +Patrick Mochel <mochel@transmeta.com> +(and others) + +--------------------------------------------------------------------------- + +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). + +There are actually two D3 states. When someone talks about D3, they usually +mean D3hot, which corresponds to an ACPI D2 state (power is reduced, the +device may lose some context). But they may also mean D3cold, which is an +ACPI D3 state (power is fully off, all state was discarded); or both. + +Bus power management is not covered in this version of this document. + +Note that all PCI devices support D0 and D3cold 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 should include disabling I/O, IRQs, and bus-mastering, as well as +physically transitioning the device to a lower power state; it may also include +calls to pci_enable_wake(). + +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 to match the suspend() parameter: + +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, except for PM-capable devices when pci_set_power_state is used. + +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, such as IRQs and bus mastering, +as well as calling pci_restore_state(). + +If the device is currently in D3, it may need to be reinitialized in resume(). + + * Some types of devices, like bus controllers, will preserve context in D3hot + (using Vcc power). Their drivers will often want to avoid re-initializing + them after re-entering D0 (perhaps to avoid resetting downstream devices). + + * Other kinds of devices in D3hot will discard device context as part of a + soft reset when re-entering the D0 state. + + * Devices resuming from D3cold always go through a power-on reset. Some + device context can also be preserved using Vaux power. + + * Some systems hide D3cold resume paths from drivers. For example, on PCs + the resume path for suspend-to-disk often runs BIOS powerup code, which + will sometimes re-initialize the device. + +To handle resets during D3 to D0 transitions, it may be convenient to share +device initialization code between probe() and resume(). Device parameters +can also be saved before the driver suspends into D3, avoiding re-probe. + +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, except for PM-capable devices when pci_set_power_state is used. + + +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 | ++------------------+ +| 11 | D0 | +| 12 | D1 | +| 13 | D2 | +| 14 | D3hot | +| 15 | 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 + |
