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diff --git a/Documentation/power/pci.txt b/Documentation/power/pci.txt deleted file mode 100644 index 8eaf9ee24d43..000000000000 --- a/Documentation/power/pci.txt +++ /dev/null @@ -1,1094 +0,0 @@ -PCI Power Management - -Copyright (c) 2010 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. - -An overview of concepts and the Linux kernel's interfaces related to PCI power -management. Based on previous work by Patrick Mochel <mochel@transmeta.com> -(and others). - -This document only covers the aspects of power management specific to PCI -devices. For general description of the kernel's interfaces related to device -power management refer to Documentation/driver-api/pm/devices.rst and -Documentation/power/runtime_pm.txt. - ---------------------------------------------------------------------------- - -1. Hardware and Platform Support for PCI Power Management -2. PCI Subsystem and Device Power Management -3. PCI Device Drivers and Power Management -4. Resources - - -1. Hardware and Platform Support for PCI Power Management -========================================================= - -1.1. Native and Platform-Based Power Management ------------------------------------------------ -In general, power management is a feature allowing one to save energy by putting -devices into states in which they draw less power (low-power states) at the -price of reduced functionality or performance. - -Usually, a device is put into a low-power state when it is underutilized or -completely inactive. However, when it is necessary to use the device once -again, it has to be put back into the "fully functional" state (full-power -state). This may happen when there are some data for the device to handle or -as a result of an external event requiring the device to be active, which may -be signaled by the device itself. - -PCI devices may be put into low-power states in two ways, by using the device -capabilities introduced by the PCI Bus Power Management Interface Specification, -or with the help of platform firmware, such as an ACPI BIOS. In the first -approach, that is referred to as the native PCI power management (native PCI PM) -in what follows, the device power state is changed as a result of writing a -specific value into one of its standard configuration registers. The second -approach requires the platform firmware to provide special methods that may be -used by the kernel to change the device's power state. - -Devices supporting the native PCI PM usually can generate wakeup signals called -Power Management Events (PMEs) to let the kernel know about external events -requiring the device to be active. After receiving a PME the kernel is supposed -to put the device that sent it into the full-power state. However, the PCI Bus -Power Management Interface Specification doesn't define any standard method of -delivering the PME from the device to the CPU and the operating system kernel. -It is assumed that the platform firmware will perform this task and therefore, -even though a PCI device is set up to generate PMEs, it also may be necessary to -prepare the platform firmware for notifying the CPU of the PMEs coming from the -device (e.g. by generating interrupts). - -In turn, if the methods provided by the platform firmware are used for changing -the power state of a device, usually the platform also provides a method for -preparing the device to generate wakeup signals. In that case, however, it -often also is necessary to prepare the device for generating PMEs using the -native PCI PM mechanism, because the method provided by the platform depends on -that. - -Thus in many situations both the native and the platform-based power management -mechanisms have to be used simultaneously to obtain the desired result. - -1.2. Native PCI Power Management --------------------------------- -The PCI Bus Power Management Interface Specification (PCI PM Spec) was -introduced between the PCI 2.1 and PCI 2.2 Specifications. It defined a -standard interface for performing various operations related to power -management. - -The implementation of the PCI PM Spec is optional for conventional PCI devices, -but it is mandatory for PCI Express devices. If a device supports the PCI PM -Spec, it has an 8 byte power management capability field in its PCI -configuration space. This field is used to describe and control the standard -features related to the native PCI power management. - -The PCI PM Spec defines 4 operating states for devices (D0-D3) and for buses -(B0-B3). The higher the number, the less power is drawn by the device or bus -in that state. However, the higher the number, the longer the latency for -the device or bus to return to the full-power state (D0 or B0, respectively). - -There are two variants of the D3 state defined by the specification. The first -one is D3hot, referred to as the software accessible D3, because devices can be -programmed to go into it. The second one, D3cold, is the state that PCI devices -are in when the supply voltage (Vcc) is removed from them. It is not possible -to program a PCI device to go into D3cold, although there may be a programmable -interface for putting the bus the device is on into a state in which Vcc is -removed from all devices on the bus. - -PCI bus power management, however, is not supported by the Linux kernel at the -time of this writing and therefore it is not covered by this document. - -Note that every PCI device can be in the full-power state (D0) or in D3cold, -regardless of whether or not it implements the PCI PM Spec. In addition to -that, if the PCI PM Spec is implemented by the device, it must support D3hot -as well as D0. The support for the D1 and D2 power states is optional. - -PCI devices supporting the PCI PM Spec can be programmed to go to any of the -supported low-power states (except for D3cold). While in D1-D3hot the -standard configuration registers of the device must be accessible to software -(i.e. the device is required to respond to PCI configuration accesses), although -its I/O and memory spaces are then disabled. This allows the device to be -programmatically put into D0. Thus the kernel can switch the device back and -forth between D0 and the supported low-power states (except for D3cold) and the -possible power state transitions the device can undergo are the following: - -+----------------------------+ -| Current State | New State | -+----------------------------+ -| D0 | D1, D2, D3 | -+----------------------------+ -| D1 | D2, D3 | -+----------------------------+ -| D2 | D3 | -+----------------------------+ -| D1, D2, D3 | D0 | -+----------------------------+ - -The transition from D3cold to D0 occurs when the supply voltage is provided to -the device (i.e. power is restored). In that case the device returns to D0 with -a full power-on reset sequence and the power-on defaults are restored to the -device by hardware just as at initial power up. - -PCI devices supporting the PCI PM Spec can be programmed to generate PMEs -while in a low-power state (D1-D3), but they are not required to be capable -of generating PMEs from all supported low-power states. In particular, the -capability of generating PMEs from D3cold is optional and depends on the -presence of additional voltage (3.3Vaux) allowing the device to remain -sufficiently active to generate a wakeup signal. - -1.3. ACPI Device Power Management ---------------------------------- -The platform firmware support for the power management of PCI devices is -system-specific. However, if the system in question is compliant with the -Advanced Configuration and Power Interface (ACPI) Specification, like the -majority of x86-based systems, it is supposed to implement device power -management interfaces defined by the ACPI standard. - -For this purpose the ACPI BIOS provides special functions called "control -methods" that may be executed by the kernel to perform specific tasks, such as -putting a device into a low-power state. These control methods are encoded -using special byte-code language called the ACPI Machine Language (AML) and -stored in the machine's BIOS. The kernel loads them from the BIOS and executes -them as needed using an AML interpreter that translates the AML byte code into -computations and memory or I/O space accesses. This way, in theory, a BIOS -writer can provide the kernel with a means to perform actions depending -on the system design in a system-specific fashion. - -ACPI control methods may be divided into global control methods, that are not -associated with any particular devices, and device control methods, that have -to be defined separately for each device supposed to be handled with the help of -the platform. This means, in particular, that ACPI device control methods can -only be used to handle devices that the BIOS writer knew about in advance. The -ACPI methods used for device power management fall into that category. - -The ACPI specification assumes that devices can be in one of four power states -labeled as D0, D1, D2, and D3 that roughly correspond to the native PCI PM -D0-D3 states (although the difference between D3hot and D3cold is not taken -into account by ACPI). Moreover, for each power state of a device there is a -set of power resources that have to be enabled for the device to be put into -that state. These power resources are controlled (i.e. enabled or disabled) -with the help of their own control methods, _ON and _OFF, that have to be -defined individually for each of them. - -To put a device into the ACPI power state Dx (where x is a number between 0 and -3 inclusive) the kernel is supposed to (1) enable the power resources required -by the device in this state using their _ON control methods and (2) execute the -_PSx control method defined for the device. In addition to that, if the device -is going to be put into a low-power state (D1-D3) and is supposed to generate -wakeup signals from that state, the _DSW (or _PSW, replaced with _DSW by ACPI -3.0) control method defined for it has to be executed before _PSx. Power -resources that are not required by the device in the target power state and are -not required any more by any other device should be disabled (by executing their -_OFF control methods). If the current power state of the device is D3, it can -only be put into D0 this way. - -However, quite often the power states of devices are changed during a -system-wide transition into a sleep state or back into the working state. ACPI -defines four system sleep states, S1, S2, S3, and S4, and denotes the system -working state as S0. In general, the target system sleep (or working) state -determines the highest power (lowest number) state the device can be put -into and the kernel is supposed to obtain this information by executing the -device's _SxD control method (where x is a number between 0 and 4 inclusive). -If the device is required to wake up the system from the target sleep state, the -lowest power (highest number) state it can be put into is also determined by the -target state of the system. The kernel is then supposed to use the device's -_SxW control method to obtain the number of that state. It also is supposed to -use the device's _PRW control method to learn which power resources need to be -enabled for the device to be able to generate wakeup signals. - -1.4. Wakeup Signaling ---------------------- -Wakeup signals generated by PCI devices, either as native PCI PMEs, or as -a result of the execution of the _DSW (or _PSW) ACPI control method before -putting the device into a low-power state, have to be caught and handled as -appropriate. If they are sent while the system is in the working state -(ACPI S0), they should be translated into interrupts so that the kernel can -put the devices generating them into the full-power state and take care of the -events that triggered them. In turn, if they are sent while the system is -sleeping, they should cause the system's core logic to trigger wakeup. - -On ACPI-based systems wakeup signals sent by conventional PCI devices are -converted into ACPI General-Purpose Events (GPEs) which are hardware signals -from the system core logic generated in response to various events that need to -be acted upon. Every GPE is associated with one or more sources of potentially -interesting events. In particular, a GPE may be associated with a PCI device -capable of signaling wakeup. The information on the connections between GPEs -and event sources is recorded in the system's ACPI BIOS from where it can be -read by the kernel. - -If a PCI device known to the system's ACPI BIOS signals wakeup, the GPE -associated with it (if there is one) is triggered. The GPEs associated with PCI -bridges may also be triggered in response to a wakeup signal from one of the -devices below the bridge (this also is the case for root bridges) and, for -example, native PCI PMEs from devices unknown to the system's ACPI BIOS may be -handled this way. - -A GPE may be triggered when the system is sleeping (i.e. when it is in one of -the ACPI S1-S4 states), in which case system wakeup is started by its core logic -(the device that was the source of the signal causing the system wakeup to occur -may be identified later). The GPEs used in such situations are referred to as -wakeup GPEs. - -Usually, however, GPEs are also triggered when the system is in the working -state (ACPI S0) and in that case the system's core logic generates a System -Control Interrupt (SCI) to notify the kernel of the event. Then, the SCI -handler identifies the GPE that caused the interrupt to be generated which, -in turn, allows the kernel to identify the source of the event (that may be -a PCI device signaling wakeup). The GPEs used for notifying the kernel of -events occurring while the system is in the working state are referred to as -runtime GPEs. - -Unfortunately, there is no standard way of handling wakeup signals sent by -conventional PCI devices on systems that are not ACPI-based, but there is one -for PCI Express devices. Namely, the PCI Express Base Specification introduced -a native mechanism for converting native PCI PMEs into interrupts generated by -root ports. For conventional PCI devices native PMEs are out-of-band, so they -are routed separately and they need not pass through bridges (in principle they -may be routed directly to the system's core logic), but for PCI Express devices -they are in-band messages that have to pass through the PCI Express hierarchy, -including the root port on the path from the device to the Root Complex. Thus -it was possible to introduce a mechanism by which a root port generates an -interrupt whenever it receives a PME message from one of the devices below it. -The PCI Express Requester ID of the device that sent the PME message is then -recorded in one of the root port's configuration registers from where it may be -read by the interrupt handler allowing the device to be identified. [PME -messages sent by PCI Express endpoints integrated with the Root Complex don't -pass through root ports, but instead they cause a Root Complex Event Collector -(if there is one) to generate interrupts.] - -In principle the native PCI Express PME signaling may also be used on ACPI-based -systems along with the GPEs, but to use it the kernel has to ask the system's -ACPI BIOS to release control of root port configuration registers. The ACPI -BIOS, however, is not required to allow the kernel to control these registers -and if it doesn't do that, the kernel must not modify their contents. Of course -the native PCI Express PME signaling cannot be used by the kernel in that case. - - -2. PCI Subsystem and Device Power Management -============================================ - -2.1. Device Power Management Callbacks --------------------------------------- -The PCI Subsystem participates in the power management of PCI devices in a -number of ways. First of all, it provides an intermediate code layer between -the device power management core (PM core) and PCI device drivers. -Specifically, the pm field of the PCI subsystem's struct bus_type object, -pci_bus_type, points to a struct dev_pm_ops object, pci_dev_pm_ops, containing -pointers to several device power management callbacks: - -const struct dev_pm_ops pci_dev_pm_ops = { - .prepare = pci_pm_prepare, - .complete = pci_pm_complete, - .suspend = pci_pm_suspend, - .resume = pci_pm_resume, - .freeze = pci_pm_freeze, - .thaw = pci_pm_thaw, - .poweroff = pci_pm_poweroff, - .restore = pci_pm_restore, - .suspend_noirq = pci_pm_suspend_noirq, - .resume_noirq = pci_pm_resume_noirq, - .freeze_noirq = pci_pm_freeze_noirq, - .thaw_noirq = pci_pm_thaw_noirq, - .poweroff_noirq = pci_pm_poweroff_noirq, - .restore_noirq = pci_pm_restore_noirq, - .runtime_suspend = pci_pm_runtime_suspend, - .runtime_resume = pci_pm_runtime_resume, - .runtime_idle = pci_pm_runtime_idle, -}; - -These callbacks are executed by the PM core in various situations related to -device power management and they, in turn, execute power management callbacks -provided by PCI device drivers. They also perform power management operations -involving some standard configuration registers of PCI devices that device -drivers need not know or care about. - -The structure representing a PCI device, struct pci_dev, contains several fields -that these callbacks operate on: - -struct pci_dev { - ... - pci_power_t current_state; /* Current operating state. */ - int pm_cap; /* PM capability offset in the - configuration space */ - unsigned int pme_support:5; /* Bitmask of states from which PME# - can be generated */ - unsigned int pme_interrupt:1;/* Is native PCIe PME signaling used? */ - unsigned int d1_support:1; /* Low power state D1 is supported */ - unsigned int d2_support:1; /* Low power state D2 is supported */ - unsigned int no_d1d2:1; /* D1 and D2 are forbidden */ - unsigned int wakeup_prepared:1; /* Device prepared for wake up */ - unsigned int d3_delay; /* D3->D0 transition time in ms */ - ... -}; - -They also indirectly use some fields of the struct device that is embedded in -struct pci_dev. - -2.2. Device Initialization --------------------------- -The PCI subsystem's first task related to device power management is to -prepare the device for power management and initialize the fields of struct -pci_dev used for this purpose. This happens in two functions defined in -drivers/pci/pci.c, pci_pm_init() and platform_pci_wakeup_init(). - -The first of these functions checks if the device supports native PCI PM -and if that's the case the offset of its power management capability structure -in the configuration space is stored in the pm_cap field of the device's struct -pci_dev object. Next, the function checks which PCI low-power states are -supported by the device and from which low-power states the device can generate -native PCI PMEs. The power management fields of the device's struct pci_dev and -the struct device embedded in it are updated accordingly and the generation of -PMEs by the device is disabled. - -The second function checks if the device can be prepared to signal wakeup with -the help of the platform firmware, such as the ACPI BIOS. If that is the case, -the function updates the wakeup fields in struct device embedded in the -device's struct pci_dev and uses the firmware-provided method to prevent the -device from signaling wakeup. - -At this point the device is ready for power management. For driverless devices, -however, this functionality is limited to a few basic operations carried out -during system-wide transitions to a sleep state and back to the working state. - -2.3. Runtime Device Power Management ------------------------------------- -The PCI subsystem plays a vital role in the runtime power management of PCI -devices. For this purpose it uses the general runtime power management -(runtime PM) framework described in Documentation/power/runtime_pm.txt. -Namely, it provides subsystem-level callbacks: - - pci_pm_runtime_suspend() - pci_pm_runtime_resume() - pci_pm_runtime_idle() - -that are executed by the core runtime PM routines. It also implements the -entire mechanics necessary for handling runtime wakeup signals from PCI devices -in low-power states, which at the time of this writing works for both the native -PCI Express PME signaling and the ACPI GPE-based wakeup signaling described in -Section 1. - -First, a PCI device is put into a low-power state, or suspended, with the help -of pm_schedule_suspend() or pm_runtime_suspend() which for PCI devices call -pci_pm_runtime_suspend() to do the actual job. For this to work, the device's -driver has to provide a pm->runtime_suspend() callback (see below), which is -run by pci_pm_runtime_suspend() as the first action. If the driver's callback -returns successfully, the device's standard configuration registers are saved, -the device is prepared to generate wakeup signals and, finally, it is put into -the target low-power state. - -The low-power state to put the device into is the lowest-power (highest number) -state from which it can signal wakeup. The exact method of signaling wakeup is -system-dependent and is determined by the PCI subsystem on the basis of the -reported capabilities of the device and the platform firmware. To prepare the -device for signaling wakeup and put it into the selected low-power state, the -PCI subsystem can use the platform firmware as well as the device's native PCI -PM capabilities, if supported. - -It is expected that the device driver's pm->runtime_suspend() callback will -not attempt to prepare the device for signaling wakeup or to put it into a -low-power state. The driver ought to leave these tasks to the PCI subsystem -that has all of the information necessary to perform them. - -A suspended device is brought back into the "active" state, or resumed, -with the help of pm_request_resume() or pm_runtime_resume() which both call -pci_pm_runtime_resume() for PCI devices. Again, this only works if the device's -driver provides a pm->runtime_resume() callback (see below). However, before -the driver's callback is executed, pci_pm_runtime_resume() brings the device -back into the full-power state, prevents it from signaling wakeup while in that -state and restores its standard configuration registers. Thus the driver's -callback need not worry about the PCI-specific aspects of the device resume. - -Note that generally pci_pm_runtime_resume() may be called in two different -situations. First, it may be called at the request of the device's driver, for -example if there are some data for it to process. Second, it may be called -as a result of a wakeup signal from the device itself (this sometimes is -referred to as "remote wakeup"). Of course, for this purpose the wakeup signal -is handled in one of the ways described in Section 1 and finally converted into -a notification for the PCI subsystem after the source device has been -identified. - -The pci_pm_runtime_idle() function, called for PCI devices by pm_runtime_idle() -and pm_request_idle(), executes the device driver's pm->runtime_idle() -callback, if defined, and if that callback doesn't return error code (or is not -present at all), suspends the device with the help of pm_runtime_suspend(). -Sometimes pci_pm_runtime_idle() is called automatically by the PM core (for -example, it is called right after the device has just been resumed), in which -cases it is expected to suspend the device if that makes sense. Usually, -however, the PCI subsystem doesn't really know if the device really can be -suspended, so it lets the device's driver decide by running its -pm->runtime_idle() callback. - -2.4. System-Wide Power Transitions ----------------------------------- -There are a few different types of system-wide power transitions, described in -Documentation/driver-api/pm/devices.rst. Each of them requires devices to be handled -in a specific way and the PM core executes subsystem-level power management -callbacks for this purpose. They are executed in phases such that each phase -involves executing the same subsystem-level callback for every device belonging -to the given subsystem before the next phase begins. These phases always run -after tasks have been frozen. - -2.4.1. System Suspend - -When the system is going into a sleep state in which the contents of memory will -be preserved, such as one of the ACPI sleep states S1-S3, the phases are: - - prepare, suspend, suspend_noirq. - -The following PCI bus type's callbacks, respectively, are used in these phases: - - pci_pm_prepare() - pci_pm_suspend() - pci_pm_suspend_noirq() - -The pci_pm_prepare() routine first puts the device into the "fully functional" -state with the help of pm_runtime_resume(). Then, it executes the device -driver's pm->prepare() callback if defined (i.e. if the driver's struct -dev_pm_ops object is present and the prepare pointer in that object is valid). - -The pci_pm_suspend() routine first checks if the device's driver implements -legacy PCI suspend routines (see Section 3), in which case the driver's legacy -suspend callback is executed, if present, and its result is returned. Next, if -the device's driver doesn't provide a struct dev_pm_ops object (containing -pointers to the driver's callbacks), pci_pm_default_suspend() is called, which -simply turns off the device's bus master capability and runs -pcibios_disable_device() to disable it, unless the device is a bridge (PCI -bridges are ignored by this routine). Next, the device driver's pm->suspend() -callback is executed, if defined, and its result is returned if it fails. -Finally, pci_fixup_device() is called to apply hardware suspend quirks related -to the device if necessary. - -Note that the suspend phase is carried out asynchronously for PCI devices, so -the pci_pm_suspend() callback may be executed in parallel for any pair of PCI -devices that don't depend on each other in a known way (i.e. none of the paths -in the device tree from the root bridge to a leaf device contains both of them). - -The pci_pm_suspend_noirq() routine is executed after suspend_device_irqs() has -been called, which means that the device driver's interrupt handler won't be -invoked while this routine is running. It first checks if the device's driver -implements legacy PCI suspends routines (Section 3), in which case the legacy -late suspend routine is called and its result is returned (the standard -configuration registers of the device are saved if the driver's callback hasn't -done that). Second, if the device driver's struct dev_pm_ops object is not -present, the device's standard configuration registers are saved and the routine -returns success. Otherwise the device driver's pm->suspend_noirq() callback is -executed, if present, and its result is returned if it fails. Next, if the -device's standard configuration registers haven't been saved yet (one of the -device driver's callbacks executed before might do that), pci_pm_suspend_noirq() -saves them, prepares the device to signal wakeup (if necessary) and puts it into -a low-power state. - -The low-power state to put the device into is the lowest-power (highest number) -state from which it can signal wakeup while the system is in the target sleep -state. Just like in the runtime PM case described above, the mechanism of -signaling wakeup is system-dependent and determined by the PCI subsystem, which -is also responsible for preparing the device to signal wakeup from the system's -target sleep state as appropriate. - -PCI device drivers (that don't implement legacy power management callbacks) are -generally not expected to prepare devices for signaling wakeup or to put them -into low-power states. However, if one of the driver's suspend callbacks -(pm->suspend() or pm->suspend_noirq()) saves the device's standard configuration -registers, pci_pm_suspend_noirq() will assume that the device has been prepared -to signal wakeup and put into a low-power state by the driver (the driver is -then assumed to have used the helper functions provided by the PCI subsystem for -this purpose). PCI device drivers are not encouraged to do that, but in some -rare cases doing that in the driver may be the optimum approach. - -2.4.2. System Resume - -When the system is undergoing a transition from a sleep state in which the -contents of memory have been preserved, such as one of the ACPI sleep states -S1-S3, into the working state (ACPI S0), the phases are: - - resume_noirq, resume, complete. - -The following PCI bus type's callbacks, respectively, are executed in these -phases: - - pci_pm_resume_noirq() - pci_pm_resume() - pci_pm_complete() - -The pci_pm_resume_noirq() routine first puts the device into the full-power -state, restores its standard configuration registers and applies early resume -hardware quirks related to the device, if necessary. This is done -unconditionally, regardless of whether or not the device's driver implements -legacy PCI power management callbacks (this way all PCI devices are in the -full-power state and their standard configuration registers have been restored -when their interrupt handlers are invoked for the first time during resume, -which allows the kernel to avoid problems with the handling of shared interrupts -by drivers whose devices are still suspended). If legacy PCI power management -callbacks (see Section 3) are implemented by the device's driver, the legacy -early resume callback is executed and its result is returned. Otherwise, the -device driver's pm->resume_noirq() callback is executed, if defined, and its -result is returned. - -The pci_pm_resume() routine first checks if the device's standard configuration -registers have been restored and restores them if that's not the case (this -only is necessary in the error path during a failing suspend). Next, resume -hardware quirks related to the device are applied, if necessary, and if the -device's driver implements legacy PCI power management callbacks (see -Section 3), the driver's legacy resume callback is executed and its result is -returned. Otherwise, the device's wakeup signaling mechanisms are blocked and -its driver's pm->resume() callback is executed, if defined (the callback's -result is then returned). - -The resume phase is carried out asynchronously for PCI devices, like the -suspend phase described above, which means that if two PCI devices don't depend -on each other in a known way, the pci_pm_resume() routine may be executed for -the both of them in parallel. - -The pci_pm_complete() routine only executes the device driver's pm->complete() -callback, if defined. - -2.4.3. System Hibernation - -System hibernation is more complicated than system suspend, because it requires -a system image to be created and written into a persistent storage medium. The -image is created atomically and all devices are quiesced, or frozen, before that -happens. - -The freezing of devices is carried out after enough memory has been freed (at -the time of this writing the image creation requires at least 50% of system RAM -to be free) in the following three phases: - - prepare, freeze, freeze_noirq - -that correspond to the PCI bus type's callbacks: - - pci_pm_prepare() - pci_pm_freeze() - pci_pm_freeze_noirq() - -This means that the prepare phase is exactly the same as for system suspend. -The other two phases, however, are different. - -The pci_pm_freeze() routine is quite similar to pci_pm_suspend(), but it runs -the device driver's pm->freeze() callback, if defined, instead of pm->suspend(), -and it doesn't apply the suspend-related hardware quirks. It is executed -asynchronously for different PCI devices that don't depend on each other in a -known way. - -The pci_pm_freeze_noirq() routine, in turn, is similar to -pci_pm_suspend_noirq(), but it calls the device driver's pm->freeze_noirq() -routine instead of pm->suspend_noirq(). It also doesn't attempt to prepare the -device for signaling wakeup and put it into a low-power state. Still, it saves -the device's standard configuration registers if they haven't been saved by one -of the driver's callbacks. - -Once the image has been created, it has to be saved. However, at this point all -devices are frozen and they cannot handle I/O, while their ability to handle -I/O is obviously necessary for the image saving. Thus they have to be brought -back to the fully functional state and this is done in the following phases: - - thaw_noirq, thaw, complete - -using the following PCI bus type's callbacks: - - pci_pm_thaw_noirq() - pci_pm_thaw() - pci_pm_complete() - -respectively. - -The first of them, pci_pm_thaw_noirq(), is analogous to pci_pm_resume_noirq(), -but it doesn't put the device into the full power state and doesn't attempt to -restore its standard configuration registers. It also executes the device -driver's pm->thaw_noirq() callback, if defined, instead of pm->resume_noirq(). - -The pci_pm_thaw() routine is similar to pci_pm_resume(), but it runs the device -driver's pm->thaw() callback instead of pm->resume(). It is executed -asynchronously for different PCI devices that don't depend on each other in a -known way. - -The complete phase it the same as for system resume. - -After saving the image, devices need to be powered down before the system can -enter the target sleep state (ACPI S4 for ACPI-based systems). This is done in -three phases: - - prepare, poweroff, poweroff_noirq - -where the prepare phase is exactly the same as for system suspend. The other -two phases are analogous to the suspend and suspend_noirq phases, respectively. -The PCI subsystem-level callbacks they correspond to - - pci_pm_poweroff() - pci_pm_poweroff_noirq() - -work in analogy with pci_pm_suspend() and pci_pm_poweroff_noirq(), respectively, -although they don't attempt to save the device's standard configuration -registers. - -2.4.4. System Restore - -System restore requires a hibernation image to be loaded into memory and the -pre-hibernation memory contents to be restored before the pre-hibernation system -activity can be resumed. - -As described in Documentation/driver-api/pm/devices.rst, the hibernation image is loaded -into memory by a fresh instance of the kernel, called the boot kernel, which in -turn is loaded and run by a boot loader in the usual way. After the boot kernel -has loaded the image, it needs to replace its own code and data with the code -and data of the "hibernated" kernel stored within the image, called the image -kernel. For this purpose all devices are frozen just like before creating -the image during hibernation, in the - - prepare, freeze, freeze_noirq - -phases described above. However, the devices affected by these phases are only -those having drivers in the boot kernel; other devices will still be in whatever -state the boot loader left them. - -Should the restoration of the pre-hibernation memory contents fail, the boot -kernel would go through the "thawing" procedure described above, using the -thaw_noirq, thaw, and complete phases (that will only affect the devices having -drivers in the boot kernel), and then continue running normally. - -If the pre-hibernation memory contents are restored successfully, which is the -usual situation, control is passed to the image kernel, which then becomes -responsible for bringing the system back to the working state. To achieve this, -it must restore the devices' pre-hibernation functionality, which is done much -like waking up from the memory sleep state, although it involves different -phases: - - restore_noirq, restore, complete - -The first two of these are analogous to the resume_noirq and resume phases -described above, respectively, and correspond to the following PCI subsystem -callbacks: - - pci_pm_restore_noirq() - pci_pm_restore() - -These callbacks work in analogy with pci_pm_resume_noirq() and pci_pm_resume(), -respectively, but they execute the device driver's pm->restore_noirq() and -pm->restore() callbacks, if available. - -The complete phase is carried out in exactly the same way as during system -resume. - - -3. PCI Device Drivers and Power Management -========================================== - -3.1. Power Management Callbacks -------------------------------- -PCI device drivers participate in power management by providing callbacks to be -executed by the PCI subsystem's power management routines described above and by -controlling the runtime power management of their devices. - -At the time of this writing there are two ways to define power management -callbacks for a PCI device driver, the recommended one, based on using a -dev_pm_ops structure described in Documentation/driver-api/pm/devices.rst, and the -"legacy" one, in which the .suspend(), .suspend_late(), .resume_early(), and -.resume() callbacks from struct pci_driver are used. The legacy approach, -however, doesn't allow one to define runtime power management callbacks and is -not really suitable for any new drivers. Therefore it is not covered by this -document (refer to the source code to learn more about it). - -It is recommended that all PCI device drivers define a struct dev_pm_ops object -containing pointers to power management (PM) callbacks that will be executed by -the PCI subsystem's PM routines in various circumstances. A pointer to the -driver's struct dev_pm_ops object has to be assigned to the driver.pm field in -its struct pci_driver object. Once that has happened, the "legacy" PM callbacks -in struct pci_driver are ignored (even if they are not NULL). - -The PM callbacks in struct dev_pm_ops are not mandatory and if they are not -defined (i.e. the respective fields of struct dev_pm_ops are unset) the PCI -subsystem will handle the device in a simplified default manner. If they are -defined, though, they are expected to behave as described in the following -subsections. - -3.1.1. prepare() - -The prepare() callback is executed during system suspend, during hibernation -(when a hibernation image is about to be created), during power-off after -saving a hibernation image and during system restore, when a hibernation image -has just been loaded into memory. - -This callback is only necessary if the driver's device has children that in -general may be registered at any time. In that case the role of the prepare() -callback is to prevent new children of the device from being registered until -one of the resume_noirq(), thaw_noirq(), or restore_noirq() callbacks is run. - -In addition to that the prepare() callback may carry out some operations -preparing the device to be suspended, although it should not allocate memory -(if additional memory is required to suspend the device, it has to be -preallocated earlier, for example in a suspend/hibernate notifier as described -in Documentation/driver-api/pm/notifiers.rst). - -3.1.2. suspend() - -The suspend() callback is only executed during system suspend, after prepare() -callbacks have been executed for all devices in the system. - -This callback is expected to quiesce the device and prepare it to be put into a -low-power state by the PCI subsystem. It is not required (in fact it even is -not recommended) that a PCI driver's suspend() callback save the standard -configuration registers of the device, prepare it for waking up the system, or -put it into a low-power state. All of these operations can very well be taken -care of by the PCI subsystem, without the driver's participation. - -However, in some rare case it is convenient to carry out these operations in -a PCI driver. Then, pci_save_state(), pci_prepare_to_sleep(), and -pci_set_power_state() should be used to save the device's standard configuration -registers, to prepare it for system wakeup (if necessary), and to put it into a -low-power state, respectively. Moreover, if the driver calls pci_save_state(), -the PCI subsystem will not execute either pci_prepare_to_sleep(), or -pci_set_power_state() for its device, so the driver is then responsible for -handling the device as appropriate. - -While the suspend() callback is being executed, the driver's interrupt handler -can be invoked to handle an interrupt from the device, so all suspend-related -operations relying on the driver's ability to handle interrupts should be -carried out in this callback. - -3.1.3. suspend_noirq() - -The suspend_noirq() callback is only executed during system suspend, after -suspend() callbacks have been executed for all devices in the system and -after device interrupts have been disabled by the PM core. - -The difference between suspend_noirq() and suspend() is that the driver's -interrupt handler will not be invoked while suspend_noirq() is running. Thus -suspend_noirq() can carry out operations that would cause race conditions to -arise if they were performed in suspend(). - -3.1.4. freeze() - -The freeze() callback is hibernation-specific and is executed in two situations, -during hibernation, after prepare() callbacks have been executed for all devices -in preparation for the creation of a system image, and during restore, -after a system image has been loaded into memory from persistent storage and the -prepare() callbacks have been executed for all devices. - -The role of this callback is analogous to the role of the suspend() callback -described above. In fact, they only need to be different in the rare cases when -the driver takes the responsibility for putting the device into a low-power -state. - -In that cases the freeze() callback should not prepare the device system wakeup -or put it into a low-power state. Still, either it or freeze_noirq() should -save the device's standard configuration registers using pci_save_state(). - -3.1.5. freeze_noirq() - -The freeze_noirq() callback is hibernation-specific. It is executed during -hibernation, after prepare() and freeze() callbacks have been executed for all -devices in preparation for the creation of a system image, and during restore, -after a system image has been loaded into memory and after prepare() and -freeze() callbacks have been executed for all devices. It is always executed -after device interrupts have been disabled by the PM core. - -The role of this callback is analogous to the role of the suspend_noirq() -callback described above and it very rarely is necessary to define -freeze_noirq(). - -The difference between freeze_noirq() and freeze() is analogous to the -difference between suspend_noirq() and suspend(). - -3.1.6. poweroff() - -The poweroff() callback is hibernation-specific. It is executed when the system -is about to be powered off after saving a hibernation image to a persistent -storage. prepare() callbacks are executed for all devices before poweroff() is -called. - -The role of this callback is analogous to the role of the suspend() and freeze() -callbacks described above, although it does not need to save the contents of -the device's registers. In particular, if the driver wants to put the device -into a low-power state itself instead of allowing the PCI subsystem to do that, -the poweroff() callback should use pci_prepare_to_sleep() and -pci_set_power_state() to prepare the device for system wakeup and to put it -into a low-power state, respectively, but it need not save the device's standard -configuration registers. - -3.1.7. poweroff_noirq() - -The poweroff_noirq() callback is hibernation-specific. It is executed after -poweroff() callbacks have been executed for all devices in the system. - -The role of this callback is analogous to the role of the suspend_noirq() and -freeze_noirq() callbacks described above, but it does not need to save the -contents of the device's registers. - -The difference between poweroff_noirq() and poweroff() is analogous to the -difference between suspend_noirq() and suspend(). - -3.1.8. resume_noirq() - -The resume_noirq() callback is only executed during system resume, after the -PM core has enabled the non-boot CPUs. The driver's interrupt handler will not -be invoked while resume_noirq() is running, so this callback can carry out -operations that might race with the interrupt handler. - -Since the PCI subsystem unconditionally puts all devices into the full power -state in the resume_noirq phase of system resume and restores their standard -configuration registers, resume_noirq() is usually not necessary. In general -it should only be used for performing operations that would lead to race -conditions if carried out by resume(). - -3.1.9. resume() - -The resume() callback is only executed during system resume, after -resume_noirq() callbacks have been executed for all devices in the system and -device interrupts have been enabled by the PM core. - -This callback is responsible for restoring the pre-suspend configuration of the -device and bringing it back to the fully functional state. The device should be -able to process I/O in a usual way after resume() has returned. - -3.1.10. thaw_noirq() - -The thaw_noirq() callback is hibernation-specific. It is executed after a -system image has been created and the non-boot CPUs have been enabled by the PM -core, in the thaw_noirq phase of hibernation. It also may be executed if the -loading of a hibernation image fails during system restore (it is then executed -after enabling the non-boot CPUs). The driver's interrupt handler will not be -invoked while thaw_noirq() is running. - -The role of this callback is analogous to the role of resume_noirq(). The -difference between these two callbacks is that thaw_noirq() is executed after -freeze() and freeze_noirq(), so in general it does not need to modify the -contents of the device's registers. - -3.1.11. thaw() - -The thaw() callback is hibernation-specific. It is executed after thaw_noirq() -callbacks have been executed for all devices in the system and after device -interrupts have been enabled by the PM core. - -This callback is responsible for restoring the pre-freeze configuration of -the device, so that it will work in a usual way after thaw() has returned. - -3.1.12. restore_noirq() - -The restore_noirq() callback is hibernation-specific. It is executed in the -restore_noirq phase of hibernation, when the boot kernel has passed control to -the image kernel and the non-boot CPUs have been enabled by the image kernel's -PM core. - -This callback is analogous to resume_noirq() with the exception that it cannot -make any assumption on the previous state of the device, even if the BIOS (or -generally the platform firmware) is known to preserve that state over a -suspend-resume cycle. - -For the vast majority of PCI device drivers there is no difference between -resume_noirq() and restore_noirq(). - -3.1.13. restore() - -The restore() callback is hibernation-specific. It is executed after -restore_noirq() callbacks have been executed for all devices in the system and -after the PM core has enabled device drivers' interrupt handlers to be invoked. - -This callback is analogous to resume(), just like restore_noirq() is analogous -to resume_noirq(). Consequently, the difference between restore_noirq() and -restore() is analogous to the difference between resume_noirq() and resume(). - -For the vast majority of PCI device drivers there is no difference between -resume() and restore(). - -3.1.14. complete() - -The complete() callback is executed in the following situations: - - during system resume, after resume() callbacks have been executed for all - devices, - - during hibernation, before saving the system image, after thaw() callbacks - have been executed for all devices, - - during system restore, when the system is going back to its pre-hibernation - state, after restore() callbacks have been executed for all devices. -It also may be executed if the loading of a hibernation image into memory fails -(in that case it is run after thaw() callbacks have been executed for all -devices that have drivers in the boot kernel). - -This callback is entirely optional, although it may be necessary if the -prepare() callback performs operations that need to be reversed. - -3.1.15. runtime_suspend() - -The runtime_suspend() callback is specific to device runtime power management -(runtime PM). It is executed by the PM core's runtime PM framework when the -device is about to be suspended (i.e. quiesced and put into a low-power state) -at run time. - -This callback is responsible for freezing the device and preparing it to be -put into a low-power state, but it must allow the PCI subsystem to perform all -of the PCI-specific actions necessary for suspending the device. - -3.1.16. runtime_resume() - -The runtime_resume() callback is specific to device runtime PM. It is executed -by the PM core's runtime PM framework when the device is about to be resumed -(i.e. put into the full-power state and programmed to process I/O normally) at -run time. - -This callback is responsible for restoring the normal functionality of the -device after it has been put into the full-power state by the PCI subsystem. -The device is expected to be able to process I/O in the usual way after -runtime_resume() has returned. - -3.1.17. runtime_idle() - -The runtime_idle() callback is specific to device runtime PM. It is executed -by the PM core's runtime PM framework whenever it may be desirable to suspend -the device according to the PM core's information. In particular, it is -automatically executed right after runtime_resume() has returned in case the -resume of the device has happened as a result of a spurious event. - -This callback is optional, but if it is not implemented or if it returns 0, the -PCI subsystem will call pm_runtime_suspend() for the device, which in turn will -cause the driver's runtime_suspend() callback to be executed. - -3.1.18. Pointing Multiple Callback Pointers to One Routine - -Although in principle each of the callbacks described in the previous -subsections can be defined as a separate function, it often is convenient to -point two or more members of struct dev_pm_ops to the same routine. There are -a few convenience macros that can be used for this purpose. - -The SIMPLE_DEV_PM_OPS macro declares a struct dev_pm_ops object with one -suspend routine pointed to by the .suspend(), .freeze(), and .poweroff() -members and one resume routine pointed to by the .resume(), .thaw(), and -.restore() members. The other function pointers in this struct dev_pm_ops are -unset. - -The UNIVERSAL_DEV_PM_OPS macro is similar to SIMPLE_DEV_PM_OPS, but it -additionally sets the .runtime_resume() pointer to the same value as -.resume() (and .thaw(), and .restore()) and the .runtime_suspend() pointer to -the same value as .suspend() (and .freeze() and .poweroff()). - -The SET_SYSTEM_SLEEP_PM_OPS can be used inside of a declaration of struct -dev_pm_ops to indicate that one suspend routine is to be pointed to by the -.suspend(), .freeze(), and .poweroff() members and one resume routine is to -be pointed to by the .resume(), .thaw(), and .restore() members. - -3.1.19. Driver Flags for Power Management - -The PM core allows device drivers to set flags that influence the handling of -power management for the devices by the core itself and by middle layer code -including the PCI bus type. The flags should be set once at the driver probe -time with the help of the dev_pm_set_driver_flags() function and they should not -be updated directly afterwards. - -The DPM_FLAG_NEVER_SKIP flag prevents the PM core from using the direct-complete -mechanism allowing device suspend/resume callbacks to be skipped if the device -is in runtime suspend when the system suspend starts. That also affects all of -the ancestors of the device, so this flag should only be used if absolutely -necessary. - -The DPM_FLAG_SMART_PREPARE flag instructs the PCI bus type to only return a -positive value from pci_pm_prepare() if the ->prepare callback provided by the -driver of the device returns a positive value. That allows the driver to opt -out from using the direct-complete mechanism dynamically. - -The DPM_FLAG_SMART_SUSPEND flag tells the PCI bus type that from the driver's -perspective the device can be safely left in runtime suspend during system -suspend. That causes pci_pm_suspend(), pci_pm_freeze() and pci_pm_poweroff() -to skip resuming the device from runtime suspend unless there are PCI-specific -reasons for doing that. Also, it causes pci_pm_suspend_late/noirq(), -pci_pm_freeze_late/noirq() and pci_pm_poweroff_late/noirq() to return early -if the device remains in runtime suspend in the beginning of the "late" phase -of the system-wide transition under way. Moreover, if the device is in -runtime suspend in pci_pm_resume_noirq() or pci_pm_restore_noirq(), its runtime -power management status will be changed to "active" (as it is going to be put -into D0 going forward), but if it is in runtime suspend in pci_pm_thaw_noirq(), -the function will set the power.direct_complete flag for it (to make the PM core -skip the subsequent "thaw" callbacks for it) and return. - -Setting the DPM_FLAG_LEAVE_SUSPENDED flag means that the driver prefers the -device to be left in suspend after system-wide transitions to the working state. -This flag is checked by the PM core, but the PCI bus type informs the PM core -which devices may be left in suspend from its perspective (that happens during -the "noirq" phase of system-wide suspend and analogous transitions) and next it -uses the dev_pm_may_skip_resume() helper to decide whether or not to return from -pci_pm_resume_noirq() early, as the PM core will skip the remaining resume -callbacks for the device during the transition under way and will set its -runtime PM status to "suspended" if dev_pm_may_skip_resume() returns "true" for -it. - -3.2. Device Runtime Power Management ------------------------------------- -In addition to providing device power management callbacks PCI device drivers -are responsible for controlling the runtime power management (runtime PM) of -their devices. - -The PCI device runtime PM is optional, but it is recommended that PCI device -drivers implement it at least in the cases where there is a reliable way of -verifying that the device is not used (like when the network cable is detached -from an Ethernet adapter or there are no devices attached to a USB controller). - -To support the PCI runtime PM the driver first needs to implement the -runtime_suspend() and runtime_resume() callbacks. It also may need to implement -the runtime_idle() callback to prevent the device from being suspended again -every time right after the runtime_resume() callback has returned -(alternatively, the runtime_suspend() callback will have to check if the -device should really be suspended and return -EAGAIN if that is not the case). - -The runtime PM of PCI devices is enabled by default by the PCI core. PCI -device drivers do not need to enable it and should not attempt to do so. -However, it is blocked by pci_pm_init() that runs the pm_runtime_forbid() -helper function. In addition to that, the runtime PM usage counter of -each PCI device is incremented by local_pci_probe() before executing the -probe callback provided by the device's driver. - -If a PCI driver implements the runtime PM callbacks and intends to use the -runtime PM framework provided by the PM core and the PCI subsystem, it needs -to decrement the device's runtime PM usage counter in its probe callback -function. If it doesn't do that, the counter will always be different from -zero for the device and it will never be runtime-suspended. The simplest -way to do that is by calling pm_runtime_put_noidle(), but if the driver -wants to schedule an autosuspend right away, for example, it may call -pm_runtime_put_autosuspend() instead for this purpose. Generally, it -just needs to call a function that decrements the devices usage counter -from its probe routine to make runtime PM work for the device. - -It is important to remember that the driver's runtime_suspend() callback -may be executed right after the usage counter has been decremented, because -user space may already have caused the pm_runtime_allow() helper function -unblocking the runtime PM of the device to run via sysfs, so the driver must -be prepared to cope with that. - -The driver itself should not call pm_runtime_allow(), though. Instead, it -should let user space or some platform-specific code do that (user space can -do it via sysfs as stated above), but it must be prepared to handle the -runtime PM of the device correctly as soon as pm_runtime_allow() is called -(which may happen at any time, even before the driver is loaded). - -When the driver's remove callback runs, it has to balance the decrementation -of the device's runtime PM usage counter at the probe time. For this reason, -if it has decremented the counter in its probe callback, it must run -pm_runtime_get_noresume() in its remove callback. [Since the core carries -out a runtime resume of the device and bumps up the device's usage counter -before running the driver's remove callback, the runtime PM of the device -is effectively disabled for the duration of the remove execution and all -runtime PM helper functions incrementing the device's usage counter are -then effectively equivalent to pm_runtime_get_noresume().] - -The runtime PM framework works by processing requests to suspend or resume -devices, or to check if they are idle (in which cases it is reasonable to -subsequently request that they be suspended). These requests are represented -by work items put into the power management workqueue, pm_wq. Although there -are a few situations in which power management requests are automatically -queued by the PM core (for example, after processing a request to resume a -device the PM core automatically queues a request to check if the device is -idle), device drivers are generally responsible for queuing power management -requests for their devices. For this purpose they should use the runtime PM -helper functions provided by the PM core, discussed in -Documentation/power/runtime_pm.txt. - -Devices can also be suspended and resumed synchronously, without placing a -request into pm_wq. In the majority of cases this also is done by their -drivers that use helper functions provided by the PM core for this purpose. - -For more information on the runtime PM of devices refer to -Documentation/power/runtime_pm.txt. - - -4. Resources -============ - -PCI Local Bus Specification, Rev. 3.0 -PCI Bus Power Management Interface Specification, Rev. 1.2 -Advanced Configuration and Power Interface (ACPI) Specification, Rev. 3.0b -PCI Express Base Specification, Rev. 2.0 -Documentation/driver-api/pm/devices.rst -Documentation/power/runtime_pm.txt |