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@@ -1,208 +1,553 @@
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+Most of the code in Linux is device drivers, so most of the Linux power
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+management code is also driver-specific. Most drivers will do very little;
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+others, especially for platforms with small batteries (like cell phones),
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+will do a lot.
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+
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+This writeup gives an overview of how drivers interact with system-wide
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+power management goals, emphasizing the models and interfaces that are
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+shared by everything that hooks up to the driver model core. Read it as
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+background for the domain-specific work you'd do with any specific driver.
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+
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+
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+Two Models for Device Power Management
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+======================================
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+Drivers will use one or both of these models to put devices into low-power
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+states:
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+
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+ System Sleep model:
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+ Drivers can enter low power states as part of entering system-wide
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+ low-power states like "suspend-to-ram", or (mostly for systems with
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+ disks) "hibernate" (suspend-to-disk).
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+
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+ This is something that device, bus, and class drivers collaborate on
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+ by implementing various role-specific suspend and resume methods to
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+ cleanly power down hardware and software subsystems, then reactivate
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+ them without loss of data.
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+
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+ Some drivers can manage hardware wakeup events, which make the system
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+ leave that low-power state. This feature may be disabled using the
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+ relevant /sys/devices/.../power/wakeup file; enabling it may cost some
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+ power usage, but let the whole system enter low power states more often.
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+
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+ Runtime Power Management model:
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+ Drivers may also enter low power states while the system is running,
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+ independently of other power management activity. Upstream drivers
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+ will normally not know (or care) if the device is in some low power
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+ state when issuing requests; the driver will auto-resume anything
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+ that's needed when it gets a request.
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+
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+ This doesn't have, or need much infrastructure; it's just something you
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+ should do when writing your drivers. For example, clk_disable() unused
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+ clocks as part of minimizing power drain for currently-unused hardware.
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+ Of course, sometimes clusters of drivers will collaborate with each
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+ other, which could involve task-specific power management.
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+
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+There's not a lot to be said about those low power states except that they
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+are very system-specific, and often device-specific. Also, that if enough
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+drivers put themselves into low power states (at "runtime"), the effect may be
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+the same as entering some system-wide low-power state (system sleep) ... and
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+that synergies exist, so that several drivers using runtime pm might put the
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+system into a state where even deeper power saving options are available.
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+
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+Most suspended devices will have quiesced all I/O: no more DMA or irqs, no
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+more data read or written, and requests from upstream drivers are no longer
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+accepted. A given bus or platform may have different requirements though.
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+
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+Examples of hardware wakeup events include an alarm from a real time clock,
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+network wake-on-LAN packets, keyboard or mouse activity, and media insertion
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+or removal (for PCMCIA, MMC/SD, USB, and so on).
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+
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+
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+Interfaces for Entering System Sleep States
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+===========================================
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+Most of the programming interfaces a device driver needs to know about
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+relate to that first model: entering a system-wide low power state,
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+rather than just minimizing power consumption by one device.
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+
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+
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+Bus Driver Methods
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+------------------
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+The core methods to suspend and resume devices reside in struct bus_type.
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+These are mostly of interest to people writing infrastructure for busses
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+like PCI or USB, or because they define the primitives that device drivers
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+may need to apply in domain-specific ways to their devices:
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-Device Power Management
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+struct bus_type {
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+ ...
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+ int (*suspend)(struct device *dev, pm_message_t state);
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+ int (*suspend_late)(struct device *dev, pm_message_t state);
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+ int (*resume_early)(struct device *dev);
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+ int (*resume)(struct device *dev);
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+};
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-Device power management encompasses two areas - the ability to save
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-state and transition a device to a low-power state when the system is
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-entering a low-power state; and the ability to transition a device to
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-a low-power state while the system is running (and independently of
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-any other power management activity).
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+Bus drivers implement those methods as appropriate for the hardware and
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+the drivers using it; PCI works differently from USB, and so on. Not many
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+people write bus drivers; most driver code is a "device driver" that
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+builds on top of bus-specific framework code.
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+
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+For more information on these driver calls, see the description later;
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+they are called in phases for every device, respecting the parent-child
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+sequencing in the driver model tree. Note that as this is being written,
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+only the suspend() and resume() are widely available; not many bus drivers
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+leverage all of those phases, or pass them down to lower driver levels.
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+
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+
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+/sys/devices/.../power/wakeup files
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+-----------------------------------
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+All devices in the driver model have two flags to control handling of
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+wakeup events, which are hardware signals that can force the device and/or
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+system out of a low power state. These are initialized by bus or device
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+driver code using device_init_wakeup(dev,can_wakeup).
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+
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+The "can_wakeup" flag just records whether the device (and its driver) can
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+physically support wakeup events. When that flag is clear, the sysfs
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+"wakeup" file is empty, and device_may_wakeup() returns false.
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+
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+For devices that can issue wakeup events, a separate flag controls whether
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+that device should try to use its wakeup mechanism. The initial value of
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+device_may_wakeup() will be true, so that the device's "wakeup" file holds
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+the value "enabled". Userspace can change that to "disabled" so that
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+device_may_wakeup() returns false; or change it back to "enabled" (so that
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+it returns true again).
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+
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+
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+EXAMPLE: PCI Device Driver Methods
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+-----------------------------------
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+PCI framework software calls these methods when the PCI device driver bound
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+to a device device has provided them:
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+
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+struct pci_driver {
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+ ...
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+ int (*suspend)(struct pci_device *pdev, pm_message_t state);
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+ int (*suspend_late)(struct pci_device *pdev, pm_message_t state);
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+
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+ int (*resume_early)(struct pci_device *pdev);
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+ int (*resume)(struct pci_device *pdev);
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+};
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+Drivers will implement those methods, and call PCI-specific procedures
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+like pci_set_power_state(), pci_enable_wake(), pci_save_state(), and
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+pci_restore_state() to manage PCI-specific mechanisms. (PCI config space
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+could be saved during driver probe, if it weren't for the fact that some
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+systems rely on userspace tweaking using setpci.) Devices are suspended
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+before their bridges enter low power states, and likewise bridges resume
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+before their devices.
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+
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+
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+Upper Layers of Driver Stacks
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+-----------------------------
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+Device drivers generally have at least two interfaces, and the methods
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+sketched above are the ones which apply to the lower level (nearer PCI, USB,
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+or other bus hardware). The network and block layers are examples of upper
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+level interfaces, as is a character device talking to userspace.
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+
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+Power management requests normally need to flow through those upper levels,
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+which often use domain-oriented requests like "blank that screen". In
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+some cases those upper levels will have power management intelligence that
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+relates to end-user activity, or other devices that work in cooperation.
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+
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+When those interfaces are structured using class interfaces, there is a
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+standard way to have the upper layer stop issuing requests to a given
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+class device (and restart later):
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+
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+struct class {
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+ ...
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+ int (*suspend)(struct device *dev, pm_message_t state);
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+ int (*resume)(struct device *dev);
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+};
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-Methods
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+Those calls are issued in specific phases of the process by which the
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+system enters a low power "suspend" state, or resumes from it.
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+
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+
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+Calling Drivers to Enter System Sleep States
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+============================================
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+When the system enters a low power state, each device's driver is asked
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+to suspend the device by putting it into state compatible with the target
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+system state. That's usually some version of "off", but the details are
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+system-specific. Also, wakeup-enabled devices will usually stay partly
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+functional in order to wake the system.
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+
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+When the system leaves that low power state, the device's driver is asked
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+to resume it. The suspend and resume operations always go together, and
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+both are multi-phase operations.
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+
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+For simple drivers, suspend might quiesce the device using the class code
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+and then turn its hardware as "off" as possible with late_suspend. The
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+matching resume calls would then completely reinitialize the hardware
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+before reactivating its class I/O queues.
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+
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+More power-aware drivers drivers will use more than one device low power
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+state, either at runtime or during system sleep states, and might trigger
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+system wakeup events.
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+
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+
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+Call Sequence Guarantees
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+------------------------
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+To ensure that bridges and similar links needed to talk to a device are
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+available when the device is suspended or resumed, the device tree is
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+walked in a bottom-up order to suspend devices. A top-down order is
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+used to resume those devices.
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+
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+The ordering of the device tree is defined by the order in which devices
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+get registered: a child can never be registered, probed or resumed before
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+its parent; and can't be removed or suspended after that parent.
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+
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+The policy is that the device tree should match hardware bus topology.
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+(Or at least the control bus, for devices which use multiple busses.)
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+
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+
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+Suspending Devices
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+------------------
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+Suspending a given device is done in several phases. Suspending the
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+system always includes every phase, executing calls for every device
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+before the next phase begins. Not all busses or classes support all
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+these callbacks; and not all drivers use all the callbacks.
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+
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+The phases are seen by driver notifications issued in this order:
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+
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+ 1 class.suspend(dev, message) is called after tasks are frozen, for
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+ devices associated with a class that has such a method. This
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+ method may sleep.
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+
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+ Since I/O activity usually comes from such higher layers, this is
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+ a good place to quiesce all drivers of a given type (and keep such
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+ code out of those drivers).
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+
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+ 2 bus.suspend(dev, message) is called next. This method may sleep,
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+ and is often morphed into a device driver call with bus-specific
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+ parameters and/or rules.
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+
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+ This call should handle parts of device suspend logic that require
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+ sleeping. It probably does work to quiesce the device which hasn't
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+ been abstracted into class.suspend() or bus.suspend_late().
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+
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+ 3 bus.suspend_late(dev, message) is called with IRQs disabled, and
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+ with only one CPU active. Until the bus.resume_early() phase
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+ completes (see later), IRQs are not enabled again. This method
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+ won't be exposed by all busses; for message based busses like USB,
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+ I2C, or SPI, device interactions normally require IRQs. This bus
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+ call may be morphed into a driver call with bus-specific parameters.
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+
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+ This call might save low level hardware state that might otherwise
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+ be lost in the upcoming low power state, and actually put the
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+ device into a low power state ... so that in some cases the device
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+ may stay partly usable until this late. This "late" call may also
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+ help when coping with hardware that behaves badly.
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+
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+The pm_message_t parameter is currently used to refine those semantics
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+(described later).
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+
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+At the end of those phases, drivers should normally have stopped all I/O
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+transactions (DMA, IRQs), saved enough state that they can re-initialize
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+or restore previous state (as needed by the hardware), and placed the
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+device into a low-power state. On many platforms they will also use
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+clk_disable() to gate off one or more clock sources; sometimes they will
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+also switch off power supplies, or reduce voltages. Drivers which have
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+runtime PM support may already have performed some or all of the steps
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+needed to prepare for the upcoming system sleep state.
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+
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+When any driver sees that its device_can_wakeup(dev), it should make sure
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+to use the relevant hardware signals to trigger a system wakeup event.
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+For example, enable_irq_wake() might identify GPIO signals hooked up to
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+a switch or other external hardware, and pci_enable_wake() does something
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+similar for PCI's PME# signal.
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+
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+If a driver (or bus, or class) fails it suspend method, the system won't
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+enter the desired low power state; it will resume all the devices it's
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+suspended so far.
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+
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+Note that drivers may need to perform different actions based on the target
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+system lowpower/sleep state. At this writing, there are only platform
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+specific APIs through which drivers could determine those target states.
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+
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+
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+Device Low Power (suspend) States
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+---------------------------------
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+Device low-power states aren't very standard. One device might only handle
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+"on" and "off, while another might support a dozen different versions of
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+"on" (how many engines are active?), plus a state that gets back to "on"
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+faster than from a full "off".
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+
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+Some busses define rules about what different suspend states mean. PCI
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+gives one example: after the suspend sequence completes, a non-legacy
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+PCI device may not perform DMA or issue IRQs, and any wakeup events it
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+issues would be issued through the PME# bus signal. Plus, there are
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+several PCI-standard device states, some of which are optional.
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+
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+In contrast, integrated system-on-chip processors often use irqs as the
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+wakeup event sources (so drivers would call enable_irq_wake) and might
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+be able to treat DMA completion as a wakeup event (sometimes DMA can stay
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+active too, it'd only be the CPU and some peripherals that sleep).
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+
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+Some details here may be platform-specific. Systems may have devices that
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+can be fully active in certain sleep states, such as an LCD display that's
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+refreshed using DMA while most of the system is sleeping lightly ... and
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+its frame buffer might even be updated by a DSP or other non-Linux CPU while
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+the Linux control processor stays idle.
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+
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+Moreover, the specific actions taken may depend on the target system state.
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+One target system state might allow a given device to be very operational;
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+another might require a hard shut down with re-initialization on resume.
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+And two different target systems might use the same device in different
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+ways; the aforementioned LCD might be active in one product's "standby",
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+but a different product using the same SOC might work differently.
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+
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+
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+Meaning of pm_message_t.event
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+-----------------------------
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+Parameters to suspend calls include the device affected and a message of
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+type pm_message_t, which has one field: the event. If driver does not
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+recognize the event code, suspend calls may abort the request and return
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+a negative errno. However, most drivers will be fine if they implement
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+PM_EVENT_SUSPEND semantics for all messages.
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+
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+The event codes are used to refine the goal of suspending the device, and
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+mostly matter when creating or resuming system memory image snapshots, as
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+used with suspend-to-disk:
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+
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+ PM_EVENT_SUSPEND -- quiesce the driver and put hardware into a low-power
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+ state. When used with system sleep states like "suspend-to-RAM" or
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+ "standby", the upcoming resume() call will often be able to rely on
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+ state kept in hardware, or issue system wakeup events. When used
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+ instead with suspend-to-disk, few devices support this capability;
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+ most are completely powered off.
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+
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+ PM_EVENT_FREEZE -- quiesce the driver, but don't necessarily change into
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+ any low power mode. A system snapshot is about to be taken, often
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+ followed by a call to the driver's resume() method. Neither wakeup
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+ events nor DMA are allowed.
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+
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+ PM_EVENT_PRETHAW -- quiesce the driver, knowing that the upcoming resume()
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+ will restore a suspend-to-disk snapshot from a different kernel image.
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+ Drivers that are smart enough to look at their hardware state during
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+ resume() processing need that state to be correct ... a PRETHAW could
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+ be used to invalidate that state (by resetting the device), like a
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+ shutdown() invocation would before a kexec() or system halt. Other
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+ drivers might handle this the same way as PM_EVENT_FREEZE. Neither
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+ wakeup events nor DMA are allowed.
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+
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+To enter "standby" (ACPI S1) or "Suspend to RAM" (STR, ACPI S3) states, or
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+the similarly named APM states, only PM_EVENT_SUSPEND is used; for "Suspend
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+to Disk" (STD, hibernate, ACPI S4), all of those event codes are used.
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+
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+There's also PM_EVENT_ON, a value which never appears as a suspend event
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+but is sometimes used to record the "not suspended" device state.
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+
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+
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+Resuming Devices
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+----------------
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+Resuming is done in multiple phases, much like suspending, with all
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+devices processing each phase's calls before the next phase begins.
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+
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+The phases are seen by driver notifications issued in this order:
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+
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+ 1 bus.resume_early(dev) is called with IRQs disabled, and with
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+ only one CPU active. As with bus.suspend_late(), this method
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+ won't be supported on busses that require IRQs in order to
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+ interact with devices.
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+
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+ This reverses the effects of bus.suspend_late().
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+
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+ 2 bus.resume(dev) is called next. This may be morphed into a device
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+ driver call with bus-specific parameters; implementations may sleep.
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+
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+ This reverses the effects of bus.suspend().
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+
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+ 3 class.resume(dev) is called for devices associated with a class
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+ that has such a method. Implementations may sleep.
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+
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+ This reverses the effects of class.suspend(), and would usually
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+ reactivate the device's I/O queue.
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+
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+At the end of those phases, drivers should normally be as functional as
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+they were before suspending: I/O can be performed using DMA and IRQs, and
|
|
|
+the relevant clocks are gated on. The device need not be "fully on"; it
|
|
|
+might be in a runtime lowpower/suspend state that acts as if it were.
|
|
|
+
|
|
|
+However, the details here may again be platform-specific. For example,
|
|
|
+some systems support multiple "run" states, and the mode in effect at
|
|
|
+the end of resume() might not be the one which preceded suspension.
|
|
|
+That means availability of certain clocks or power supplies changed,
|
|
|
+which could easily affect how a driver works.
|
|
|
+
|
|
|
+
|
|
|
+Drivers need to be able to handle hardware which has been reset since the
|
|
|
+suspend methods were called, for example by complete reinitialization.
|
|
|
+This may be the hardest part, and the one most protected by NDA'd documents
|
|
|
+and chip errata. It's simplest if the hardware state hasn't changed since
|
|
|
+the suspend() was called, but that can't always be guaranteed.
|
|
|
+
|
|
|
+Drivers must also be prepared to notice that the device has been removed
|
|
|
+while the system was powered off, whenever that's physically possible.
|
|
|
+PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
|
|
|
+where common Linux platforms will see such removal. Details of how drivers
|
|
|
+will notice and handle such removals are currently bus-specific, and often
|
|
|
+involve a separate thread.
|
|
|
|
|
|
-The methods to suspend and resume devices reside in struct bus_type:
|
|
|
|
|
|
-struct bus_type {
|
|
|
- ...
|
|
|
- int (*suspend)(struct device * dev, pm_message_t state);
|
|
|
- int (*resume)(struct device * dev);
|
|
|
-};
|
|
|
+Note that the bus-specific runtime PM wakeup mechanism can exist, and might
|
|
|
+be defined to share some of the same driver code as for system wakeup. For
|
|
|
+example, a bus-specific device driver's resume() method might be used there,
|
|
|
+so it wouldn't only be called from bus.resume() during system-wide wakeup.
|
|
|
+See bus-specific information about how runtime wakeup events are handled.
|
|
|
|
|
|
-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
|
|
|
+--------------
|
|
|
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.
|
|
|
+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.
|
|
|
|
|
|
+That is, IRQs are disabled, the suspend_late() phase begins, then the
|
|
|
+sysdev_driver.suspend() phase, and the system enters a sleep state. Then
|
|
|
+the sysdev_driver.resume() phase begins, followed by the resume_early()
|
|
|
+phase, after which IRQs are enabled.
|
|
|
|
|
|
-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'.
|
|
|
+Code to actually enter and exit the system-wide low power state sometimes
|
|
|
+involves hardware details that are only known to the boot firmware, and
|
|
|
+may leave a CPU running software (from SRAM or flash memory) that monitors
|
|
|
+the system and manages its wakeup sequence.
|
|
|
|
|
|
-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.
|
|
|
-
|
|
|
-pm_message_t meaning
|
|
|
-
|
|
|
-pm_message_t has two fields. event ("major"), and flags. If driver
|
|
|
-does not know event code, it aborts the request, returning error. Some
|
|
|
-drivers may need to deal with special cases based on the actual type
|
|
|
-of suspend operation being done at the system level. This is why
|
|
|
-there are flags.
|
|
|
-
|
|
|
-Event codes are:
|
|
|
-
|
|
|
-ON -- no need to do anything except special cases like broken
|
|
|
-HW.
|
|
|
-
|
|
|
-# NOTIFICATION -- pretty much same as ON?
|
|
|
-
|
|
|
-FREEZE -- stop DMA and interrupts, and be prepared to reinit HW from
|
|
|
-scratch. That probably means stop accepting upstream requests, the
|
|
|
-actual policy of what to do with them being specific to a given
|
|
|
-driver. It's acceptable for a network driver to just drop packets
|
|
|
-while a block driver is expected to block the queue so no request is
|
|
|
-lost. (Use IDE as an example on how to do that). FREEZE requires no
|
|
|
-power state change, and it's expected for drivers to be able to
|
|
|
-quickly transition back to operating state.
|
|
|
-
|
|
|
-SUSPEND -- like FREEZE, but also put hardware into low-power state. If
|
|
|
-there's need to distinguish several levels of sleep, additional flag
|
|
|
-is probably best way to do that.
|
|
|
-
|
|
|
-Transitions are only from a resumed state to a suspended state, never
|
|
|
-between 2 suspended states. (ON -> FREEZE or ON -> SUSPEND can happen,
|
|
|
-FREEZE -> SUSPEND or SUSPEND -> FREEZE can not).
|
|
|
-
|
|
|
-All events are:
|
|
|
-
|
|
|
-[NOTE NOTE NOTE: If you are driver author, you should not care; you
|
|
|
-should only look at event, and ignore flags.]
|
|
|
-
|
|
|
-#Prepare for suspend -- userland is still running but we are going to
|
|
|
-#enter suspend state. This gives drivers chance to load firmware from
|
|
|
-#disk and store it in memory, or do other activities taht require
|
|
|
-#operating userland, ability to kmalloc GFP_KERNEL, etc... All of these
|
|
|
-#are forbiden once the suspend dance is started.. event = ON, flags =
|
|
|
-#PREPARE_TO_SUSPEND
|
|
|
-
|
|
|
-Apm standby -- prepare for APM event. Quiesce devices to make life
|
|
|
-easier for APM BIOS. event = FREEZE, flags = APM_STANDBY
|
|
|
-
|
|
|
-Apm suspend -- same as APM_STANDBY, but it we should probably avoid
|
|
|
-spinning down disks. event = FREEZE, flags = APM_SUSPEND
|
|
|
-
|
|
|
-System halt, reboot -- quiesce devices to make life easier for BIOS. event
|
|
|
-= FREEZE, flags = SYSTEM_HALT or SYSTEM_REBOOT
|
|
|
-
|
|
|
-System shutdown -- at least disks need to be spun down, or data may be
|
|
|
-lost. Quiesce devices, just to make life easier for BIOS. event =
|
|
|
-FREEZE, flags = SYSTEM_SHUTDOWN
|
|
|
-
|
|
|
-Kexec -- turn off DMAs and put hardware into some state where new
|
|
|
-kernel can take over. event = FREEZE, flags = KEXEC
|
|
|
-
|
|
|
-Powerdown at end of swsusp -- very similar to SYSTEM_SHUTDOWN, except wake
|
|
|
-may need to be enabled on some devices. This actually has at least 3
|
|
|
-subtypes, system can reboot, enter S4 and enter S5 at the end of
|
|
|
-swsusp. event = FREEZE, flags = SWSUSP and one of SYSTEM_REBOOT,
|
|
|
-SYSTEM_SHUTDOWN, SYSTEM_S4
|
|
|
-
|
|
|
-Suspend to ram -- put devices into low power state. event = SUSPEND,
|
|
|
-flags = SUSPEND_TO_RAM
|
|
|
-
|
|
|
-Freeze for swsusp snapshot -- stop DMA and interrupts. No need to put
|
|
|
-devices into low power mode, but you must be able to reinitialize
|
|
|
-device from scratch in resume method. This has two flavors, its done
|
|
|
-once on suspending kernel, once on resuming kernel. event = FREEZE,
|
|
|
-flags = DURING_SUSPEND or DURING_RESUME
|
|
|
-
|
|
|
-Device detach requested from /sys -- deinitialize device; proably same as
|
|
|
-SYSTEM_SHUTDOWN, I do not understand this one too much. probably event
|
|
|
-= FREEZE, flags = DEV_DETACH.
|
|
|
-
|
|
|
-#These are not really events sent:
|
|
|
-#
|
|
|
-#System fully on -- device is working normally; this is probably never
|
|
|
-#passed to suspend() method... event = ON, flags = 0
|
|
|
-#
|
|
|
-#Ready after resume -- userland is now running, again. Time to free any
|
|
|
-#memory you ate during prepare to suspend... event = ON, flags =
|
|
|
-#READY_AFTER_RESUME
|
|
|
-#
|
|
|
+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. These devices
|
|
|
+often support a range of runtime power states, which might use names such
|
|
|
+as "off", "sleep", "idle", "active", and so on. Those states will in some
|
|
|
+cases (like PCI) be partially constrained by a bus the device uses, and will
|
|
|
+usually include hardware states that are also used in system sleep states.
|
|
|
+
|
|
|
+However, note that if a driver puts a device into a runtime low power state
|
|
|
+and the system then goes into a system-wide sleep state, it normally ought
|
|
|
+to resume into that runtime low power state rather than "full on". Such
|
|
|
+distinctions would be part of the driver-internal state machine for that
|
|
|
+hardware; the whole point of runtime power management is to be sure that
|
|
|
+drivers are decoupled in that way from the state machine governing phases
|
|
|
+of the system-wide power/sleep state transitions.
|
|
|
+
|
|
|
+
|
|
|
+Power Saving Techniques
|
|
|
+-----------------------
|
|
|
+Normally runtime power management is handled by the drivers without specific
|
|
|
+userspace or kernel intervention, by device-aware use of techniques like:
|
|
|
+
|
|
|
+ Using information provided by other system layers
|
|
|
+ - stay deeply "off" except between open() and close()
|
|
|
+ - if transceiver/PHY indicates "nobody connected", stay "off"
|
|
|
+ - application protocols may include power commands or hints
|
|
|
+
|
|
|
+ Using fewer CPU cycles
|
|
|
+ - using DMA instead of PIO
|
|
|
+ - removing timers, or making them lower frequency
|
|
|
+ - shortening "hot" code paths
|
|
|
+ - eliminating cache misses
|
|
|
+ - (sometimes) offloading work to device firmware
|
|
|
+
|
|
|
+ Reducing other resource costs
|
|
|
+ - gating off unused clocks in software (or hardware)
|
|
|
+ - switching off unused power supplies
|
|
|
+ - eliminating (or delaying/merging) IRQs
|
|
|
+ - tuning DMA to use word and/or burst modes
|
|
|
+
|
|
|
+ Using device-specific low power states
|
|
|
+ - using lower voltages
|
|
|
+ - avoiding needless DMA transfers
|
|
|
+
|
|
|
+Read your hardware documentation carefully to see the opportunities that
|
|
|
+may be available. If you can, measure the actual power usage and check
|
|
|
+it against the budget established for your project.
|
|
|
+
|
|
|
+
|
|
|
+Examples: USB hosts, system timer, system CPU
|
|
|
+----------------------------------------------
|
|
|
+USB host controllers make interesting, if complex, examples. In many cases
|
|
|
+these have no work to do: no USB devices are connected, or all of them are
|
|
|
+in the USB "suspend" state. Linux host controller drivers can then disable
|
|
|
+periodic DMA transfers that would otherwise be a constant power drain on the
|
|
|
+memory subsystem, and enter a suspend state. In power-aware controllers,
|
|
|
+entering that suspend state may disable the clock used with USB signaling,
|
|
|
+saving a certain amount of power.
|
|
|
+
|
|
|
+The controller will be woken from that state (with an IRQ) by changes to the
|
|
|
+signal state on the data lines of a given port, for example by an existing
|
|
|
+peripheral requesting "remote wakeup" or by plugging a new peripheral. The
|
|
|
+same wakeup mechanism usually works from "standby" sleep states, and on some
|
|
|
+systems also from "suspend to RAM" (or even "suspend to disk") states.
|
|
|
+(Except that ACPI may be involved instead of normal IRQs, on some hardware.)
|
|
|
+
|
|
|
+System devices like timers and CPUs may have special roles in the platform
|
|
|
+power management scheme. For example, system timers using a "dynamic tick"
|
|
|
+approach don't just save CPU cycles (by eliminating needless timer IRQs),
|
|
|
+but they may also open the door to using lower power CPU "idle" states that
|
|
|
+cost more than a jiffie to enter and exit. On x86 systems these are states
|
|
|
+like "C3"; note that periodic DMA transfers from a USB host controller will
|
|
|
+also prevent entry to a C3 state, much like a periodic timer IRQ.
|
|
|
+
|
|
|
+That kind of runtime mechanism interaction is common. "System On Chip" (SOC)
|
|
|
+processors often have low power idle modes that can't be entered unless
|
|
|
+certain medium-speed clocks (often 12 or 48 MHz) are gated off. When the
|
|
|
+drivers gate those clocks effectively, then the system idle task may be able
|
|
|
+to use the lower power idle modes and thereby increase battery life.
|
|
|
+
|
|
|
+If the CPU can have a "cpufreq" driver, there also may be opportunities
|
|
|
+to shift to lower voltage settings and reduce the power cost of executing
|
|
|
+a given number of instructions. (Without voltage adjustment, it's rare
|
|
|
+for cpufreq to save much power; the cost-per-instruction must go down.)
|
|
|
+
|
|
|
+
|
|
|
+/sys/devices/.../power/state files
|
|
|
+==================================
|
|
|
+For now you can also test some of this functionality using sysfs.
|
|
|
+
|
|
|
+ DEPRECATED: USE "power/state" ONLY FOR DRIVER TESTING, AND
|
|
|
+ AVOID USING dev->power.power_state IN DRIVERS.
|
|
|
+
|
|
|
+ THESE WILL BE REMOVED. IF THE "power/state" FILE GETS REPLACED,
|
|
|
+ IT WILL BECOME SOMETHING COUPLED TO THE BUS OR DRIVER.
|
|
|
+
|
|
|
+In each device's directory, there is a 'power' directory, which contains
|
|
|
+at least a 'state' file. The value of this field is effectively boolean,
|
|
|
+PM_EVENT_ON or PM_EVENT_SUSPEND.
|
|
|
+
|
|
|
+ * Reading from this file displays a value corresponding to
|
|
|
+ the power.power_state.event field. All nonzero values are
|
|
|
+ displayed as "2", corresponding to a low power state; zero
|
|
|
+ is displayed as "0", corresponding to normal operation.
|
|
|
+
|
|
|
+ * Writing to this file initiates a transition using the
|
|
|
+ specified event code number; only '0', '2', and '3' are
|
|
|
+ accepted (without a newline); '2' and '3' are both
|
|
|
+ mapped to PM_EVENT_SUSPEND.
|
|
|
+
|
|
|
+On writes, the PM core relies on that recorded event code and the device/bus
|
|
|
+capabilities to determine whether it uses a partial suspend() or resume()
|
|
|
+sequence to change things so that the recorded event corresponds to the
|
|
|
+numeric parameter.
|
|
|
+
|
|
|
+ - If the bus requires the irqs-disabled suspend_late()/resume_early()
|
|
|
+ phases, writes fail because those operations are not supported here.
|
|
|
+
|
|
|
+ - If the recorded value is the expected value, nothing is done.
|
|
|
+
|
|
|
+ - If the recorded value is nonzero, the device is partially resumed,
|
|
|
+ using the bus.resume() and/or class.resume() methods.
|
|
|
+
|
|
|
+ - If the target value is nonzero, the device is partially suspended,
|
|
|
+ using the class.suspend() and/or bus.suspend() methods and the
|
|
|
+ PM_EVENT_SUSPEND message.
|
|
|
+
|
|
|
+Drivers have no way to tell whether their suspend() and resume() calls
|
|
|
+have come through the sysfs power/state file or as part of entering a
|
|
|
+system sleep state, except that when accessed through sysfs the normal
|
|
|
+parent/child sequencing rules are ignored. Drivers (such as bus, bridge,
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+or hub drivers) which expose child devices may need to enforce those rules
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+on their own.
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