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- Linux Power Management Support
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-
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-This document briefly describes how to use power management with your
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-Linux system and how to add power management support to Linux drivers.
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-
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-APM or ACPI?
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-------------
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-If you have a relatively recent x86 mobile, desktop, or server system,
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-odds are it supports either Advanced Power Management (APM) or
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-Advanced Configuration and Power Interface (ACPI). ACPI is the newer
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-of the two technologies and puts power management in the hands of the
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-operating system, allowing for more intelligent power management than
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-is possible with BIOS controlled APM.
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-
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-The best way to determine which, if either, your system supports is to
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-build a kernel with both ACPI and APM enabled (as of 2.3.x ACPI is
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-enabled by default). If a working ACPI implementation is found, the
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-ACPI driver will override and disable APM, otherwise the APM driver
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-will be used.
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-
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-No, sorry, you cannot have both ACPI and APM enabled and running at
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-once. Some people with broken ACPI or broken APM implementations
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-would like to use both to get a full set of working features, but you
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-simply cannot mix and match the two. Only one power management
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-interface can be in control of the machine at once. Think about it..
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-
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-User-space Daemons
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-------------------
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-Both APM and ACPI rely on user-space daemons, apmd and acpid
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-respectively, to be completely functional. Obtain both of these
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-daemons from your Linux distribution or from the Internet (see below)
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-and be sure that they are started sometime in the system boot process.
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-Go ahead and start both. If ACPI or APM is not available on your
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-system the associated daemon will exit gracefully.
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-
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- apmd: http://worldvisions.ca/~apenwarr/apmd/
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- acpid: http://acpid.sf.net/
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-
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-Driver Interface -- OBSOLETE, DO NOT USE!
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-----------------*************************
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-
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-Note: pm_register(), pm_access(), pm_dev_idle() and friends are
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-obsolete. Please do not use them. Instead you should properly hook
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-your driver into the driver model, and use its suspend()/resume()
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-callbacks to do this kind of stuff.
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-
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-If you are writing a new driver or maintaining an old driver, it
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-should include power management support. Without power management
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-support, a single driver may prevent a system with power management
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-capabilities from ever being able to suspend (safely).
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-
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-Overview:
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-1) Register each instance of a device with "pm_register"
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-2) Call "pm_access" before accessing the hardware.
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- (this will ensure that the hardware is awake and ready)
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-3) Your "pm_callback" is called before going into a
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- suspend state (ACPI D1-D3) or after resuming (ACPI D0)
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- from a suspend.
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-4) Call "pm_dev_idle" when the device is not being used
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- (optional but will improve device idle detection)
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-5) When unloaded, unregister the device with "pm_unregister"
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-
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-/*
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- * Description: Register a device with the power-management subsystem
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- *
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- * Parameters:
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- * type - device type (PCI device, system device, ...)
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- * id - instance number or unique identifier
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- * cback - request handler callback (suspend, resume, ...)
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- *
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- * Returns: Registered PM device or NULL on error
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- *
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- * Examples:
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- * dev = pm_register(PM_SYS_DEV, PM_SYS_VGA, vga_callback);
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- *
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- * struct pci_dev *pci_dev = pci_find_dev(...);
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- * dev = pm_register(PM_PCI_DEV, PM_PCI_ID(pci_dev), callback);
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- */
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-struct pm_dev *pm_register(pm_dev_t type, unsigned long id, pm_callback cback);
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-
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-/*
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- * Description: Unregister a device with the power management subsystem
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- *
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- * Parameters:
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- * dev - PM device previously returned from pm_register
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- */
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-void pm_unregister(struct pm_dev *dev);
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-
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-/*
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- * Description: Unregister all devices with a matching callback function
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- *
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- * Parameters:
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- * cback - previously registered request callback
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- *
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- * Notes: Provided for easier porting from old APM interface
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- */
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-void pm_unregister_all(pm_callback cback);
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-
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-/*
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- * Power management request callback
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- *
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- * Parameters:
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- * dev - PM device previously returned from pm_register
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- * rqst - request type
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- * data - data, if any, associated with the request
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- *
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- * Returns: 0 if the request is successful
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- * EINVAL if the request is not supported
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- * EBUSY if the device is now busy and cannot handle the request
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- * ENOMEM if the device was unable to handle the request due to memory
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- *
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- * Details: The device request callback will be called before the
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- * device/system enters a suspend state (ACPI D1-D3) or
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- * or after the device/system resumes from suspend (ACPI D0).
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- * For PM_SUSPEND, the ACPI D-state being entered is passed
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- * as the "data" argument to the callback. The device
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- * driver should save (PM_SUSPEND) or restore (PM_RESUME)
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- * device context when the request callback is called.
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- *
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- * Once a driver returns 0 (success) from a suspend
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- * request, it should not process any further requests or
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- * access the device hardware until a call to "pm_access" is made.
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- */
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-typedef int (*pm_callback)(struct pm_dev *dev, pm_request_t rqst, void *data);
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-
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-Driver Details
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---------------
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-This is just a quick Q&A as a stopgap until a real driver writers'
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-power management guide is available.
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-
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-Q: When is a device suspended?
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-
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-Devices can be suspended based on direct user request (eg. laptop lid
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-closes), system power policy (eg. sleep after 30 minutes of console
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-inactivity), or device power policy (eg. power down device after 5
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-minutes of inactivity)
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-
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-Q: Must a driver honor a suspend request?
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-
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-No, a driver can return -EBUSY from a suspend request and this
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-will stop the system from suspending. When a suspend request
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-fails, all suspended devices are resumed and the system continues
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-to run. Suspend can be retried at a later time.
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-
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-Q: Can the driver block suspend/resume requests?
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-
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-Yes, a driver can delay its return from a suspend or resume
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-request until the device is ready to handle requests. It
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-is advantageous to return as quickly as possible from a
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-request as suspend/resume are done serially.
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-
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-Q: What context is a suspend/resume initiated from?
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-
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-A suspend or resume is initiated from a kernel thread context.
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-It is safe to block, allocate memory, initiate requests
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-or anything else you can do within the kernel.
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-
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-Q: Will requests continue to arrive after a suspend?
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-
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-Possibly. It is the driver's responsibility to queue(*),
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-fail, or drop any requests that arrive after returning
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-success to a suspend request. It is important that the
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-driver not access its device until after it receives
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-a resume request as the device's bus may no longer
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-be active.
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-
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-(*) If a driver queues requests for processing after
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- resume be aware that the device, network, etc.
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- might be in a different state than at suspend time.
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- It's probably better to drop requests unless
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- the driver is a storage device.
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-
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-Q: Do I have to manage bus-specific power management registers
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-
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-No. It is the responsibility of the bus driver to manage
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-PCI, USB, etc. power management registers. The bus driver
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-or the power management subsystem will also enable any
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-wake-on functionality that the device has.
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-
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-Q: So, really, what do I need to do to support suspend/resume?
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-
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-You need to save any device context that would
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-be lost if the device was powered off and then restore
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-it at resume time. When ACPI is active, there are
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-three levels of device suspend states; D1, D2, and D3.
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-(The suspend state is passed as the "data" argument
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-to the device callback.) With D3, the device is powered
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-off and loses all context, D1 and D2 are shallower power
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-states and require less device context to be saved. To
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-play it safe, just save everything at suspend and restore
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-everything at resume.
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-
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-Q: Where do I store device context for suspend?
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-
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-Anywhere in memory, kmalloc a buffer or store it
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-in the device descriptor. You are guaranteed that the
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-contents of memory will be restored and accessible
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-before resume, even when the system suspends to disk.
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-
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-Q: What do I need to do for ACPI vs. APM vs. etc?
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-
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-Drivers need not be aware of the specific power management
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-technology that is active. They just need to be aware
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-of when the overlying power management system requests
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-that they suspend or resume.
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-
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-Q: What about device dependencies?
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-
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-When a driver registers a device, the power management
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-subsystem uses the information provided to build a
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-tree of device dependencies (eg. USB device X is on
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-USB controller Y which is on PCI bus Z) When power
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-management wants to suspend a device, it first sends
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-a suspend request to its driver, then the bus driver,
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-and so on up to the system bus. Device resumes
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-proceed in the opposite direction.
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-
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-Q: Who do I contact for additional information about
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- enabling power management for my specific driver/device?
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-
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-ACPI Development mailing list: linux-acpi@vger.kernel.org
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-
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-System Interface -- OBSOLETE, DO NOT USE!
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-----------------*************************
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-If you are providing new power management support to Linux (ie.
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-adding support for something like APM or ACPI), you should
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-communicate with drivers through the existing generic power
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-management interface.
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-
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-/*
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- * Send a request to all devices
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- *
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- * Parameters:
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- * rqst - request type
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- * data - data, if any, associated with the request
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- *
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- * Returns: 0 if the request is successful
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- * See "pm_callback" return for errors
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- *
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- * Details: Walk list of registered devices and call pm_send
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- * for each until complete or an error is encountered.
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- * If an error is encountered for a suspend request,
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- * return all devices to the state they were in before
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- * the suspend request.
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- */
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-int pm_send_all(pm_request_t rqst, void *data);
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-
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-/*
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- * Find a matching device
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- *
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- * Parameters:
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- * type - device type (PCI device, system device, or 0 to match all devices)
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- * from - previous match or NULL to start from the beginning
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- *
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- * Returns: Matching device or NULL if none found
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- */
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-struct pm_dev *pm_find(pm_dev_t type, struct pm_dev *from);
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Rafael J. Wysocki