252 lines
9.5 KiB
Text
252 lines
9.5 KiB
Text
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Linux Power Management Support
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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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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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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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No sorry, you can not 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 can not 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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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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apmd: http://worldvisions.ca/~apenwarr/apmd/
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acpid: http://acpid.sf.net/
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Driver Interface -- OBSOLETE, DO NOT USE!
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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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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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* 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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* 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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* 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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* 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 can not 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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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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Q: When is a device suspended?
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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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Q: Must a driver honor a suspend request?
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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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Q: Can the driver block suspend/resume requests?
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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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Q: What context is a suspend/resume initiated from?
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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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Q: Will requests continue to arrive after a suspend?
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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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(*) 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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Q: Do I have to manage bus-specific power management registers
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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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Q: So, really, what do I need to do to support suspend/resume?
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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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Q: Where do I store device context for suspend?
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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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Q: What do I need to do for ACPI vs. APM vs. etc?
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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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Q: What about device dependencies?
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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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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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ACPI Development mailing list: acpi-devel@lists.sourceforge.net
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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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* 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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* 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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