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