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diff --git a/Documentation/pm.txt b/Documentation/pm.txt new file mode 100644 index 00000000000..cc63ae18d14 --- /dev/null +++ b/Documentation/pm.txt @@ -0,0 +1,251 @@ + 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 can not 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 can not 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! +----------------************************* +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 can not 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: acpi-devel@lists.sourceforge.net + +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); |