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+
+Device Power Management
+
+
+Device power management encompasses two areas - the ability to save
+state and transition a device to a low-power state when the system is
+entering a low-power state; and the ability to transition a device to
+a low-power state while the system is running (and independently of
+any other power management activity).
+
+
+Methods
+
+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);
+};
+
+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 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.
+
+
+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'.
+
+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 beeing 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
+#
+
+Driver Detach Power Management
+
+The kernel now supports the ability to place a device in a low-power
+state when it is detached from its driver, which happens when its
+module is removed.
+
+Each device contains a 'detach_state' file in its sysfs directory
+which can be used to control this state. Reading from this file
+displays what the current detach state is set to. This is 0 (On) by
+default. A user may write a positive integer value to this file in the
+range of 1-4 inclusive.
+
+A value of 1-3 will indicate the device should be placed in that
+low-power state, which will cause ->suspend() to be called for that
+device. A value of 4 indicates that the device should be shutdown, so
+->shutdown() will be called for that device.
+
+The driver is responsible for reinitializing the device when the
+module is re-inserted during it's ->probe() (or equivalent) method.
+The driver core will not call any extra functions when binding the
+device to the driver.
+
+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
+#