Linux Cross Reference
Linux-2.6.17/Documentation/power/devices.txt

   
   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
 #
 
 
 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
 #