209 lines
8.5 KiB
Plaintext
209 lines
8.5 KiB
Plaintext
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Device Power Management
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Device power management encompasses two areas - the ability to save
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state and transition a device to a low-power state when the system is
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entering a low-power state; and the ability to transition a device to
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a low-power state while the system is running (and independently of
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any other power management activity).
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Methods
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The methods to suspend and resume devices reside in struct bus_type:
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struct bus_type {
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...
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int (*suspend)(struct device * dev, pm_message_t state);
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int (*resume)(struct device * dev);
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};
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Each bus driver is responsible implementing these methods, translating
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the call into a bus-specific request and forwarding the call to the
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bus-specific drivers. For example, PCI drivers implement suspend() and
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resume() methods in struct pci_driver. The PCI core is simply
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responsible for translating the pointers to PCI-specific ones and
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calling the low-level driver.
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This is done to a) ease transition to the new power management methods
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and leverage the existing PM code in various bus drivers; b) allow
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buses to implement generic and default PM routines for devices, and c)
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make the flow of execution obvious to the reader.
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System Power Management
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When the system enters a low-power state, the device tree is walked in
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a depth-first fashion to transition each device into a low-power
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state. The ordering of the device tree is guaranteed by the order in
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which devices get registered - children are never registered before
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their ancestors, and devices are placed at the back of the list when
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registered. By walking the list in reverse order, we are guaranteed to
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suspend devices in the proper order.
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Devices are suspended once with interrupts enabled. Drivers are
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expected to stop I/O transactions, save device state, and place the
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device into a low-power state. Drivers may sleep, allocate memory,
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etc. at will.
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Some devices are broken and will inevitably have problems powering
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down or disabling themselves with interrupts enabled. For these
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special cases, they may return -EAGAIN. This will put the device on a
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list to be taken care of later. When interrupts are disabled, before
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we enter the low-power state, their drivers are called again to put
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their device to sleep.
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On resume, the devices that returned -EAGAIN will be called to power
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themselves back on with interrupts disabled. Once interrupts have been
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re-enabled, the rest of the drivers will be called to resume their
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devices. On resume, a driver is responsible for powering back on each
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device, restoring state, and re-enabling I/O transactions for that
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device.
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System devices follow a slightly different API, which can be found in
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include/linux/sysdev.h
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drivers/base/sys.c
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System devices will only be suspended with interrupts disabled, and
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after all other devices have been suspended. On resume, they will be
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resumed before any other devices, and also with interrupts disabled.
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Runtime Power Management
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Many devices are able to dynamically power down while the system is
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still running. This feature is useful for devices that are not being
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used, and can offer significant power savings on a running system.
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In each device's directory, there is a 'power' directory, which
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contains at least a 'state' file. Reading from this file displays what
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power state the device is currently in. Writing to this file initiates
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a transition to the specified power state, which must be a decimal in
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the range 1-3, inclusive; or 0 for 'On'.
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The PM core will call the ->suspend() method in the bus_type object
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that the device belongs to if the specified state is not 0, or
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->resume() if it is.
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Nothing will happen if the specified state is the same state the
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device is currently in.
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If the device is already in a low-power state, and the specified state
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is another, but different, low-power state, the ->resume() method will
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first be called to power the device back on, then ->suspend() will be
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called again with the new state.
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The driver is responsible for saving the working state of the device
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and putting it into the low-power state specified. If this was
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successful, it returns 0, and the device's power_state field is
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updated.
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The driver must take care to know whether or not it is able to
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properly resume the device, including all step of reinitialization
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necessary. (This is the hardest part, and the one most protected by
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NDA'd documents).
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The driver must also take care not to suspend a device that is
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currently in use. It is their responsibility to provide their own
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exclusion mechanisms.
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The runtime power transition happens with interrupts enabled. If a
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device cannot support being powered down with interrupts, it may
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return -EAGAIN (as it would during a system power management
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transition), but it will _not_ be called again, and the transaction
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will fail.
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There is currently no way to know what states a device or driver
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supports a priori. This will change in the future.
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pm_message_t meaning
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pm_message_t has two fields. event ("major"), and flags. If driver
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does not know event code, it aborts the request, returning error. Some
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drivers may need to deal with special cases based on the actual type
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of suspend operation being done at the system level. This is why
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there are flags.
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Event codes are:
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ON -- no need to do anything except special cases like broken
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HW.
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# NOTIFICATION -- pretty much same as ON?
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FREEZE -- stop DMA and interrupts, and be prepared to reinit HW from
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scratch. That probably means stop accepting upstream requests, the
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actual policy of what to do with them being specific to a given
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driver. It's acceptable for a network driver to just drop packets
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while a block driver is expected to block the queue so no request is
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lost. (Use IDE as an example on how to do that). FREEZE requires no
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power state change, and it's expected for drivers to be able to
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quickly transition back to operating state.
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SUSPEND -- like FREEZE, but also put hardware into low-power state. If
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there's need to distinguish several levels of sleep, additional flag
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is probably best way to do that.
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Transitions are only from a resumed state to a suspended state, never
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between 2 suspended states. (ON -> FREEZE or ON -> SUSPEND can happen,
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FREEZE -> SUSPEND or SUSPEND -> FREEZE can not).
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All events are:
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[NOTE NOTE NOTE: If you are driver author, you should not care; you
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should only look at event, and ignore flags.]
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#Prepare for suspend -- userland is still running but we are going to
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#enter suspend state. This gives drivers chance to load firmware from
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#disk and store it in memory, or do other activities taht require
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#operating userland, ability to kmalloc GFP_KERNEL, etc... All of these
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#are forbiden once the suspend dance is started.. event = ON, flags =
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#PREPARE_TO_SUSPEND
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Apm standby -- prepare for APM event. Quiesce devices to make life
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easier for APM BIOS. event = FREEZE, flags = APM_STANDBY
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Apm suspend -- same as APM_STANDBY, but it we should probably avoid
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spinning down disks. event = FREEZE, flags = APM_SUSPEND
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System halt, reboot -- quiesce devices to make life easier for BIOS. event
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= FREEZE, flags = SYSTEM_HALT or SYSTEM_REBOOT
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System shutdown -- at least disks need to be spun down, or data may be
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lost. Quiesce devices, just to make life easier for BIOS. event =
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FREEZE, flags = SYSTEM_SHUTDOWN
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Kexec -- turn off DMAs and put hardware into some state where new
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kernel can take over. event = FREEZE, flags = KEXEC
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Powerdown at end of swsusp -- very similar to SYSTEM_SHUTDOWN, except wake
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may need to be enabled on some devices. This actually has at least 3
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subtypes, system can reboot, enter S4 and enter S5 at the end of
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swsusp. event = FREEZE, flags = SWSUSP and one of SYSTEM_REBOOT,
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SYSTEM_SHUTDOWN, SYSTEM_S4
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Suspend to ram -- put devices into low power state. event = SUSPEND,
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flags = SUSPEND_TO_RAM
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Freeze for swsusp snapshot -- stop DMA and interrupts. No need to put
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devices into low power mode, but you must be able to reinitialize
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device from scratch in resume method. This has two flavors, its done
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once on suspending kernel, once on resuming kernel. event = FREEZE,
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flags = DURING_SUSPEND or DURING_RESUME
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Device detach requested from /sys -- deinitialize device; proably same as
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SYSTEM_SHUTDOWN, I do not understand this one too much. probably event
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= FREEZE, flags = DEV_DETACH.
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#These are not really events sent:
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#
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#System fully on -- device is working normally; this is probably never
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#passed to suspend() method... event = ON, flags = 0
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#
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#Ready after resume -- userland is now running, again. Time to free any
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#memory you ate during prepare to suspend... event = ON, flags =
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#READY_AFTER_RESUME
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#
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