On Monday 10 July 2006 10:56 pm, Andrew Morton wrote:
> It's all very suspend-the-whole-machine centric. We don't presently help
> that NIC driver to put itself into a low-power state if there's been no net
> activity for 100 milliseconds. I guess that comes later.
Though on more thought, there actually are a bunch of mechanisms that
can kick in at runtime, and some useful things that can be said there.
Here's an updated version that covers a bit more of the runtime issues,
albeit without a network driver example. (As well as addressing the
issues noted by Pavel and Richard.)
It adds examples of runtime suspend mechanisms: the system timer and
dynamic tick, USB host controllers entering low power modes, and one of
the way those can interact to make it easier to use x86 C3 states ...
- Dave
Most of the code in Linux is device drivers, so most of the Linux power
management code is also driver-specific. Most drivers will do very little;
others, especially for platforms with small batteries (like cell phones),
will do a lot.
This writeup gives an overview of how drivers interact with system-wide
power management goals, emphasizing the models and interfaces that are
shared by everything that hooks up to the driver model core. Read it as
background for the domain-specific work you'd do with any specific driver.
Two Models for Device Power Management
======================================
Drivers will use one of these models to put devices into low-power states:
1 As part of entering system-wide low-power states like "suspend-to-ram",
or (mostly for systems with disks) "hibernate" (suspend-to-disk).
This is something that device, bus, and class drivers collaborate on
by implementing various role-specific suspend and resume methods to
cleanly power down hardware and software subsystems, then reactivate
them without loss of data.
Some drivers can manage hardware wakeup events, which make the system
leave that low-power state. This feature may be disabled using the
device's power/wakeup file; enabling it may cost some power usage,
but let the whole system enter low power states more often.
2 While the system is running, independently of other power management
activity. Upstream drivers will normally not know (or care) if the
device is in some low power state when issuing requests.
This doesn't have, or need much infrastructure; it's just something you
should do when writing your drivers. For example, clk_disable() unused
clocks as part of minimizing power drain for currently-unused hardware.
Of course, sometimes clusters of drivers will collaborate with each
other, which could involve task-specific power management.
There's not a lot to be said about those low power states except that they
are very system-specific, and often device-specific. Also, that if enough
drivers put themselves into low power states (model #2), the effect may be
the same as entering some system-wide low-power states (model #1) ... and
that synergies exist, so that several drivers using model #2 can put the
system into a state where even deeper power saving options are available.
Most suspended devices will have quiesced all I/O: no more DMA or irqs, no
more data read or written, and requests from upstream drivers are no longer
accepted. A given bus or platform may have different requirements though.
Examples of hardware wakeup events include an alarm from a real time clock,
network wake-on-LAN packets, keyboard or mouse activity, and media insertion
or removal (for PCMCIA, MMC/SD, USB, and so on).
Interfaces for Entering System Suspend States
=============================================
Most of the programming interfaces a device driver needs to know about
relate to that first model: entering a system-wide low power state,
rather than just minimizing power consumption by one device.
Bus driver methods
------------------
The core methods to suspend and resume devices reside in struct bus_type.
These are mostly of interest to people writing infrastructure for busses
like PCI or USB, or because they define the primitives that device drivers
may need to apply in domain-specific ways to their devices:
struct bus_type {
...
int (*suspend_prepare)(struct device *dev, pm_message_t state);
int (*suspend)(struct device *dev, pm_message_t state);
int (*suspend_late)(struct device *dev, pm_message_t state);
int (*resume_early)(struct device *dev);
int (*resume)(struct device *dev);
};
Bus drivers implement those methods as appropriate for the hardware and
the drivers using it; PCI works differently from USB, and so on. Not many
people write bus drivers; most driver code is a "device driver" that
builds on top of bus-specific framework code.
For more information on these driver calls, see the description later;
they are called in phases for every device, respecting the parent-child
sequencing in the driver model treee. Note that as this is being written,
only the suspend() and resume() are widely available; not many bus drivers
leverage all of those phases, or pass them down to lower driver levels.
EXAMPLE: PCI device driver methods
-----------------------------------
PCI framework software calls these methods when the pci device driver bound
to a device device has provided them:
struct pci_driver {
...
int (*suspend_prepare)(struct pci_device *pdev, pm_message_t state);
int (*suspend)(struct pci_device *pdev, pm_message_t state);
int (*suspend_late)(struct pci_device *pdev, pm_message_t state);
int (*resume_early)(struct pci_device *pdev);
int (*resume)(struct pci_device *pdev);
};
Drivers will implement those methods, and call PCI-specific procedures
like pci_set_power_state() and pci_enable_wake() to manage PCI-specific
mechanisms. Devices are suspended before their bridges enter low power
states, and likewise bridges resume before their devices.
Upper layers of driver stacks
-----------------------------
Device drivers generally have at least two interfaces, and the methods
sketched above are the ones which apply to the lower level (nearer PCI, USB,
or other bus hardware). The network and block layers are examples of upper
level interfaces, as is a character device talking to userspace.
Power management requests normally need to flow through those upper levels,
which often use domain-oriented requests like "blank that screen". In
some cases those upper levels will have power management intelligence that
relates to end-user activity, or other devices that work in cooperation.
When those interfaces are structured using class interfaces, there is a
standard way to have the upper layer stop issuing requests to a given
class device (and restart later):
struct class {
...
int (*suspend)(struct device *dev, pm_message_t state);
int (*resume)(struct device *dev);
};
Those calls are issued in specific phases of the process by which the
system enters a low power "suspend" state, or resumes from it.
Calling Drivers to enter System Sleep States
============================================
When the system enters a low power state, each device's driver is asked
to suspend the device by putting it into state compatible with the target
system state. That's usually some version of "off", but the details are
system-specific. Also, wakeup-enabled devices will usually stay partly
functional in order to wake the system.
When the system leaves that low power state, the device's driver is asked
to resume it. The suspend and resume operations always go together, and
both are multi-phase operations.
For simple drivers, suspend might quiesce the device using the class code
and then turn its hardware as "off" as possible with late_suspend. The
matching resume calls would then completely reinitialize the hardware
before reactivating its class I/O queues.
More power-aware drivers drivers will use more than one device low power
state, either at runtime or during system sleep states, and might trigger
system wakeup events.
Call Sequence Guarantees
------------------------
To ensure that bridges and similar links needed to talk to a device are
available when the device is suspended or resumed, the device tree is
walked in a bottom-up order to suspend devices. A top-down order is
used to resume those devices.
The ordering of the device tree is defined by the order in which devices
get registered: a child can never be registered/probed or resumed before
its parent, or removed/suspended after that parent.
The policy is that the device tree should match hardware bus topology.
(Or at least the control bus, for devices which use multiple busses.)
Suspending Devices
------------------
Suspending a given device is done in several phases. Each phase will be
omitted if it's not relevant for that device. Other devices will often
be suspending at the same time, so each phase will normally be done for
all devices before the next phase begins.
In order, the phases are:
1 bus.suspend_prepare(dev) is called early, with all tasks active.
This method may sleep, and is often morphed into a device
driver call with bus-specific parameters.
Drivers that need to synchronize their suspension with various
management activities could synchronize here. This may be a good
point to arrange that predictable actions, like removing media from
suspended systems, will not cause trouble on resume.
2 class.suspend(dev) is called after tasks are frozen, for devices
associated with a class that has such a method.
Since I/O activity usually comes from such higher layers, this is
a good place to quiesce all drivers of a given type (and keep such
code out of those drivers).
3 bus.suspend(dev) is called next. This method may sleep, and is often
morphed into a device driver call with bus-specific parameters.
This call should handle parts of device suspend logic that require
sleeping. It probably does work to quiesce the device which hasn't
been abstracted into class.suspend() or bus.suspend_late().
4 bus.suspend_late(dev) is called with IRQs disabled, and with only
one CPU active. This may be morphed into a driver call
with bus-specific parameters.
This call might save low level hardware state that might otherwise
be lost in the upcoming low power state, and actually put the
device into a lowpower state ... so that in some cases the device
may stay partly usable until this late. This "late" call may also
help when coping with hardware that behaves badly.
At the end of those phases, drivers should normally have stopped all I/O
transactions (DMA, IRQs), saved enough state that they can re-initialize
or restore previous state (as needed by the hardware), and placed the
device into a low-power state. On many platforms they will also use
clk_disable() to gate off one or more clock sources; sometimes they will
also switch off power supplies, or reduce voltages. A pm_message_t
parameter is currently used to nuance those semantics (described later).
When any driver sees that its device_can_wakeup(dev), it should make sure
to use the relevant hardware signals to trigger a system wakeup event.
For example, enable_irq_wake() might identify GPIO signals hooked up to
a switch or other external hardware, and pci_enable_wake() does something
similar for PCI's PME# signal.
Low Power (suspend) States
--------------------------
Device low-power states aren't very standard. One device might only handle
"on" and "off, while another might support a dozen different versions of
"on" (how many engines are active?), plus a state that gets back to "on"
faster than from a full "off".
Some busses define rules about what different suspend states mean. PCI
gives one example: after the suspend sequence completes, a non-legacy
PCI device may not perform DMA or issue IRQs, and any wakeup events it
issues would be issued through the PME# bus signal. Plus, there are
several PCI-standard device states, some of which are optional.
In contrast, integrated system-on-chip processors often use irqs as the
wakeup event sources (so drivers would call enable_irq_wake) and might
be able to treat DMA completion as a wakeup event (sometimes DMA can stay
active too, it'd only be the CPU and some peripherals that sleeps).
Some details here may be platform-specific. Systems may have devices that
can be fully active in certain sleep states, such as an LCD display that's
refreshed using DMA while most of the system is sleeping lightly ... and
its framebuffer might even be updated by a DSP or other non-Linux CPU while
the Linux control processor stays idle.
Moreover, the specific actions taken may depend on the target system state.
One target system state might allow a given device to be very operational;
another might require a hard shut down with re-initialization on resume.
And two different target systems might use the same device in different
ways; the aforementioned LCD might be active in one product's "standby",
but a different product using the same SOC might work differently.
Meaning of pm_message_t.event
-----------------------------
Parameters to suspend calls include the device affected and a message of
type pm_message_t, which has one field: the event. If driver does not
recognize the event code, suspend calls may abort the request and return
a negative errno. However, most drivers will be fine if they implement
PM_EVENT_SUSPEND semantics for all messages.
The event codes are used to nuance the goal of suspending the device,
and mostly matter when creating or resuming suspend-to-disk snapshots:
PM_EVENT_SUSPEND -- quiesce the driver and put hardware into a low-power
state. When used with system sleep states like "suspend-to-RAM" or
"standby", the upcoming resume() call will often be able to rely on
state kept in hardware, or issue system wakeup events. When used
instead with suspend-to-disk, few devices support this capability;
most are completely powered off.
PM_EVENT_FREEZE -- quiesce the driver, but don't necessarily change into
any low power mode. The driver's resume() will often be called soon.
Neither wakeup events nor DMA are allowed.
PM_EVENT_PRETHAW -- quiesce the driver, knowing that the upcoming resume()
will restore a suspend-to-disk snapshot from a different kernel image.
Drivers that are smart enough to look at their hardware state during
resume() processing need that state to be correct ... a PRETHAW could
be used to invalidate that state (by resetting the device). Other
drivers might handle this the same way as PM_EVENT_FREEZE.
There's also PM_EVENT_ON, a value which never appears as a suspend event
but is sometimes used to record the "not suspended" device state.
Resuming Devices
----------------
Resuming is similarly done in multiple phases:
1 bus.resume_early(dev) is called with IRQs disabled, and with
only one CPU active.
This reverses the effects of bus.suspend_late().
2 bus.resume(dev) is called next. This may be morphed into a device
driver call with bus-specific parameters.
This reverses the effects of bus.suspend().
3 class.resume(dev) is called for devices associated with a class
that has such a method.
This reverses the effects of class.suspend(), and would usually
reactivate the device's I/O queue.
Drivers need to be able to handle hardware which has been reset since
the suspend methods were called, for example by complete reinitialization.
(This is the hardest part, and the one most protected by NDA'd documents
and chip errata.) At the end of those phases, drivers should normally be
as functional as they were before resume(): I/O can be performed using DMA
and IRQs, and the relevant clocks are gated on.
However, the details here may again be platform-specific. For example,
some systems support multiple "run" states, and the mode in effect at
the end of resume() might not be the one which preceded suspension.
That means availability of certain clocks or power supplies changed,
which could easily affect how a driver works.
System Devices
--------------
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.
That is, sysdev_driver.suspend() is called right after the suspend_late()
phase; sysdev_driver.resume() is called before the resume_early() phase.
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; these devices
often support a range of runtime power states. Those states will in some
cases (like PCI) be constrained by a bus the device uses, and will usually
be hardware states that are also used in system sleep states.
However, note that if a driver puts a device into a runtime low power state
and the system then goes into a system-wide sleep state, it normally ought
to resume into that runtime low power state rather than "full on". That
distinction would be part of the driver-internal state machine for that
hardware.
Power Saving Techniques
-----------------------
Normally runtime power management is handled by the drivers without specific
userspace or kernel intervention, by device-aware use of techniques like:
Using fewer CPU cycles
- shortening "hot" code paths
- using DMA instead of PIO
- (sometimes) offloading work to device firmware
- removing timers, or making them lower frequency
- eliminating cache misses
Reducing other resource costs
- disabling unused clocks in software
- letting hardware gate off unused clocks
- switching off unused power supplies
- eliminating (or delaying/merging) IRQs
- avoiding needless DMA transfers
- tuning DMA to avoid byte transfers (word and/or burst modes)
- using lower voltages
- switching off unused intermediate busses
Using device-specific low power states
Read your hardware documentation carefully to see the opportunities that
may be available. If you can, measure the actual power usage and check
it against the budget established for your project.
Examples: USB hosts, system timer, system CPU
----------------------------------------------
USB host controllers make interesting, if complex, examples. In many cases
these have no work to do: no USB devices are connected, or all of them are
in the USB "suspend" state. Linux host controller drivers can then disable
periodic DMA transfers that would otherwise be a constant power drain on the
memory subsystem, and enter a suspend state. In power-aware controllers,
entering that suspend state may disable the clock used with USB signaling
(12 or 480 MHz), saving a certain amount of power.
The controller will be woken from that state (with an IRQ) by changes to the
signal state on the data lines of a given port, for example by an existing
peripheral requestin "remote wakeup" or by plugging a new peripheral. The
same wakeup mechanism usually works from "standby" sleep states, and on some
systems also from "suspend to RAM" (or even "suspend to disk") states.
System devices like clocks and CPUs may have special roles in the platform
power management scheme. For example, system timers using a "dynamic tick"
approach don't just save CPU cycles (by eliminating needless timer IRQs),
but they may also open the door to using lower power CPU "idle" states that
cost more than a jiffie to enter or exit. On x86 systems these are states
like "C3"; note that periodic DMA transfers from a USB host controller will
also prevent entry to a C3 state, much like a periodic timer IRQ. That kind
of interaction among runtime mechanisms is common.
If the CPU has "cpufreq" support, there also may be opportunities to shift
to lower voltage settings, which reduces the power of executing a given
number of instructions.
/sys/devices/.../power/state files
----------------------------------
For now you can also test some of this functionality using sysfs.
DEPRECATED: USE "power/state" ONLY FOR DRIVER TESTING.
IT WILL BE REMOVED, AND REPLACED WITH SOMETHING WHICH
GIVES MORE APPROPRIATE ACCESS TO THE RANGE OF POWER STATES
EACH TYPE OF DEVICE AND BUS SUPPORTS THROUGH ITS DRIVER.
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 currently be a
number: '0' for 'on', or either '2' or '3' for 'suspended' (which
sometimes also implies reset, especially in conjunction with deeper
system sleep states).
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.
When using this interface to suspend, the PM core does not call the
bus.suspend_prepare() or bus.suspend_late() methods. When using it
to resume, the PM core does not call the bus.resume_early() method.
Nothing will happen if the specified state is the same state the
device is currently in.
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 drivers must also take care not to suspend a device that is
currently in use. It is their responsibility to provide their own
exclusion mechanisms.
There is currently no way to know what states a device or driver
supports a priori. This may change in the future.
