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- Power Management for USB
-
- Alan Stern <stern@rowland.harvard.edu>
-
- October 28, 2010
-
-
-
- What is Power Management?
- -------------------------
-
-Power Management (PM) is the practice of saving energy by suspending
-parts of a computer system when they aren't being used. While a
-component is "suspended" it is in a nonfunctional low-power state; it
-might even be turned off completely. A suspended component can be
-"resumed" (returned to a functional full-power state) when the kernel
-needs to use it. (There also are forms of PM in which components are
-placed in a less functional but still usable state instead of being
-suspended; an example would be reducing the CPU's clock rate. This
-document will not discuss those other forms.)
-
-When the parts being suspended include the CPU and most of the rest of
-the system, we speak of it as a "system suspend". When a particular
-device is turned off while the system as a whole remains running, we
-call it a "dynamic suspend" (also known as a "runtime suspend" or
-"selective suspend"). This document concentrates mostly on how
-dynamic PM is implemented in the USB subsystem, although system PM is
-covered to some extent (see Documentation/power/*.txt for more
-information about system PM).
-
-Note: Dynamic PM support for USB is present only if the kernel was
-built with CONFIG_USB_SUSPEND enabled (which depends on
-CONFIG_PM_RUNTIME). System PM support is present only if the kernel
-was built with CONFIG_SUSPEND or CONFIG_HIBERNATION enabled.
-
-
- What is Remote Wakeup?
- ----------------------
-
-When a device has been suspended, it generally doesn't resume until
-the computer tells it to. Likewise, if the entire computer has been
-suspended, it generally doesn't resume until the user tells it to, say
-by pressing a power button or opening the cover.
-
-However some devices have the capability of resuming by themselves, or
-asking the kernel to resume them, or even telling the entire computer
-to resume. This capability goes by several names such as "Wake On
-LAN"; we will refer to it generically as "remote wakeup". When a
-device is enabled for remote wakeup and it is suspended, it may resume
-itself (or send a request to be resumed) in response to some external
-event. Examples include a suspended keyboard resuming when a key is
-pressed, or a suspended USB hub resuming when a device is plugged in.
-
-
- When is a USB device idle?
- --------------------------
-
-A device is idle whenever the kernel thinks it's not busy doing
-anything important and thus is a candidate for being suspended. The
-exact definition depends on the device's driver; drivers are allowed
-to declare that a device isn't idle even when there's no actual
-communication taking place. (For example, a hub isn't considered idle
-unless all the devices plugged into that hub are already suspended.)
-In addition, a device isn't considered idle so long as a program keeps
-its usbfs file open, whether or not any I/O is going on.
-
-If a USB device has no driver, its usbfs file isn't open, and it isn't
-being accessed through sysfs, then it definitely is idle.
-
-
- Forms of dynamic PM
- -------------------
-
-Dynamic suspends occur when the kernel decides to suspend an idle
-device. This is called "autosuspend" for short. In general, a device
-won't be autosuspended unless it has been idle for some minimum period
-of time, the so-called idle-delay time.
-
-Of course, nothing the kernel does on its own initiative should
-prevent the computer or its devices from working properly. If a
-device has been autosuspended and a program tries to use it, the
-kernel will automatically resume the device (autoresume). For the
-same reason, an autosuspended device will usually have remote wakeup
-enabled, if the device supports remote wakeup.
-
-It is worth mentioning that many USB drivers don't support
-autosuspend. In fact, at the time of this writing (Linux 2.6.23) the
-only drivers which do support it are the hub driver, kaweth, asix,
-usblp, usblcd, and usb-skeleton (which doesn't count). If a
-non-supporting driver is bound to a device, the device won't be
-autosuspended. In effect, the kernel pretends the device is never
-idle.
-
-We can categorize power management events in two broad classes:
-external and internal. External events are those triggered by some
-agent outside the USB stack: system suspend/resume (triggered by
-userspace), manual dynamic resume (also triggered by userspace), and
-remote wakeup (triggered by the device). Internal events are those
-triggered within the USB stack: autosuspend and autoresume. Note that
-all dynamic suspend events are internal; external agents are not
-allowed to issue dynamic suspends.
-
-
- The user interface for dynamic PM
- ---------------------------------
-
-The user interface for controlling dynamic PM is located in the power/
-subdirectory of each USB device's sysfs directory, that is, in
-/sys/bus/usb/devices/.../power/ where "..." is the device's ID. The
-relevant attribute files are: wakeup, control, and
-autosuspend_delay_ms. (There may also be a file named "level"; this
-file was deprecated as of the 2.6.35 kernel and replaced by the
-"control" file. In 2.6.38 the "autosuspend" file will be deprecated
-and replaced by the "autosuspend_delay_ms" file. The only difference
-is that the newer file expresses the delay in milliseconds whereas the
-older file uses seconds. Confusingly, both files are present in 2.6.37
-but only "autosuspend" works.)
-
- power/wakeup
-
- This file is empty if the device does not support
- remote wakeup. Otherwise the file contains either the
- word "enabled" or the word "disabled", and you can
- write those words to the file. The setting determines
- whether or not remote wakeup will be enabled when the
- device is next suspended. (If the setting is changed
- while the device is suspended, the change won't take
- effect until the following suspend.)
-
- power/control
-
- This file contains one of two words: "on" or "auto".
- You can write those words to the file to change the
- device's setting.
-
- "on" means that the device should be resumed and
- autosuspend is not allowed. (Of course, system
- suspends are still allowed.)
-
- "auto" is the normal state in which the kernel is
- allowed to autosuspend and autoresume the device.
-
- (In kernels up to 2.6.32, you could also specify
- "suspend", meaning that the device should remain
- suspended and autoresume was not allowed. This
- setting is no longer supported.)
-
- power/autosuspend_delay_ms
-
- This file contains an integer value, which is the
- number of milliseconds the device should remain idle
- before the kernel will autosuspend it (the idle-delay
- time). The default is 2000. 0 means to autosuspend
- as soon as the device becomes idle, and negative
- values mean never to autosuspend. You can write a
- number to the file to change the autosuspend
- idle-delay time.
-
-Writing "-1" to power/autosuspend_delay_ms and writing "on" to
-power/control do essentially the same thing -- they both prevent the
-device from being autosuspended. Yes, this is a redundancy in the
-API.
-
-(In 2.6.21 writing "0" to power/autosuspend would prevent the device
-from being autosuspended; the behavior was changed in 2.6.22. The
-power/autosuspend attribute did not exist prior to 2.6.21, and the
-power/level attribute did not exist prior to 2.6.22. power/control
-was added in 2.6.34, and power/autosuspend_delay_ms was added in
-2.6.37 but did not become functional until 2.6.38.)
-
-
- Changing the default idle-delay time
- ------------------------------------
-
-The default autosuspend idle-delay time (in seconds) is controlled by
-a module parameter in usbcore. You can specify the value when usbcore
-is loaded. For example, to set it to 5 seconds instead of 2 you would
-do:
-
- modprobe usbcore autosuspend=5
-
-Equivalently, you could add to a configuration file in /etc/modprobe.d
-a line saying:
-
- options usbcore autosuspend=5
-
-Some distributions load the usbcore module very early during the boot
-process, by means of a program or script running from an initramfs
-image. To alter the parameter value you would have to rebuild that
-image.
-
-If usbcore is compiled into the kernel rather than built as a loadable
-module, you can add
-
- usbcore.autosuspend=5
-
-to the kernel's boot command line.
-
-Finally, the parameter value can be changed while the system is
-running. If you do:
-
- echo 5 >/sys/module/usbcore/parameters/autosuspend
-
-then each new USB device will have its autosuspend idle-delay
-initialized to 5. (The idle-delay values for already existing devices
-will not be affected.)
-
-Setting the initial default idle-delay to -1 will prevent any
-autosuspend of any USB device. This is a simple alternative to
-disabling CONFIG_USB_SUSPEND and rebuilding the kernel, and it has the
-added benefit of allowing you to enable autosuspend for selected
-devices.
-
-
- Warnings
- --------
-
-The USB specification states that all USB devices must support power
-management. Nevertheless, the sad fact is that many devices do not
-support it very well. You can suspend them all right, but when you
-try to resume them they disconnect themselves from the USB bus or
-they stop working entirely. This seems to be especially prevalent
-among printers and scanners, but plenty of other types of device have
-the same deficiency.
-
-For this reason, by default the kernel disables autosuspend (the
-power/control attribute is initialized to "on") for all devices other
-than hubs. Hubs, at least, appear to be reasonably well-behaved in
-this regard.
-
-(In 2.6.21 and 2.6.22 this wasn't the case. Autosuspend was enabled
-by default for almost all USB devices. A number of people experienced
-problems as a result.)
-
-This means that non-hub devices won't be autosuspended unless the user
-or a program explicitly enables it. As of this writing there aren't
-any widespread programs which will do this; we hope that in the near
-future device managers such as HAL will take on this added
-responsibility. In the meantime you can always carry out the
-necessary operations by hand or add them to a udev script. You can
-also change the idle-delay time; 2 seconds is not the best choice for
-every device.
-
-If a driver knows that its device has proper suspend/resume support,
-it can enable autosuspend all by itself. For example, the video
-driver for a laptop's webcam might do this (in recent kernels they
-do), since these devices are rarely used and so should normally be
-autosuspended.
-
-Sometimes it turns out that even when a device does work okay with
-autosuspend there are still problems. For example, the usbhid driver,
-which manages keyboards and mice, has autosuspend support. Tests with
-a number of keyboards show that typing on a suspended keyboard, while
-causing the keyboard to do a remote wakeup all right, will nonetheless
-frequently result in lost keystrokes. Tests with mice show that some
-of them will issue a remote-wakeup request in response to button
-presses but not to motion, and some in response to neither.
-
-The kernel will not prevent you from enabling autosuspend on devices
-that can't handle it. It is even possible in theory to damage a
-device by suspending it at the wrong time. (Highly unlikely, but
-possible.) Take care.
-
-
- The driver interface for Power Management
- -----------------------------------------
-
-The requirements for a USB driver to support external power management
-are pretty modest; the driver need only define
-
- .suspend
- .resume
- .reset_resume
-
-methods in its usb_driver structure, and the reset_resume method is
-optional. The methods' jobs are quite simple:
-
- The suspend method is called to warn the driver that the
- device is going to be suspended. If the driver returns a
- negative error code, the suspend will be aborted. Normally
- the driver will return 0, in which case it must cancel all
- outstanding URBs (usb_kill_urb()) and not submit any more.
-
- The resume method is called to tell the driver that the
- device has been resumed and the driver can return to normal
- operation. URBs may once more be submitted.
-
- The reset_resume method is called to tell the driver that
- the device has been resumed and it also has been reset.
- The driver should redo any necessary device initialization,
- since the device has probably lost most or all of its state
- (although the interfaces will be in the same altsettings as
- before the suspend).
-
-If the device is disconnected or powered down while it is suspended,
-the disconnect method will be called instead of the resume or
-reset_resume method. This is also quite likely to happen when
-waking up from hibernation, as many systems do not maintain suspend
-current to the USB host controllers during hibernation. (It's
-possible to work around the hibernation-forces-disconnect problem by
-using the USB Persist facility.)
-
-The reset_resume method is used by the USB Persist facility (see
-Documentation/usb/persist.txt) and it can also be used under certain
-circumstances when CONFIG_USB_PERSIST is not enabled. Currently, if a
-device is reset during a resume and the driver does not have a
-reset_resume method, the driver won't receive any notification about
-the resume. Later kernels will call the driver's disconnect method;
-2.6.23 doesn't do this.
-
-USB drivers are bound to interfaces, so their suspend and resume
-methods get called when the interfaces are suspended or resumed. In
-principle one might want to suspend some interfaces on a device (i.e.,
-force the drivers for those interface to stop all activity) without
-suspending the other interfaces. The USB core doesn't allow this; all
-interfaces are suspended when the device itself is suspended and all
-interfaces are resumed when the device is resumed. It isn't possible
-to suspend or resume some but not all of a device's interfaces. The
-closest you can come is to unbind the interfaces' drivers.
-
-
- The driver interface for autosuspend and autoresume
- ---------------------------------------------------
-
-To support autosuspend and autoresume, a driver should implement all
-three of the methods listed above. In addition, a driver indicates
-that it supports autosuspend by setting the .supports_autosuspend flag
-in its usb_driver structure. It is then responsible for informing the
-USB core whenever one of its interfaces becomes busy or idle. The
-driver does so by calling these six functions:
-
- int usb_autopm_get_interface(struct usb_interface *intf);
- void usb_autopm_put_interface(struct usb_interface *intf);
- int usb_autopm_get_interface_async(struct usb_interface *intf);
- void usb_autopm_put_interface_async(struct usb_interface *intf);
- void usb_autopm_get_interface_no_resume(struct usb_interface *intf);
- void usb_autopm_put_interface_no_suspend(struct usb_interface *intf);
-
-The functions work by maintaining a usage counter in the
-usb_interface's embedded device structure. When the counter is > 0
-then the interface is deemed to be busy, and the kernel will not
-autosuspend the interface's device. When the usage counter is = 0
-then the interface is considered to be idle, and the kernel may
-autosuspend the device.
-
-Drivers need not be concerned about balancing changes to the usage
-counter; the USB core will undo any remaining "get"s when a driver
-is unbound from its interface. As a corollary, drivers must not call
-any of the usb_autopm_* functions after their disconnect() routine has
-returned.
-
-Drivers using the async routines are responsible for their own
-synchronization and mutual exclusion.
-
- usb_autopm_get_interface() increments the usage counter and
- does an autoresume if the device is suspended. If the
- autoresume fails, the counter is decremented back.
-
- usb_autopm_put_interface() decrements the usage counter and
- attempts an autosuspend if the new value is = 0.
-
- usb_autopm_get_interface_async() and
- usb_autopm_put_interface_async() do almost the same things as
- their non-async counterparts. The big difference is that they
- use a workqueue to do the resume or suspend part of their
- jobs. As a result they can be called in an atomic context,
- such as an URB's completion handler, but when they return the
- device will generally not yet be in the desired state.
-
- usb_autopm_get_interface_no_resume() and
- usb_autopm_put_interface_no_suspend() merely increment or
- decrement the usage counter; they do not attempt to carry out
- an autoresume or an autosuspend. Hence they can be called in
- an atomic context.
-
-The simplest usage pattern is that a driver calls
-usb_autopm_get_interface() in its open routine and
-usb_autopm_put_interface() in its close or release routine. But other
-patterns are possible.
-
-The autosuspend attempts mentioned above will often fail for one
-reason or another. For example, the power/control attribute might be
-set to "on", or another interface in the same device might not be
-idle. This is perfectly normal. If the reason for failure was that
-the device hasn't been idle for long enough, a timer is scheduled to
-carry out the operation automatically when the autosuspend idle-delay
-has expired.
-
-Autoresume attempts also can fail, although failure would mean that
-the device is no longer present or operating properly. Unlike
-autosuspend, there's no idle-delay for an autoresume.
-
-
- Other parts of the driver interface
- -----------------------------------
-
-Drivers can enable autosuspend for their devices by calling
-
- usb_enable_autosuspend(struct usb_device *udev);
-
-in their probe() routine, if they know that the device is capable of
-suspending and resuming correctly. This is exactly equivalent to
-writing "auto" to the device's power/control attribute. Likewise,
-drivers can disable autosuspend by calling
-
- usb_disable_autosuspend(struct usb_device *udev);
-
-This is exactly the same as writing "on" to the power/control attribute.
-
-Sometimes a driver needs to make sure that remote wakeup is enabled
-during autosuspend. For example, there's not much point
-autosuspending a keyboard if the user can't cause the keyboard to do a
-remote wakeup by typing on it. If the driver sets
-intf->needs_remote_wakeup to 1, the kernel won't autosuspend the
-device if remote wakeup isn't available. (If the device is already
-autosuspended, though, setting this flag won't cause the kernel to
-autoresume it. Normally a driver would set this flag in its probe
-method, at which time the device is guaranteed not to be
-autosuspended.)
-
-If a driver does its I/O asynchronously in interrupt context, it
-should call usb_autopm_get_interface_async() before starting output and
-usb_autopm_put_interface_async() when the output queue drains. When
-it receives an input event, it should call
-
- usb_mark_last_busy(struct usb_device *udev);
-
-in the event handler. This tells the PM core that the device was just
-busy and therefore the next autosuspend idle-delay expiration should
-be pushed back. Many of the usb_autopm_* routines also make this call,
-so drivers need to worry only when interrupt-driven input arrives.
-
-Asynchronous operation is always subject to races. For example, a
-driver may call the usb_autopm_get_interface_async() routine at a time
-when the core has just finished deciding the device has been idle for
-long enough but not yet gotten around to calling the driver's suspend
-method. The suspend method must be responsible for synchronizing with
-the I/O request routine and the URB completion handler; it should
-cause autosuspends to fail with -EBUSY if the driver needs to use the
-device.
-
-External suspend calls should never be allowed to fail in this way,
-only autosuspend calls. The driver can tell them apart by applying
-the PMSG_IS_AUTO() macro to the message argument to the suspend
-method; it will return True for internal PM events (autosuspend) and
-False for external PM events.
-
-
- Mutual exclusion
- ----------------
-
-For external events -- but not necessarily for autosuspend or
-autoresume -- the device semaphore (udev->dev.sem) will be held when a
-suspend or resume method is called. This implies that external
-suspend/resume events are mutually exclusive with calls to probe,
-disconnect, pre_reset, and post_reset; the USB core guarantees that
-this is true of autosuspend/autoresume events as well.
-
-If a driver wants to block all suspend/resume calls during some
-critical section, the best way is to lock the device and call
-usb_autopm_get_interface() (and do the reverse at the end of the
-critical section). Holding the device semaphore will block all
-external PM calls, and the usb_autopm_get_interface() will prevent any
-internal PM calls, even if it fails. (Exercise: Why?)
-
-
- Interaction between dynamic PM and system PM
- --------------------------------------------
-
-Dynamic power management and system power management can interact in
-a couple of ways.
-
-Firstly, a device may already be autosuspended when a system suspend
-occurs. Since system suspends are supposed to be as transparent as
-possible, the device should remain suspended following the system
-resume. But this theory may not work out well in practice; over time
-the kernel's behavior in this regard has changed. As of 2.6.37 the
-policy is to resume all devices during a system resume and let them
-handle their own runtime suspends afterward.
-
-Secondly, a dynamic power-management event may occur as a system
-suspend is underway. The window for this is short, since system
-suspends don't take long (a few seconds usually), but it can happen.
-For example, a suspended device may send a remote-wakeup signal while
-the system is suspending. The remote wakeup may succeed, which would
-cause the system suspend to abort. If the remote wakeup doesn't
-succeed, it may still remain active and thus cause the system to
-resume as soon as the system suspend is complete. Or the remote
-wakeup may fail and get lost. Which outcome occurs depends on timing
-and on the hardware and firmware design.
-
-
- xHCI hardware link PM
- ---------------------
-
-xHCI host controller provides hardware link power management to usb2.0
-(xHCI 1.0 feature) and usb3.0 devices which support link PM. By
-enabling hardware LPM, the host can automatically put the device into
-lower power state(L1 for usb2.0 devices, or U1/U2 for usb3.0 devices),
-which state device can enter and resume very quickly.
-
-The user interface for controlling USB2 hardware LPM is located in the
-power/ subdirectory of each USB device's sysfs directory, that is, in
-/sys/bus/usb/devices/.../power/ where "..." is the device's ID. The
-relevant attribute files is usb2_hardware_lpm.
-
- power/usb2_hardware_lpm
-
- When a USB2 device which support LPM is plugged to a
- xHCI host root hub which support software LPM, the
- host will run a software LPM test for it; if the device
- enters L1 state and resume successfully and the host
- supports USB2 hardware LPM, this file will show up and
- driver will enable hardware LPM for the device. You
- can write y/Y/1 or n/N/0 to the file to enable/disable
- USB2 hardware LPM manually. This is for test purpose mainly.