Lines Matching +full:power +full:- +full:on
1 .. _usb-power-management:
3 Power Management for USB
7 :Date: Last-updated: February 2014
11 ---------
12 * What is Power Management?
17 * Changing the default idle-delay time
19 * The driver interface for Power Management
25 * USB Port Power Control
26 * User Interface for Port Power Control
27 * Suggested Userspace Port Power Policy
30 What is Power Management?
31 -------------------------
33 Power Management (PM) is the practice of saving energy by suspending
35 component is ``suspended`` it is in a nonfunctional low-power state; it
37 ``resumed`` (returned to a functional full-power state) when the kernel
47 "selective suspend"). This document concentrates mostly on how
49 covered to some extent (see ``Documentation/power/*.rst`` for more
59 kernel had been built with ``CONFIG_USB_SUSPEND`` enabled (which depended on
67 ----------------------
72 by pressing a power button or opening the cover.
76 to resume. This capability goes by several names such as "Wake On
85 --------------------------
89 exact definition depends on the device's driver; drivers are allowed
94 its usbfs file open, whether or not any I/O is going on.
101 -------------------
106 of time, the so-called idle-delay time.
108 Of course, nothing the kernel does on its own initiative should
118 usblp, usblcd, and usb-skeleton (which doesn't count). If a
119 non-supporting driver is bound to a device, the device won't be
123 We can categorize power management events in two broad classes:
134 ---------------------------------
136 The user interface for controlling dynamic PM is located in the ``power/``
138 ``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
148 ``power/wakeup``
159 ``power/control``
161 This file contains one of two words: ``on`` or ``auto``.
165 - ``on`` means that the device should be resumed and
169 - ``auto`` is the normal state in which the kernel is
177 ``power/autosuspend_delay_ms``
181 before the kernel will autosuspend it (the idle-delay
186 idle-delay time.
188 Writing ``-1`` to ``power/autosuspend_delay_ms`` and writing ``on`` to
189 ``power/control`` do essentially the same thing -- they both prevent the
193 (In 2.6.21 writing ``0`` to ``power/autosuspend`` would prevent the device
195 ``power/autosuspend`` attribute did not exist prior to 2.6.21, and the
196 ``power/level`` attribute did not exist prior to 2.6.22. ``power/control``
197 was added in 2.6.34, and ``power/autosuspend_delay_ms`` was added in
201 Changing the default idle-delay time
202 ------------------------------------
204 The default autosuspend idle-delay time (in seconds) is controlled by
233 then each new USB device will have its autosuspend idle-delay
234 initialized to 5. (The idle-delay values for already existing devices
237 Setting the initial default idle-delay to -1 will prevent any
243 --------
245 The USB specification states that all USB devices must support power
254 ``power/control`` attribute is initialized to ``on``) for all devices other
255 than hubs. Hubs, at least, appear to be reasonably well-behaved in
262 This means that non-hub devices won't be autosuspended unless the user
265 future device managers such as HAL will take on this added
268 also change the idle-delay time; 2 seconds is not the best choice for
280 a number of keyboards show that typing on a suspended keyboard, while
283 of them will issue a remote-wakeup request in response to button
286 The kernel will not prevent you from enabling autosuspend on devices
292 The driver interface for Power Management
293 -----------------------------------------
295 The requirements for a USB driver to support external power management
305 - The ``suspend`` method is called to warn the driver that the
311 - The ``resume`` method is called to tell the driver that the
315 - The ``reset_resume`` method is called to tell the driver that
327 possible to work around the hibernation-forces-disconnect problem by
331 :ref:`usb-persist`) and it can also be used under certain
340 principle one might want to suspend some interfaces on a device (i.e.,
350 ---------------------------------------------------
376 runtime suspend should the interface be bound to a driver again. On
379 has returned -- say from within a work-queue routine -- provided they
395 their non-async counterparts. The big difference is that they
413 reason or another. For example, the ``power/control`` attribute might be
414 set to ``on``, or another interface in the same device might not be
417 carry out the operation automatically when the autosuspend idle-delay
422 autosuspend, there's no idle-delay for an autoresume.
426 -----------------------------------
434 writing ``auto`` to the device's ``power/control`` attribute. Likewise,
439 This is exactly the same as writing ``on`` to the ``power/control`` attribute.
444 remote wakeup by typing on it. If the driver sets
445 ``intf->needs_remote_wakeup`` to 1, the kernel won't autosuspend the
460 busy and therefore the next autosuspend idle-delay expiration should
462 so drivers need to worry only when interrupt-driven input arrives.
470 cause autosuspends to fail with -EBUSY if the driver needs to use the
481 ----------------
483 For external events -- but not necessarily for autosuspend or
484 autoresume -- the device semaphore (udev->dev.sem) will be held when a
499 --------------------------------------------
501 Dynamic power management and system power management can interact in
512 Secondly, a dynamic power-management event may occur as a system
515 For example, a suspended device may send a remote-wakeup signal while
520 wakeup may fail and get lost. Which outcome occurs depends on timing
521 and on the hardware and firmware design.
525 ---------------------
527 xHCI host controller provides hardware link power management to usb2.0
530 lower power state(L1 for usb2.0 devices, or U1/U2 for usb3.0 devices),
534 ``power/`` subdirectory of each USB device's sysfs directory, that is, in
535 ``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
538 ``power/usb2_hardware_lpm``
549 ``power/usb3_hardware_lpm_u1``
550 ``power/usb3_hardware_lpm_u2``
552 When a USB 3.0 lpm-capable device is plugged in to a
562 USB Port Power Control
563 ----------------------
566 controlled link power management, the USB subsystem also has the
567 capability to disable power to ports under some conditions. Power is
569 In the case of a root or platform-internal hub the host controller
571 method calls to set the port power state. For more background see the
577 a shared power well causing power to remain until all ports in the gang
586 Please see "User Interface for Port Power Control" for details.
588 As far as the effect on the device itself it is similar to what a device
589 goes through during system suspend, i.e. the power session is lost. Any
591 similarly affected by a port power cycle event. For this reason the
597 http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf
601 http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/
608 power control implementation will block poweroff attempts on that
612 User Interface for Port Power Control
613 -------------------------------------
615 The port power control mechanism uses the PM runtime system. Poweroff is
616 requested by clearing the ``power/pm_qos_no_power_off`` flag of the port device
620 suspended. This mechanism is dependent on the hub advertising port power
621 switching in its hub descriptor (wHubCharacteristics logical power switching
631 lost and all attached child-devices will disconnect. A good rule of thumb is
633 ``/sys/module/usbcore`` then unbinding it will interfere with port power
636 Example of the relevant files for port power control. Note, in this example
639 prefix=/sys/devices/pci0000:00/0000:00:14.0/usb3/3-1
645 $prefix/3-1:1.0/3-1-port1/device
647 $prefix/3-1:1.0/3-1-port1/power/pm_qos_no_power_off
648 $prefix/3-1:1.0/3-1-port1/device/power/control
649 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf0>/driver/unbind
650 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf1>/driver/unbind
652 $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intfN>/driver/unbind
654 In addition to these files some ports may have a 'peer' link to a port on
656 hi-speed peer::
658 $prefix/3-1:1.0/3-1-port1/peer -> ../../../../usb2/2-1/2-1:1.0/2-1-port1
659 ../../../../usb2/2-1/2-1:1.0/2-1-port1/peer -> ../../../../usb3/3-1/3-1:1.0/3-1-port1
662 peer ports are simply the hi-speed and superspeed interface pins that
667 connection and attempt to connect to the hi-speed pins. The
670 1. Port suspend is sequenced to guarantee that hi-speed ports are powered-off
671 before their superspeed peer is permitted to power-off. The implication is
672 that the setting ``pm_qos_no_power_off`` to zero on a superspeed port may
673 not cause the port to power-off until its highspeed peer has gone to its
675 if it wants to guarantee that a superspeed port will power-off.
677 2. Port resume is sequenced to force a superspeed port to power-on prior to its
681 power session is lost the device may have been removed, or need reset.
682 Resuming the child device when the parent port regains power resolves those
683 states and clamps the maximum port power cycle frequency at the rate the
684 child device can suspend (autosuspend-delay) and resume (reset-resume
687 Sysfs files relevant for port power control:
689 ``<hubdev-portX>/power/pm_qos_no_power_off``:
697 ``<hubdev-portX>/power/runtime_status``:
698 This file reflects whether the port is 'active' (power is on)
702 ``<hubdev-portX>/connect_type``:
703 An advisory read-only flag to userspace indicating the
710 port on the platform. Typically userspace would choose
728 exposed on a platform. Considered safe to be
729 powered-off at all times.
738 - since we are relying on the BIOS to get this ACPI
742 - Take care in clearing ``pm_qos_no_power_off``. Once
743 power is off this port will
749 ``<child>/power/control``:
751 power down until ``<child>/power/runtime_status``
755 ``<child>/power/persist``:
758 power session loss (suspend / port-power event). When
764 this time the only mechanism to clear the usb-internal
765 wakeup-capability for an interface device is to unbind
768 Summary of poweroff pre-requisite settings relative to a port device::
770 echo 0 > power/pm_qos_no_power_off
771 echo 0 > peer/power/pm_qos_no_power_off # if it exists
772 echo auto > power/control # this is the default value
773 echo auto > <child>/power/control
774 echo 1 > <child>/power/persist # this is the default value
776 Suggested Userspace Port Power Policy
777 -------------------------------------
783 ``power/pm_qos_no_power_off`` set to ``1`` causing ports to always remain
792 A more aggressive userspace policy is to enable USB port power off for
793 all ports (set ``<hubdev-portX>/power/pm_qos_no_power_off`` to ``0``) when
795 system. For example, a distro may want to enable power off all USB
796 ports when the screen blanks, and re-power them when the screen becomes
797 active. Smart phones and tablets may want to power off USB ports when
798 the user pushes the power button.