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usb: documentation for usb port power off mechanisms
describe the mechanisms for controlling port power policy and discovering the port power state. [oliver]: fixes, clarification of wakeup vs port-power-control [sarah]: wordsmithing [djbw]: updates for peer port changes [alan]: review and fixes Cc: Oliver Neukum <oneukum@suse.de> Signed-off-by: Lan Tianyu <tianyu.lan@intel.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Acked-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Lan Tianyu
Greg Kroah-Hartman
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@ -2,9 +2,28 @@
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Alan Stern <stern@rowland.harvard.edu>
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October 28, 2010
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Last-updated: February 2014
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Contents:
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---------
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* What is Power Management?
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* What is Remote Wakeup?
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* When is a USB device idle?
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* Forms of dynamic PM
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* The user interface for dynamic PM
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* Changing the default idle-delay time
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* Warnings
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* The driver interface for Power Management
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* The driver interface for autosuspend and autoresume
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* Other parts of the driver interface
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* Mutual exclusion
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* Interaction between dynamic PM and system PM
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* xHCI hardware link PM
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* USB Port Power Control
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* User Interface for Port Power Control
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* Suggested Userspace Port Power Policy
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What is Power Management?
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-------------------------
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@ -516,3 +535,225 @@ relevant attribute files is usb2_hardware_lpm.
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driver will enable hardware LPM for the device. You
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can write y/Y/1 or n/N/0 to the file to enable/disable
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USB2 hardware LPM manually. This is for test purpose mainly.
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USB Port Power Control
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----------------------
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In addition to suspending endpoint devices and enabling hardware
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controlled link power management, the USB subsystem also has the
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capability to disable power to ports under some conditions. Power is
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controlled through Set/ClearPortFeature(PORT_POWER) requests to a hub.
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In the case of a root or platform-internal hub the host controller
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driver translates PORT_POWER requests into platform firmware (ACPI)
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method calls to set the port power state. For more background see the
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Linux Plumbers Conference 2012 slides [1] and video [2]:
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Upon receiving a ClearPortFeature(PORT_POWER) request a USB port is
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logically off, and may trigger the actual loss of VBUS to the port [3].
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VBUS may be maintained in the case where a hub gangs multiple ports into
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a shared power well causing power to remain until all ports in the gang
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are turned off. VBUS may also be maintained by hub ports configured for
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a charging application. In any event a logically off port will lose
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connection with its device, not respond to hotplug events, and not
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respond to remote wakeup events*.
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WARNING: turning off a port may result in the inability to hot add a device.
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Please see "User Interface for Port Power Control" for details.
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As far as the effect on the device itself it is similar to what a device
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goes through during system suspend, i.e. the power session is lost. Any
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USB device or driver that misbehaves with system suspend will be
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similarly affected by a port power cycle event. For this reason the
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implementation shares the same device recovery path (and honors the same
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quirks) as the system resume path for the hub.
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[1]: http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf
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[2]: http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/
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[3]: USB 3.1 Section 10.12
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* wakeup note: if a device is configured to send wakeup events the port
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power control implementation will block poweroff attempts on that
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port.
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User Interface for Port Power Control
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-------------------------------------
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The port power control mechanism uses the PM runtime system. Poweroff is
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requested by clearing the power/pm_qos_no_power_off flag of the port device
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(defaults to 1). If the port is disconnected it will immediately receive a
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ClearPortFeature(PORT_POWER) request. Otherwise, it will honor the pm runtime
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rules and require the attached child device and all descendants to be suspended.
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This mechanism is dependent on the hub advertising port power switching in its
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hub descriptor (wHubCharacteristics logical power switching mode field).
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Note, some interface devices/drivers do not support autosuspend. Userspace may
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need to unbind the interface drivers before the usb_device will suspend. An
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unbound interface device is suspended by default. When unbinding, be careful
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to unbind interface drivers, not the driver of the parent usb device. Also,
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leave hub interface drivers bound. If the driver for the usb device (not
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interface) is unbound the kernel is no longer able to resume the device. If a
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hub interface driver is unbound, control of its child ports is lost and all
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attached child-devices will disconnect. A good rule of thumb is that if the
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'driver/module' link for a device points to /sys/module/usbcore then unbinding
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it will interfere with port power control.
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Example of the relevant files for port power control. Note, in this example
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these files are relative to a usb hub device (prefix).
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prefix=/sys/devices/pci0000:00/0000:00:14.0/usb3/3-1
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attached child device +
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hub port device + |
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hub interface device + | |
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v v v
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$prefix/3-1:1.0/3-1-port1/device
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$prefix/3-1:1.0/3-1-port1/power/pm_qos_no_power_off
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$prefix/3-1:1.0/3-1-port1/device/power/control
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$prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf0>/driver/unbind
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$prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf1>/driver/unbind
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...
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$prefix/3-1:1.0/3-1-port1/device/3-1.1:<intfN>/driver/unbind
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In addition to these files some ports may have a 'peer' link to a port on
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another hub. The expectation is that all superspeed ports have a
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hi-speed peer.
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$prefix/3-1:1.0/3-1-port1/peer -> ../../../../usb2/2-1/2-1:1.0/2-1-port1
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../../../../usb2/2-1/2-1:1.0/2-1-port1/peer -> ../../../../usb3/3-1/3-1:1.0/3-1-port1
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Distinct from 'companion ports', or 'ehci/xhci shared switchover ports'
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peer ports are simply the hi-speed and superspeed interface pins that
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are combined into a single usb3 connector. Peer ports share the same
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ancestor XHCI device.
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While a superspeed port is powered off a device may downgrade its
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connection and attempt to connect to the hi-speed pins. The
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implementation takes steps to prevent this:
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1/ Port suspend is sequenced to guarantee that hi-speed ports are powered-off
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before their superspeed peer is permitted to power-off. The implication is
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that the setting pm_qos_no_power_off to zero on a superspeed port may not cause
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the port to power-off until its highspeed peer has gone to its runtime suspend
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state. Userspace must take care to order the suspensions if it wants to
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guarantee that a superspeed port will power-off.
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2/ Port resume is sequenced to force a superspeed port to power-on prior to its
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highspeed peer.
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3/ Port resume always triggers an attached child device to resume. After a
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power session is lost the device may have been removed, or need reset.
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Resuming the child device when the parent port regains power resolves those
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states and clamps the maximum port power cycle frequency at the rate the child
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device can suspend (autosuspend-delay) and resume (reset-resume latency).
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Sysfs files relevant for port power control:
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<hubdev-portX>/power/pm_qos_no_power_off:
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This writable flag controls the state of an idle port.
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Once all children and descendants have suspended the
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port may suspend/poweroff provided that
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pm_qos_no_power_off is '0'. If pm_qos_no_power_off is
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'1' the port will remain active/powered regardless of
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the stats of descendants. Defaults to 1.
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<hubdev-portX>/power/runtime_status:
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This file reflects whether the port is 'active' (power is on)
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or 'suspended' (logically off). There is no indication to
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userspace whether VBUS is still supplied.
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<hubdev-portX>/connect_type:
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An advisory read-only flag to userspace indicating the
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location and connection type of the port. It returns
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one of four values 'hotplug', 'hardwired', 'not used',
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and 'unknown'. All values, besides unknown, are set by
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platform firmware.
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"hotplug" indicates an externally connectable/visible
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port on the platform. Typically userspace would choose
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to keep such a port powered to handle new device
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connection events.
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"hardwired" refers to a port that is not visible but
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connectable. Examples are internal ports for USB
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bluetooth that can be disconnected via an external
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switch or a port with a hardwired USB camera. It is
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expected to be safe to allow these ports to suspend
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provided pm_qos_no_power_off is coordinated with any
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switch that gates connections. Userspace must arrange
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for the device to be connected prior to the port
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powering off, or to activate the port prior to enabling
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connection via a switch.
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"not used" refers to an internal port that is expected
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to never have a device connected to it. These may be
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empty internal ports, or ports that are not physically
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exposed on a platform. Considered safe to be
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powered-off at all times.
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"unknown" means platform firmware does not provide
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information for this port. Most commonly refers to
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external hub ports which should be considered 'hotplug'
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for policy decisions.
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NOTE1: since we are relying on the BIOS to get this ACPI
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information correct, the USB port descriptions may be
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missing or wrong.
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NOTE2: Take care in clearing pm_qos_no_power_off. Once
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power is off this port will
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not respond to new connect events.
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Once a child device is attached additional constraints are
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applied before the port is allowed to poweroff.
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<child>/power/control:
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Must be 'auto', and the port will not
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power down until <child>/power/runtime_status
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reflects the 'suspended' state. Default
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value is controlled by child device driver.
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<child>/power/persist:
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This defaults to '1' for most devices and indicates if
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kernel can persist the device's configuration across a
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power session loss (suspend / port-power event). When
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this value is '0' (quirky devices), port poweroff is
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disabled.
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<child>/driver/unbind:
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Wakeup capable devices will block port poweroff. At
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this time the only mechanism to clear the usb-internal
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wakeup-capability for an interface device is to unbind
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its driver.
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Summary of poweroff pre-requisite settings relative to a port device:
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echo 0 > power/pm_qos_no_power_off
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echo 0 > peer/power/pm_qos_no_power_off # if it exists
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echo auto > power/control # this is the default value
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echo auto > <child>/power/control
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echo 1 > <child>/power/persist # this is the default value
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Suggested Userspace Port Power Policy
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-------------------------------------
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As noted above userspace needs to be careful and deliberate about what
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ports are enabled for poweroff.
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The default configuration is that all ports start with
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power/pm_qos_no_power_off set to '1' causing ports to always remain
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active.
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Given confidence in the platform firmware's description of the ports
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(ACPI _PLD record for a port populates 'connect_type') userspace can
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clear pm_qos_no_power_off for all 'not used' ports. The same can be
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done for 'hardwired' ports provided poweroff is coordinated with any
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connection switch for the port.
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A more aggressive userspace policy is to enable USB port power off for
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all ports (set <hubdev-portX>/power/pm_qos_no_power_off to '0') when
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some external factor indicates the user has stopped interacting with the
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system. For example, a distro may want to enable power off all USB
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ports when the screen blanks, and re-power them when the screen becomes
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active. Smart phones and tablets may want to power off USB ports when
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the user pushes the power button.
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