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Diffstat (limited to 'Documentation/usb/power-management.txt')
| -rw-r--r-- | Documentation/usb/power-management.txt | 245 |
1 files changed, 243 insertions, 2 deletions
diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt index 1392b61d6ebe..7b90fe034c4b 100644 --- a/Documentation/usb/power-management.txt +++ b/Documentation/usb/power-management.txt @@ -2,8 +2,27 @@ Alan Stern <stern@rowland.harvard.edu> - October 28, 2010 - + Last-updated: February 2014 + + + Contents: + --------- + * What is Power Management? + * What is Remote Wakeup? + * When is a USB device idle? + * Forms of dynamic PM + * The user interface for dynamic PM + * Changing the default idle-delay time + * Warnings + * The driver interface for Power Management + * The driver interface for autosuspend and autoresume + * Other parts of the driver interface + * Mutual exclusion + * Interaction between dynamic PM and system PM + * xHCI hardware link PM + * USB Port Power Control + * User Interface for Port Power Control + * Suggested Userspace Port Power Policy What is Power Management? @@ -516,3 +535,225 @@ relevant attribute files is usb2_hardware_lpm. 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. + + + USB Port Power Control + ---------------------- + +In addition to suspending endpoint devices and enabling hardware +controlled link power management, the USB subsystem also has the +capability to disable power to ports under some conditions. Power is +controlled through Set/ClearPortFeature(PORT_POWER) requests to a hub. +In the case of a root or platform-internal hub the host controller +driver translates PORT_POWER requests into platform firmware (ACPI) +method calls to set the port power state. For more background see the +Linux Plumbers Conference 2012 slides [1] and video [2]: + +Upon receiving a ClearPortFeature(PORT_POWER) request a USB port is +logically off, and may trigger the actual loss of VBUS to the port [3]. +VBUS may be maintained in the case where a hub gangs multiple ports into +a shared power well causing power to remain until all ports in the gang +are turned off. VBUS may also be maintained by hub ports configured for +a charging application. In any event a logically off port will lose +connection with its device, not respond to hotplug events, and not +respond to remote wakeup events*. + +WARNING: turning off a port may result in the inability to hot add a device. +Please see "User Interface for Port Power Control" for details. + +As far as the effect on the device itself it is similar to what a device +goes through during system suspend, i.e. the power session is lost. Any +USB device or driver that misbehaves with system suspend will be +similarly affected by a port power cycle event. For this reason the +implementation shares the same device recovery path (and honors the same +quirks) as the system resume path for the hub. + +[1]: http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf +[2]: http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/ +[3]: USB 3.1 Section 10.12 +* wakeup note: if a device is configured to send wakeup events the port + power control implementation will block poweroff attempts on that + port. + + + User Interface for Port Power Control + ------------------------------------- + +The port power control mechanism uses the PM runtime system. Poweroff is +requested by clearing the power/pm_qos_no_power_off flag of the port device +(defaults to 1). If the port is disconnected it will immediately receive a +ClearPortFeature(PORT_POWER) request. Otherwise, it will honor the pm runtime +rules and require the attached child device and all descendants to be suspended. +This mechanism is dependent on the hub advertising port power switching in its +hub descriptor (wHubCharacteristics logical power switching mode field). + +Note, some interface devices/drivers do not support autosuspend. Userspace may +need to unbind the interface drivers before the usb_device will suspend. An +unbound interface device is suspended by default. When unbinding, be careful +to unbind interface drivers, not the driver of the parent usb device. Also, +leave hub interface drivers bound. If the driver for the usb device (not +interface) is unbound the kernel is no longer able to resume the device. If a +hub interface driver is unbound, control of its child ports is lost and all +attached child-devices will disconnect. A good rule of thumb is that if the +'driver/module' link for a device points to /sys/module/usbcore then unbinding +it will interfere with port power control. + +Example of the relevant files for port power control. Note, in this example +these files are relative to a usb hub device (prefix). + + prefix=/sys/devices/pci0000:00/0000:00:14.0/usb3/3-1 + + attached child device + + hub port device + | + hub interface device + | | + v v v + $prefix/3-1:1.0/3-1-port1/device + + $prefix/3-1:1.0/3-1-port1/power/pm_qos_no_power_off + $prefix/3-1:1.0/3-1-port1/device/power/control + $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf0>/driver/unbind + $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf1>/driver/unbind + ... + $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intfN>/driver/unbind + +In addition to these files some ports may have a 'peer' link to a port on +another hub. The expectation is that all superspeed ports have a +hi-speed peer. + +$prefix/3-1:1.0/3-1-port1/peer -> ../../../../usb2/2-1/2-1:1.0/2-1-port1 +../../../../usb2/2-1/2-1:1.0/2-1-port1/peer -> ../../../../usb3/3-1/3-1:1.0/3-1-port1 + +Distinct from 'companion ports', or 'ehci/xhci shared switchover ports' +peer ports are simply the hi-speed and superspeed interface pins that +are combined into a single usb3 connector. Peer ports share the same +ancestor XHCI device. + +While a superspeed port is powered off a device may downgrade its +connection and attempt to connect to the hi-speed pins. The +implementation takes steps to prevent this: + +1/ Port suspend is sequenced to guarantee that hi-speed ports are powered-off + before their superspeed peer is permitted to power-off. The implication is + that the setting pm_qos_no_power_off to zero on a superspeed port may not cause + the port to power-off until its highspeed peer has gone to its runtime suspend + state. Userspace must take care to order the suspensions if it wants to + guarantee that a superspeed port will power-off. + +2/ Port resume is sequenced to force a superspeed port to power-on prior to its + highspeed peer. + +3/ Port resume always triggers an attached child device to resume. After a + power session is lost the device may have been removed, or need reset. + Resuming the child device when the parent port regains power resolves those + states and clamps the maximum port power cycle frequency at the rate the child + device can suspend (autosuspend-delay) and resume (reset-resume latency). + +Sysfs files relevant for port power control: + <hubdev-portX>/power/pm_qos_no_power_off: + This writable flag controls the state of an idle port. + Once all children and descendants have suspended the + port may suspend/poweroff provided that + pm_qos_no_power_off is '0'. If pm_qos_no_power_off is + '1' the port will remain active/powered regardless of + the stats of descendants. Defaults to 1. + + <hubdev-portX>/power/runtime_status: + This file reflects whether the port is 'active' (power is on) + or 'suspended' (logically off). There is no indication to + userspace whether VBUS is still supplied. + + <hubdev-portX>/connect_type: + An advisory read-only flag to userspace indicating the + location and connection type of the port. It returns + one of four values 'hotplug', 'hardwired', 'not used', + and 'unknown'. All values, besides unknown, are set by + platform firmware. + + "hotplug" indicates an externally connectable/visible + port on the platform. Typically userspace would choose + to keep such a port powered to handle new device + connection events. + + "hardwired" refers to a port that is not visible but + connectable. Examples are internal ports for USB + bluetooth that can be disconnected via an external + switch or a port with a hardwired USB camera. It is + expected to be safe to allow these ports to suspend + provided pm_qos_no_power_off is coordinated with any + switch that gates connections. Userspace must arrange + for the device to be connected prior to the port + powering off, or to activate the port prior to enabling + connection via a switch. + + "not used" refers to an internal port that is expected + to never have a device connected to it. These may be + empty internal ports, or ports that are not physically + exposed on a platform. Considered safe to be + powered-off at all times. + + "unknown" means platform firmware does not provide + information for this port. Most commonly refers to + external hub ports which should be considered 'hotplug' + for policy decisions. + + NOTE1: since we are relying on the BIOS to get this ACPI + information correct, the USB port descriptions may be + missing or wrong. + + NOTE2: Take care in clearing pm_qos_no_power_off. Once + power is off this port will + not respond to new connect events. + + Once a child device is attached additional constraints are + applied before the port is allowed to poweroff. + + <child>/power/control: + Must be 'auto', and the port will not + power down until <child>/power/runtime_status + reflects the 'suspended' state. Default + value is controlled by child device driver. + + <child>/power/persist: + This defaults to '1' for most devices and indicates if + kernel can persist the device's configuration across a + power session loss (suspend / port-power event). When + this value is '0' (quirky devices), port poweroff is + disabled. + + <child>/driver/unbind: + Wakeup capable devices will block port poweroff. At + this time the only mechanism to clear the usb-internal + wakeup-capability for an interface device is to unbind + its driver. + +Summary of poweroff pre-requisite settings relative to a port device: + + echo 0 > power/pm_qos_no_power_off + echo 0 > peer/power/pm_qos_no_power_off # if it exists + echo auto > power/control # this is the default value + echo auto > <child>/power/control + echo 1 > <child>/power/persist # this is the default value + + Suggested Userspace Port Power Policy + ------------------------------------- + +As noted above userspace needs to be careful and deliberate about what +ports are enabled for poweroff. + +The default configuration is that all ports start with +power/pm_qos_no_power_off set to '1' causing ports to always remain +active. + +Given confidence in the platform firmware's description of the ports +(ACPI _PLD record for a port populates 'connect_type') userspace can +clear pm_qos_no_power_off for all 'not used' ports. The same can be +done for 'hardwired' ports provided poweroff is coordinated with any +connection switch for the port. + +A more aggressive userspace policy is to enable USB port power off for +all ports (set <hubdev-portX>/power/pm_qos_no_power_off to '0') when +some external factor indicates the user has stopped interacting with the +system. For example, a distro may want to enable power off all USB +ports when the screen blanks, and re-power them when the screen becomes +active. Smart phones and tablets may want to power off USB ports when +the user pushes the power button. |