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diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt new file mode 100644 index 000000000..b5f839117 --- /dev/null +++ b/Documentation/usb/power-management.txt @@ -0,0 +1,760 @@ + Power Management for USB + + Alan Stern <stern@rowland.harvard.edu> + + 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? + ------------------------- + +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). + +System PM support is present only if the kernel was built with CONFIG_SUSPEND +or CONFIG_HIBERNATION enabled. Dynamic PM support for USB is present whenever +the kernel was built with CONFIG_PM enabled. + +[Historically, dynamic PM support for USB was present only if the +kernel had been built with CONFIG_USB_SUSPEND enabled (which depended on +CONFIG_PM_RUNTIME). Starting with the 3.10 kernel release, dynamic PM support +for USB was present whenever the kernel was built with CONFIG_PM_RUNTIME +enabled. The CONFIG_USB_SUSPEND option had been eliminated.] + + + 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 has the benefit of allowing you +then 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. + + + 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. |