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diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt deleted file mode 100644 index 00e706997130..000000000000 --- a/Documentation/usb/power-management.txt +++ /dev/null @@ -1,772 +0,0 @@ - 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 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 are usb2_hardware_lpm and usb3_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. - - power/usb3_hardware_lpm_u1 - power/usb3_hardware_lpm_u2 - - When a USB 3.0 lpm-capable device is plugged in to a - xHCI host which supports link PM, it will check if U1 - and U2 exit latencies have been set in the BOS - descriptor; if the check is passed and the host - supports USB3 hardware LPM, USB3 hardware LPM will be - enabled for the device and these files will be created. - The files hold a string value (enable or disable) - indicating whether or not USB3 hardware LPM U1 or U2 - is enabled for the device. - - 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. |