| Power Management for USB |
| |
| Alan Stern <stern@rowland.harvard.edu> |
| |
| October 28, 2010 |
| |
| |
| |
| 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). |
| |
| Note: Dynamic PM support for USB is present only if the kernel was |
| built with CONFIG_USB_SUSPEND enabled (which depends on |
| CONFIG_PM_RUNTIME). System PM support is present only if the kernel |
| was built with CONFIG_SUSPEND or CONFIG_HIBERNATION enabled. |
| |
| |
| 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 /etc/modprobe.conf 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 is a simple alternative to |
| disabling CONFIG_USB_SUSPEND and rebuilding the kernel, and it has the |
| added benefit of allowing you 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 diconnect() 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. |