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diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt new file mode 100644 index 0000000..e48ea1d --- /dev/null +++ b/Documentation/usb/power-management.txt @@ -0,0 +1,530 @@ + Power Management for USB + + Alan Stern <stern@rowland.harvard.edu> + + October 5, 2007 + + + + 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. 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 can occur in two ways: manual and automatic. +"Manual" means that the user has told the kernel to suspend a device, +whereas "automatic" means that the kernel has decided all by itself to +suspend a device. Automatic suspend 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 suspend/resume (also triggered by +userspace), and remote wakeup (triggered by the device). Internal +events are those triggered within the USB stack: autosuspend and +autoresume. + + + 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, level, and autosuspend. + + 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/level + + This file contains one of three words: "on", "auto", + or "suspend". 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. + + "suspend" means that the device should remain + suspended, and autoresume is not allowed. (But remote + wakeup may still be allowed, since it is controlled + separately by the power/wakeup attribute.) + + power/autosuspend + + This file contains an integer value, which is the + number of seconds the device should remain idle before + the kernel will autosuspend it (the idle-delay time). + The default is 2. 0 means to autosuspend as soon as + the device becomes idle, and -1 means never to + autosuspend. You can write a number to the file to + change the autosuspend idle-delay time. + +Writing "-1" to power/autosuspend and writing "on" to power/level 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.) + + + Changing the default idle-delay time + ------------------------------------ + +The default autosuspend idle-delay time 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/level 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. + +Sometimes it turns out that even when a device does work okay with +autosuspend there are still problems. For example, there are +experimental patches adding autosuspend support to the usbhid driver, +which manages keyboards and mice, among other things. Tests with a +number of keyboards showed that typing on a suspended keyboard, while +causing the keyboard to do a remote wakeup all right, would +nonetheless frequently result in lost keystrokes. Tests with mice +showed that some of them would 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 -- for example, suspending a +USB hard disk might cause it to spin down without parking the heads. +(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 three functions: + + int usb_autopm_get_interface(struct usb_interface *intf); + void usb_autopm_put_interface(struct usb_interface *intf); + int usb_autopm_set_interface(struct usb_interface *intf); + +The functions work by maintaining a counter in the usb_interface +structure. When intf->pm_usage_count is > 0 then the interface is +deemed to be busy, and the kernel will not autosuspend the interface's +device. When intf->pm_usage_count is <= 0 then the interface is +considered to be idle, and the kernel may autosuspend the device. + +(There is a similar pm_usage_count field in struct usb_device, +associated with the device itself rather than any of its interfaces. +This field is used only by the USB core.) + +The driver owns intf->pm_usage_count; it can modify the value however +and whenever it likes. A nice aspect of the usb_autopm_* routines is +that the changes they make are protected by the usb_device structure's +PM mutex (udev->pm_mutex); however drivers may change pm_usage_count +without holding the mutex. + + usb_autopm_get_interface() increments pm_usage_count and + attempts an autoresume if the new value is > 0 and the + device is suspended. + + usb_autopm_put_interface() decrements pm_usage_count and + attempts an autosuspend if the new value is <= 0 and the + device isn't suspended. + + usb_autopm_set_interface() leaves pm_usage_count alone. + It attempts an autoresume if the value is > 0 and the device + is suspended, and it attempts an autosuspend if the value is + <= 0 and the device isn't suspended. + +There also are a couple of utility routines drivers can use: + + usb_autopm_enable() sets pm_usage_cnt to 0 and then calls + usb_autopm_set_interface(), which will attempt an autosuspend. + + usb_autopm_disable() sets pm_usage_cnt to 1 and then calls + usb_autopm_set_interface(), which will attempt an autoresume. + +The conventional 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/level 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 delayed workqueue +routine is automatically set up to carry out the operation when the +autosuspend idle-delay has expired. + +Autoresume attempts also can fail. This will happen if power/level is +set to "suspend" or if the device doesn't manage to resume properly. +Unlike autosuspend, there's no delay for an autoresume. + + + Other parts of the driver interface + ----------------------------------- + +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 or has been disabled through +the power/wakeup attribute. (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.) + +The usb_autopm_* routines have to run in a sleepable process context; +they must not be called from an interrupt handler or while holding a +spinlock. In fact, the entire autosuspend mechanism is not well geared +toward interrupt-driven operation. However there is one thing a +driver can do in an interrupt handler: + + usb_mark_last_busy(struct usb_device *udev); + +This sets udev->last_busy to the current time. udev->last_busy is the +field used for idle-delay calculations; updating it will cause any +pending autosuspend to be moved back. The usb_autopm_* routines will +also set the last_busy field to the current time. + +Calling urb_mark_last_busy() from within an URB completion handler is +subject to races: The kernel may have just finished deciding the +device has been idle for long enough but not yet gotten around to +calling the driver's suspend method. The driver would have to be +responsible for synchronizing its suspend method with its URB +completion handler and causing the autosuspend to fail with -EBUSY if +an URB had completed too recently. + +External suspend calls should never be allowed to fail in this way, +only autosuspend calls. The driver can tell them apart by checking +udev->auto_pm; this flag will be set to 1 for internal PM events +(autosuspend or autoresume) and 0 for external PM events. + +Many of the ingredients in the autosuspend framework are oriented +towards interfaces: The usb_interface structure contains the +pm_usage_cnt field, and the usb_autopm_* routines take an interface +pointer as their argument. But somewhat confusingly, a few of the +pieces (usb_mark_last_busy() and udev->auto_pm) use the usb_device +structure instead. Drivers need to keep this straight; they can call +interface_to_usbdev() to find the device structure for a given +interface. + + + Locking requirements + -------------------- + +All three suspend/resume methods are always called while holding the +usb_device's PM mutex. For external events -- but not necessarily for +autosuspend or autoresume -- the device semaphore (udev->dev.sem) will +also be held. 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 internal +suspend/resume events as well. + +If a driver wants to block all suspend/resume calls during some +critical section, it can simply acquire udev->pm_mutex. Note that +calls to resume may be triggered indirectly. Block IO due to memory +allocations can make the vm subsystem resume a device. Thus while +holding this lock you must not allocate memory with GFP_KERNEL or +GFP_NOFS. + +Alternatively, if the critical section might call some of the +usb_autopm_* routines, the driver can avoid deadlock by doing: + + down(&udev->dev.sem); + rc = usb_autopm_get_interface(intf); + +and at the end of the critical section: + + if (!rc) + usb_autopm_put_interface(intf); + up(&udev->dev.sem); + +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?) + +The rules for locking order are: + + Never acquire any device semaphore while holding any PM mutex. + + Never acquire udev->pm_mutex while holding the PM mutex for + a device that isn't a descendant of udev. + +In other words, PM mutexes should only be acquired going up the device +tree, and they should be acquired only after locking all the device +semaphores you need to hold. These rules don't matter to drivers very +much; they usually affect just the USB core. + +Still, drivers do need to be careful. For example, many drivers use a +private mutex to synchronize their normal I/O activities with their +disconnect method. Now if the driver supports autosuspend then it +must call usb_autopm_put_interface() from somewhere -- maybe from its +close method. It should make the call while holding the private mutex, +since a driver shouldn't call any of the usb_autopm_* functions for an +interface from which it has been unbound. + +But the usb_autpm_* routines always acquire the device's PM mutex, and +consequently the locking order has to be: private mutex first, PM +mutex second. Since the suspend method is always called with the PM +mutex held, it mustn't try to acquire the private mutex. It has to +synchronize with the driver's I/O activities in some other way. + + + 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 manually suspended or 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. The 2.6.23 kernel obeys this principle +for manually suspended devices but not for autosuspended devices; they +do get resumed when the system wakes up. (Presumably they will be +autosuspended again after their idle-delay time expires.) In later +kernels this behavior will be fixed. + +(There is an exception. If a device would undergo a reset-resume +instead of a normal resume, and the device is enabled for remote +wakeup, then the reset-resume takes place even if the device was +already suspended when the system suspend began. The justification is +that a reset-resume is a kind of remote-wakeup event. Or to put it +another way, a device which needs a reset won't be able to generate +normal remote-wakeup signals, so it ought to be resumed immediately.) + +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. + +More interestingly, a device might undergo a manual resume or +autoresume during system suspend. With current kernels this shouldn't +happen, because manual resumes must be initiated by userspace and +autoresumes happen in response to I/O requests, but all user processes +and I/O should be quiescent during a system suspend -- thanks to the +freezer. However there are plans to do away with the freezer, which +would mean these things would become possible. If and when this comes +about, the USB core will carefully arrange matters so that either type +of resume will block until the entire system has resumed. |
