diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/usb/power-management.txt | 217 |
1 files changed, 77 insertions, 140 deletions
diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt index e3fa189..2790ad4 100644 --- a/Documentation/usb/power-management.txt +++ b/Documentation/usb/power-management.txt @@ -2,7 +2,7 @@ Alan Stern <stern@rowland.harvard.edu> - November 10, 2009 + December 11, 2009 @@ -29,9 +29,9 @@ 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. +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? @@ -326,64 +326,63 @@ driver does so by calling these six functions: 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 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. +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. -(There is a similar pm_usage_count field in struct usb_device, +(There is a similar usage counter 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.) - -Drivers must not modify intf->pm_usage_count directly; its value -should be changed only be using the functions listed above. Drivers -are responsible for insuring that the overall change to pm_usage_count -during their lifetime balances out to 0 (it may be necessary for the -disconnect method to call usb_autopm_put_interface() one or more times -to fulfill this requirement). The first two routines use the PM mutex -in struct usb_device for mutual exclusion; drivers using the async -routines are responsible for their own synchronization and mutual -exclusion. - - 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. +This counter is used only by the USB core.) + +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 differences are: they do - not acquire the PM mutex, and they use a workqueue to do their + 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 not generally not yet be in the desired state. + 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 pm_usage_count value; they do not attempt to - carry out an autoresume or an autosuspend. Hence they can be - called in an atomic context. + 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 conventional usage pattern is that a driver calls +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. +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. +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 delay for an autoresume. +autosuspend, there's no idle-delay for an autoresume. Other parts of the driver interface @@ -413,26 +412,27 @@ 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 synchronous 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: +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); -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. +in the event handler. 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. Most +of the usb_autopm_* routines will also set the last_busy field to the +current time. + +Asynchronous operation is always subject to races. For example, a +driver may call one of the usb_autopm_*_interface_async() routines 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 output 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 checking @@ -440,75 +440,23 @@ the PM_EVENT_AUTO bit in the message.event argument to the suspend method; this bit will be set for internal PM events (autosuspend) and clear 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 (i.e., usb_mark_last_busy()) 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. + Mutual exclusion + ---------------- - 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. +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, 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. +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 @@ -517,22 +465,11 @@ synchronize with the driver's I/O activities in some other way. 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.) +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. Secondly, a dynamic power-management event may occur as a system suspend is underway. The window for this is short, since system |