diff options
Diffstat (limited to 'Documentation/power')
| -rw-r--r-- | Documentation/power/devices.txt | 37 | ||||
| -rw-r--r-- | Documentation/power/freezing-of-tasks.txt | 39 | ||||
| -rw-r--r-- | Documentation/power/runtime_pm.txt | 130 |
3 files changed, 121 insertions, 85 deletions
diff --git a/Documentation/power/devices.txt b/Documentation/power/devices.txt index 3139fb505dce..20af7def23c8 100644 --- a/Documentation/power/devices.txt +++ b/Documentation/power/devices.txt @@ -126,7 +126,9 @@ The core methods to suspend and resume devices reside in struct dev_pm_ops pointed to by the ops member of struct dev_pm_domain, or by the pm member of struct bus_type, struct device_type and struct class. They are mostly of interest to the people writing infrastructure for platforms and buses, like PCI -or USB, or device type and device class drivers. +or USB, or device type and device class drivers. They also are relevant to the +writers of device drivers whose subsystems (PM domains, device types, device +classes and bus types) don't provide all power management methods. Bus drivers implement these methods as appropriate for the hardware and the drivers using it; PCI works differently from USB, and so on. Not many people @@ -268,32 +270,35 @@ various phases always run after tasks have been frozen and before they are unfrozen. Furthermore, the *_noirq phases run at a time when IRQ handlers have been disabled (except for those marked with the IRQF_NO_SUSPEND flag). -All phases use PM domain, bus, type, or class callbacks (that is, methods -defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, or dev->class->pm). -These callbacks are regarded by the PM core as mutually exclusive. Moreover, -PM domain callbacks always take precedence over bus, type and class callbacks, -while type callbacks take precedence over bus and class callbacks, and class -callbacks take precedence over bus callbacks. To be precise, the following -rules are used to determine which callback to execute in the given phase: +All phases use PM domain, bus, type, class or driver callbacks (that is, methods +defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or +dev->driver->pm). These callbacks are regarded by the PM core as mutually +exclusive. Moreover, PM domain callbacks always take precedence over all of the +other callbacks and, for example, type callbacks take precedence over bus, class +and driver callbacks. To be precise, the following rules are used to determine +which callback to execute in the given phase: - 1. If dev->pm_domain is present, the PM core will attempt to execute the - callback included in dev->pm_domain->ops. If that callback is not - present, no action will be carried out for the given device. + 1. If dev->pm_domain is present, the PM core will choose the callback + included in dev->pm_domain->ops for execution 2. Otherwise, if both dev->type and dev->type->pm are present, the callback - included in dev->type->pm will be executed. + included in dev->type->pm will be chosen for execution. 3. Otherwise, if both dev->class and dev->class->pm are present, the - callback included in dev->class->pm will be executed. + callback included in dev->class->pm will be chosen for execution. 4. Otherwise, if both dev->bus and dev->bus->pm are present, the callback - included in dev->bus->pm will be executed. + included in dev->bus->pm will be chosen for execution. This allows PM domains and device types to override callbacks provided by bus types or device classes if necessary. -These callbacks may in turn invoke device- or driver-specific methods stored in -dev->driver->pm, but they don't have to. +The PM domain, type, class and bus callbacks may in turn invoke device- or +driver-specific methods stored in dev->driver->pm, but they don't have to do +that. + +If the subsystem callback chosen for execution is not present, the PM core will +execute the corresponding method from dev->driver->pm instead if there is one. Entering System Suspend diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt index 316c2ba187f4..6ccb68f68da6 100644 --- a/Documentation/power/freezing-of-tasks.txt +++ b/Documentation/power/freezing-of-tasks.txt @@ -21,7 +21,7 @@ freeze_processes() (defined in kernel/power/process.c) is called. It executes try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and either wakes them up, if they are kernel threads, or sends fake signals to them, if they are user space processes. A task that has TIF_FREEZE set, should react -to it by calling the function called refrigerator() (defined in +to it by calling the function called __refrigerator() (defined in kernel/freezer.c), which sets the task's PF_FROZEN flag, changes its state to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is cleared for it. Then, we say that the task is 'frozen' and therefore the set of functions @@ -29,10 +29,10 @@ handling this mechanism is referred to as 'the freezer' (these functions are defined in kernel/power/process.c, kernel/freezer.c & include/linux/freezer.h). User space processes are generally frozen before kernel threads. -It is not recommended to call refrigerator() directly. Instead, it is -recommended to use the try_to_freeze() function (defined in -include/linux/freezer.h), that checks the task's TIF_FREEZE flag and makes the -task enter refrigerator() if the flag is set. +__refrigerator() must not be called directly. Instead, use the +try_to_freeze() function (defined in include/linux/freezer.h), that checks +the task's TIF_FREEZE flag and makes the task enter __refrigerator() if the +flag is set. For user space processes try_to_freeze() is called automatically from the signal-handling code, but the freezable kernel threads need to call it @@ -61,13 +61,13 @@ wait_event_freezable() and wait_event_freezable_timeout() macros. After the system memory state has been restored from a hibernation image and devices have been reinitialized, the function thaw_processes() is called in order to clear the PF_FROZEN flag for each frozen task. Then, the tasks that -have been frozen leave refrigerator() and continue running. +have been frozen leave __refrigerator() and continue running. III. Which kernel threads are freezable? Kernel threads are not freezable by default. However, a kernel thread may clear PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE -directly is strongly discouraged). From this point it is regarded as freezable +directly is not allowed). From this point it is regarded as freezable and must call try_to_freeze() in a suitable place. IV. Why do we do that? @@ -176,3 +176,28 @@ tasks, since it generally exists anyway. A driver must have all firmwares it may need in RAM before suspend() is called. If keeping them is not practical, for example due to their size, they must be requested early enough using the suspend notifier API described in notifiers.txt. + +VI. Are there any precautions to be taken to prevent freezing failures? + +Yes, there are. + +First of all, grabbing the 'pm_mutex' lock to mutually exclude a piece of code +from system-wide sleep such as suspend/hibernation is not encouraged. +If possible, that piece of code must instead hook onto the suspend/hibernation +notifiers to achieve mutual exclusion. Look at the CPU-Hotplug code +(kernel/cpu.c) for an example. + +However, if that is not feasible, and grabbing 'pm_mutex' is deemed necessary, +it is strongly discouraged to directly call mutex_[un]lock(&pm_mutex) since +that could lead to freezing failures, because if the suspend/hibernate code +successfully acquired the 'pm_mutex' lock, and hence that other entity failed +to acquire the lock, then that task would get blocked in TASK_UNINTERRUPTIBLE +state. As a consequence, the freezer would not be able to freeze that task, +leading to freezing failure. + +However, the [un]lock_system_sleep() APIs are safe to use in this scenario, +since they ask the freezer to skip freezing this task, since it is anyway +"frozen enough" as it is blocked on 'pm_mutex', which will be released +only after the entire suspend/hibernation sequence is complete. +So, to summarize, use [un]lock_system_sleep() instead of directly using +mutex_[un]lock(&pm_mutex). That would prevent freezing failures. diff --git a/Documentation/power/runtime_pm.txt b/Documentation/power/runtime_pm.txt index c2ae8bf77d46..4abe83e1045a 100644 --- a/Documentation/power/runtime_pm.txt +++ b/Documentation/power/runtime_pm.txt @@ -57,6 +57,10 @@ the following: 4. Bus type of the device, if both dev->bus and dev->bus->pm are present. +If the subsystem chosen by applying the above rules doesn't provide the relevant +callback, the PM core will invoke the corresponding driver callback stored in +dev->driver->pm directly (if present). + The PM core always checks which callback to use in the order given above, so the priority order of callbacks from high to low is: PM domain, device type, class and bus type. Moreover, the high-priority one will always take precedence over @@ -64,86 +68,88 @@ a low-priority one. The PM domain, bus type, device type and class callbacks are referred to as subsystem-level callbacks in what follows. By default, the callbacks are always invoked in process context with interrupts -enabled. However, subsystems can use the pm_runtime_irq_safe() helper function -to tell the PM core that their ->runtime_suspend(), ->runtime_resume() and -->runtime_idle() callbacks may be invoked in atomic context with interrupts -disabled for a given device. This implies that the callback routines in -question must not block or sleep, but it also means that the synchronous helper -functions listed at the end of Section 4 may be used for that device within an -interrupt handler or generally in an atomic context. - -The subsystem-level suspend callback is _entirely_ _responsible_ for handling -the suspend of the device as appropriate, which may, but need not include -executing the device driver's own ->runtime_suspend() callback (from the +enabled. However, the pm_runtime_irq_safe() helper function can be used to tell +the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume() +and ->runtime_idle() callbacks for the given device in atomic context with +interrupts disabled. This implies that the callback routines in question must +not block or sleep, but it also means that the synchronous helper functions +listed at the end of Section 4 may be used for that device within an interrupt +handler or generally in an atomic context. + +The subsystem-level suspend callback, if present, is _entirely_ _responsible_ +for handling the suspend of the device as appropriate, which may, but need not +include executing the device driver's own ->runtime_suspend() callback (from the PM core's point of view it is not necessary to implement a ->runtime_suspend() callback in a device driver as long as the subsystem-level suspend callback knows what to do to handle the device). - * Once the subsystem-level suspend callback has completed successfully - for given device, the PM core regards the device as suspended, which need - not mean that the device has been put into a low power state. It is - supposed to mean, however, that the device will not process data and will - not communicate with the CPU(s) and RAM until the subsystem-level resume - callback is executed for it. The runtime PM status of a device after - successful execution of the subsystem-level suspend callback is 'suspended'. - - * If the subsystem-level suspend callback returns -EBUSY or -EAGAIN, - the device's runtime PM status is 'active', which means that the device - _must_ be fully operational afterwards. - - * If the subsystem-level suspend callback returns an error code different - from -EBUSY or -EAGAIN, the PM core regards this as a fatal error and will - refuse to run the helper functions described in Section 4 for the device, - until the status of it is directly set either to 'active', or to 'suspended' - (the PM core provides special helper functions for this purpose). - -In particular, if the driver requires remote wake-up capability (i.e. hardware + * Once the subsystem-level suspend callback (or the driver suspend callback, + if invoked directly) has completed successfully for the given device, the PM + core regards the device as suspended, which need not mean that it has been + put into a low power state. It is supposed to mean, however, that the + device will not process data and will not communicate with the CPU(s) and + RAM until the appropriate resume callback is executed for it. The runtime + PM status of a device after successful execution of the suspend callback is + 'suspended'. + + * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM + status remains 'active', which means that the device _must_ be fully + operational afterwards. + + * If the suspend callback returns an error code different from -EBUSY and + -EAGAIN, the PM core regards this as a fatal error and will refuse to run + the helper functions described in Section 4 for the device until its status + is directly set to either'active', or 'suspended' (the PM core provides + special helper functions for this purpose). + +In particular, if the driver requires remote wakeup capability (i.e. hardware mechanism allowing the device to request a change of its power state, such as PCI PME) for proper functioning and device_run_wake() returns 'false' for the device, then ->runtime_suspend() should return -EBUSY. On the other hand, if -device_run_wake() returns 'true' for the device and the device is put into a low -power state during the execution of the subsystem-level suspend callback, it is -expected that remote wake-up will be enabled for the device. Generally, remote -wake-up should be enabled for all input devices put into a low power state at -run time. - -The subsystem-level resume callback is _entirely_ _responsible_ for handling the -resume of the device as appropriate, which may, but need not include executing -the device driver's own ->runtime_resume() callback (from the PM core's point of -view it is not necessary to implement a ->runtime_resume() callback in a device -driver as long as the subsystem-level resume callback knows what to do to handle -the device). - - * Once the subsystem-level resume callback has completed successfully, the PM - core regards the device as fully operational, which means that the device - _must_ be able to complete I/O operations as needed. The runtime PM status - of the device is then 'active'. - - * If the subsystem-level resume callback returns an error code, the PM core - regards this as a fatal error and will refuse to run the helper functions - described in Section 4 for the device, until its status is directly set - either to 'active' or to 'suspended' (the PM core provides special helper - functions for this purpose). - -The subsystem-level idle callback is executed by the PM core whenever the device -appears to be idle, which is indicated to the PM core by two counters, the -device's usage counter and the counter of 'active' children of the device. +device_run_wake() returns 'true' for the device and the device is put into a +low-power state during the execution of the suspend callback, it is expected +that remote wakeup will be enabled for the device. Generally, remote wakeup +should be enabled for all input devices put into low-power states at run time. + +The subsystem-level resume callback, if present, is _entirely_ _responsible_ for +handling the resume of the device as appropriate, which may, but need not +include executing the device driver's own ->runtime_resume() callback (from the +PM core's point of view it is not necessary to implement a ->runtime_resume() +callback in a device driver as long as the subsystem-level resume callback knows +what to do to handle the device). + + * Once the subsystem-level resume callback (or the driver resume callback, if + invoked directly) has completed successfully, the PM core regards the device + as fully operational, which means that the device _must_ be able to complete + I/O operations as needed. The runtime PM status of the device is then + 'active'. + + * If the resume callback returns an error code, the PM core regards this as a + fatal error and will refuse to run the helper functions described in Section + 4 for the device, until its status is directly set to either 'active', or + 'suspended' (by means of special helper functions provided by the PM core + for this purpose). + +The idle callback (a subsystem-level one, if present, or the driver one) is +executed by the PM core whenever the device appears to be idle, which is +indicated to the PM core by two counters, the device's usage counter and the +counter of 'active' children of the device. * If any of these counters is decreased using a helper function provided by the PM core and it turns out to be equal to zero, the other counter is checked. If that counter also is equal to zero, the PM core executes the - subsystem-level idle callback with the device as an argument. + idle callback with the device as its argument. -The action performed by a subsystem-level idle callback is totally dependent on -the subsystem in question, but the expected and recommended action is to check +The action performed by the idle callback is totally dependent on the subsystem +(or driver) in question, but the expected and recommended action is to check if the device can be suspended (i.e. if all of the conditions necessary for suspending the device are satisfied) and to queue up a suspend request for the device in that case. The value returned by this callback is ignored by the PM core. The helper functions provided by the PM core, described in Section 4, guarantee -that the following constraints are met with respect to the bus type's runtime -PM callbacks: +that the following constraints are met with respect to runtime PM callbacks for +one device: (1) The callbacks are mutually exclusive (e.g. it is forbidden to execute ->runtime_suspend() in parallel with ->runtime_resume() or with another |