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Diffstat (limited to 'Documentation/driver-api/pm/devices.rst')
| -rw-r--r-- | Documentation/driver-api/pm/devices.rst | 199 |
1 files changed, 127 insertions, 72 deletions
diff --git a/Documentation/driver-api/pm/devices.rst b/Documentation/driver-api/pm/devices.rst index f66c7b9126ea..946ad0b94e31 100644 --- a/Documentation/driver-api/pm/devices.rst +++ b/Documentation/driver-api/pm/devices.rst @@ -349,7 +349,7 @@ the phases are: ``prepare``, ``suspend``, ``suspend_late``, ``suspend_noirq``. PM core will skip the ``suspend``, ``suspend_late`` and ``suspend_noirq`` phases as well as all of the corresponding phases of the subsequent device resume for all of these devices. In that case, - the ``->complete`` callback will be invoked directly after the + the ``->complete`` callback will be the next one invoked after the ``->prepare`` callback and is entirely responsible for putting the device into a consistent state as appropriate. @@ -361,9 +361,9 @@ the phases are: ``prepare``, ``suspend``, ``suspend_late``, ``suspend_noirq``. runtime PM disabled. This feature also can be controlled by device drivers by using the - ``DPM_FLAG_NEVER_SKIP`` and ``DPM_FLAG_SMART_PREPARE`` driver power - management flags. [Typically, they are set at the time the driver is - probed against the device in question by passing them to the + ``DPM_FLAG_NO_DIRECT_COMPLETE`` and ``DPM_FLAG_SMART_PREPARE`` driver + power management flags. [Typically, they are set at the time the driver + is probed against the device in question by passing them to the :c:func:`dev_pm_set_driver_flags` helper function.] If the first of these flags is set, the PM core will not apply the direct-complete procedure described above to the given device and, consequenty, to any @@ -383,11 +383,15 @@ the phases are: ``prepare``, ``suspend``, ``suspend_late``, ``suspend_noirq``. ``->suspend`` methods provided by subsystems (bus types and PM domains in particular) must follow an additional rule regarding what can be done to the devices before their drivers' ``->suspend`` methods are called. - Namely, they can only resume the devices from runtime suspend by - calling :c:func:`pm_runtime_resume` for them, if that is necessary, and + Namely, they may resume the devices from runtime suspend by + calling :c:func:`pm_runtime_resume` for them, if that is necessary, but they must not update the state of the devices in any other way at that time (in case the drivers need to resume the devices from runtime - suspend in their ``->suspend`` methods). + suspend in their ``->suspend`` methods). In fact, the PM core prevents + subsystems or drivers from putting devices into runtime suspend at + these times by calling :c:func:`pm_runtime_get_noresume` before issuing + the ``->prepare`` callback (and calling :c:func:`pm_runtime_put` after + issuing the ``->complete`` callback). 3. For a number of devices it is convenient to split suspend into the "quiesce device" and "save device state" phases, in which cases @@ -459,22 +463,22 @@ When resuming from freeze, standby or memory sleep, the phases are: Note, however, that new children may be registered below the device as soon as the ``->resume`` callbacks occur; it's not necessary to wait - until the ``complete`` phase with that. + until the ``complete`` phase runs. Moreover, if the preceding ``->prepare`` callback returned a positive number, the device may have been left in runtime suspend throughout the - whole system suspend and resume (the ``suspend``, ``suspend_late``, - ``suspend_noirq`` phases of system suspend and the ``resume_noirq``, - ``resume_early``, ``resume`` phases of system resume may have been - skipped for it). In that case, the ``->complete`` callback is entirely + whole system suspend and resume (its ``->suspend``, ``->suspend_late``, + ``->suspend_noirq``, ``->resume_noirq``, + ``->resume_early``, and ``->resume`` callbacks may have been + skipped). In that case, the ``->complete`` callback is entirely responsible for putting the device into a consistent state after system suspend if necessary. [For example, it may need to queue up a runtime resume request for the device for this purpose.] To check if that is the case, the ``->complete`` callback can consult the device's - ``power.direct_complete`` flag. Namely, if that flag is set when the - ``->complete`` callback is being run, it has been called directly after - the preceding ``->prepare`` and special actions may be required - to make the device work correctly afterward. + ``power.direct_complete`` flag. If that flag is set when the + ``->complete`` callback is being run then the direct-complete mechanism + was used, and special actions may be required to make the device work + correctly afterward. At the end of these phases, drivers should be as functional as they were before suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are @@ -575,10 +579,12 @@ and the phases are similar. The ``->poweroff``, ``->poweroff_late`` and ``->poweroff_noirq`` callbacks should do essentially the same things as the ``->suspend``, ``->suspend_late`` -and ``->suspend_noirq`` callbacks, respectively. The only notable difference is +and ``->suspend_noirq`` callbacks, respectively. A notable difference is that they need not store the device register values, because the registers should already have been stored during the ``freeze``, ``freeze_late`` or -``freeze_noirq`` phases. +``freeze_noirq`` phases. Also, on many machines the firmware will power-down +the entire system, so it is not necessary for the callback to put the device in +a low-power state. Leaving Hibernation @@ -764,70 +770,119 @@ device driver in question. If it is necessary to resume a device from runtime suspend during a system-wide transition into a sleep state, that can be done by calling -:c:func:`pm_runtime_resume` for it from the ``->suspend`` callback (or its -couterpart for transitions related to hibernation) of either the device's driver -or a subsystem responsible for it (for example, a bus type or a PM domain). -That is guaranteed to work by the requirement that subsystems must not change -the state of devices (possibly except for resuming them from runtime suspend) +:c:func:`pm_runtime_resume` from the ``->suspend`` callback (or the ``->freeze`` +or ``->poweroff`` callback for transitions related to hibernation) of either the +device's driver or its subsystem (for example, a bus type or a PM domain). +However, subsystems must not otherwise change the runtime status of devices from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before* invoking device drivers' ``->suspend`` callbacks (or equivalent). +.. _smart_suspend_flag: + +The ``DPM_FLAG_SMART_SUSPEND`` Driver Flag +------------------------------------------ + Some bus types and PM domains have a policy to resume all devices from runtime suspend upfront in their ``->suspend`` callbacks, but that may not be really -necessary if the driver of the device can cope with runtime-suspended devices. -The driver can indicate that by setting ``DPM_FLAG_SMART_SUSPEND`` in -:c:member:`power.driver_flags` at the probe time, by passing it to the -:c:func:`dev_pm_set_driver_flags` helper. That also may cause middle-layer code +necessary if the device's driver can cope with runtime-suspended devices. +The driver can indicate this by setting ``DPM_FLAG_SMART_SUSPEND`` in +:c:member:`power.driver_flags` at probe time, with the assistance of the +:c:func:`dev_pm_set_driver_flags` helper routine. + +Setting that flag causes the PM core and middle-layer code (bus types, PM domains etc.) to skip the ``->suspend_late`` and ``->suspend_noirq`` callbacks provided by the driver if the device remains in -runtime suspend at the beginning of the ``suspend_late`` phase of system-wide -suspend (or in the ``poweroff_late`` phase of hibernation), when runtime PM -has been disabled for it, under the assumption that its state should not change -after that point until the system-wide transition is over (the PM core itself -does that for devices whose "noirq", "late" and "early" system-wide PM callbacks -are executed directly by it). If that happens, the driver's system-wide resume -callbacks, if present, may still be invoked during the subsequent system-wide -resume transition and the device's runtime power management status may be set -to "active" before enabling runtime PM for it, so the driver must be prepared to -cope with the invocation of its system-wide resume callbacks back-to-back with -its ``->runtime_suspend`` one (without the intervening ``->runtime_resume`` and -so on) and the final state of the device must reflect the "active" runtime PM -status in that case. +runtime suspend throughout those phases of the system-wide suspend (and +similarly for the "freeze" and "poweroff" parts of system hibernation). +[Otherwise the same driver +callback might be executed twice in a row for the same device, which would not +be valid in general.] If the middle-layer system-wide PM callbacks are present +for the device then they are responsible for skipping these driver callbacks; +if not then the PM core skips them. The subsystem callback routines can +determine whether they need to skip the driver callbacks by testing the return +value from the :c:func:`dev_pm_skip_suspend` helper function. + +In addition, with ``DPM_FLAG_SMART_SUSPEND`` set, the driver's ``->thaw_noirq`` +and ``->thaw_early`` callbacks are skipped in hibernation if the device remained +in runtime suspend throughout the preceding "freeze" transition. Again, if the +middle-layer callbacks are present for the device, they are responsible for +doing this, otherwise the PM core takes care of it. + + +The ``DPM_FLAG_MAY_SKIP_RESUME`` Driver Flag +-------------------------------------------- During system-wide resume from a sleep state it's easiest to put devices into the full-power state, as explained in :file:`Documentation/power/runtime_pm.rst`. [Refer to that document for more information regarding this particular issue as well as for information on the device runtime power management framework in -general.] - -However, it often is desirable to leave devices in suspend after system -transitions to the working state, especially if those devices had been in +general.] However, it often is desirable to leave devices in suspend after +system transitions to the working state, especially if those devices had been in runtime suspend before the preceding system-wide suspend (or analogous) -transition. Device drivers can use the ``DPM_FLAG_LEAVE_SUSPENDED`` flag to -indicate to the PM core (and middle-layer code) that they prefer the specific -devices handled by them to be left suspended and they have no problems with -skipping their system-wide resume callbacks for this reason. Whether or not the -devices will actually be left in suspend may depend on their state before the -given system suspend-resume cycle and on the type of the system transition under -way. In particular, devices are not left suspended if that transition is a -restore from hibernation, as device states are not guaranteed to be reflected -by the information stored in the hibernation image in that case. - -The middle-layer code involved in the handling of the device is expected to -indicate to the PM core if the device may be left in suspend by setting its -:c:member:`power.may_skip_resume` status bit which is checked by the PM core -during the "noirq" phase of the preceding system-wide suspend (or analogous) -transition. The middle layer is then responsible for handling the device as -appropriate in its "noirq" resume callback, which is executed regardless of -whether or not the device is left suspended, but the other resume callbacks -(except for ``->complete``) will be skipped automatically by the PM core if the -device really can be left in suspend. - -For devices whose "noirq", "late" and "early" driver callbacks are invoked -directly by the PM core, all of the system-wide resume callbacks are skipped if -``DPM_FLAG_LEAVE_SUSPENDED`` is set and the device is in runtime suspend during -the ``suspend_noirq`` (or analogous) phase or the transition under way is a -proper system suspend (rather than anything related to hibernation) and the -device's wakeup settings are suitable for runtime PM (that is, it cannot -generate wakeup signals at all or it is allowed to wake up the system from -sleep). +transition. + +To that end, device drivers can use the ``DPM_FLAG_MAY_SKIP_RESUME`` flag to +indicate to the PM core and middle-layer code that they allow their "noirq" and +"early" resume callbacks to be skipped if the device can be left in suspend +after system-wide PM transitions to the working state. Whether or not that is +the case generally depends on the state of the device before the given system +suspend-resume cycle and on the type of the system transition under way. +In particular, the "thaw" and "restore" transitions related to hibernation are +not affected by ``DPM_FLAG_MAY_SKIP_RESUME`` at all. [All callbacks are +issued during the "restore" transition regardless of the flag settings, +and whether or not any driver callbacks +are skipped during the "thaw" transition depends whether or not the +``DPM_FLAG_SMART_SUSPEND`` flag is set (see `above <smart_suspend_flag_>`_). +In addition, a device is not allowed to remain in runtime suspend if any of its +children will be returned to full power.] + +The ``DPM_FLAG_MAY_SKIP_RESUME`` flag is taken into account in combination with +the :c:member:`power.may_skip_resume` status bit set by the PM core during the +"suspend" phase of suspend-type transitions. If the driver or the middle layer +has a reason to prevent the driver's "noirq" and "early" resume callbacks from +being skipped during the subsequent system resume transition, it should +clear :c:member:`power.may_skip_resume` in its ``->suspend``, ``->suspend_late`` +or ``->suspend_noirq`` callback. [Note that the drivers setting +``DPM_FLAG_SMART_SUSPEND`` need to clear :c:member:`power.may_skip_resume` in +their ``->suspend`` callback in case the other two are skipped.] + +Setting the :c:member:`power.may_skip_resume` status bit along with the +``DPM_FLAG_MAY_SKIP_RESUME`` flag is necessary, but generally not sufficient, +for the driver's "noirq" and "early" resume callbacks to be skipped. Whether or +not they should be skipped can be determined by evaluating the +:c:func:`dev_pm_skip_resume` helper function. + +If that function returns ``true``, the driver's "noirq" and "early" resume +callbacks should be skipped and the device's runtime PM status will be set to +"suspended" by the PM core. Otherwise, if the device was runtime-suspended +during the preceding system-wide suspend transition and its +``DPM_FLAG_SMART_SUSPEND`` is set, its runtime PM status will be set to +"active" by the PM core. [Hence, the drivers that do not set +``DPM_FLAG_SMART_SUSPEND`` should not expect the runtime PM status of their +devices to be changed from "suspended" to "active" by the PM core during +system-wide resume-type transitions.] + +If the ``DPM_FLAG_MAY_SKIP_RESUME`` flag is not set for a device, but +``DPM_FLAG_SMART_SUSPEND`` is set and the driver's "late" and "noirq" suspend +callbacks are skipped, its system-wide "noirq" and "early" resume callbacks, if +present, are invoked as usual and the device's runtime PM status is set to +"active" by the PM core before enabling runtime PM for it. In that case, the +driver must be prepared to cope with the invocation of its system-wide resume +callbacks back-to-back with its ``->runtime_suspend`` one (without the +intervening ``->runtime_resume`` and system-wide suspend callbacks) and the +final state of the device must reflect the "active" runtime PM status in that +case. [Note that this is not a problem at all if the driver's +``->suspend_late`` callback pointer points to the same function as its +``->runtime_suspend`` one and its ``->resume_early`` callback pointer points to +the same function as the ``->runtime_resume`` one, while none of the other +system-wide suspend-resume callbacks of the driver are present, for example.] + +Likewise, if ``DPM_FLAG_MAY_SKIP_RESUME`` is set for a device, its driver's +system-wide "noirq" and "early" resume callbacks may be skipped while its "late" +and "noirq" suspend callbacks may have been executed (in principle, regardless +of whether or not ``DPM_FLAG_SMART_SUSPEND`` is set). In that case, the driver +needs to be able to cope with the invocation of its ``->runtime_resume`` +callback back-to-back with its "late" and "noirq" suspend ones. [For instance, +that is not a concern if the driver sets both ``DPM_FLAG_SMART_SUSPEND`` and +``DPM_FLAG_MAY_SKIP_RESUME`` and uses the same pair of suspend/resume callback +functions for runtime PM and system-wide suspend/resume.] |