On Wed, Jan 9, 2019 at 11:54 AM Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote: > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > > Replace the remaining documents under Documentation/cpuidle/ > with one more complete governor and driver API document for cpuidle > under Documentation/driver-api/pm/. > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> In the face of the lack of any feedback I'm assuming that this is fine by everyone. > --- > > On top of https://patchwork.kernel.org/patch/10747185/ > > --- > Documentation/cpuidle/driver.txt | 37 ---- > Documentation/cpuidle/governor.txt | 28 --- > Documentation/driver-api/pm/cpuidle.rst | 282 ++++++++++++++++++++++++++++++++ > Documentation/driver-api/pm/index.rst | 7 > MAINTAINERS | 1 > 5 files changed, 287 insertions(+), 68 deletions(-) > > Index: linux-pm/Documentation/driver-api/pm/index.rst > =================================================================== > --- linux-pm.orig/Documentation/driver-api/pm/index.rst > +++ linux-pm/Documentation/driver-api/pm/index.rst > @@ -1,9 +1,10 @@ > -======================= > -Device Power Management > -======================= > +=============================== > +CPU and Device Power Management > +=============================== > > .. toctree:: > > + cpuidle > devices > notifiers > types > Index: linux-pm/Documentation/driver-api/pm/cpuidle.rst > =================================================================== > --- /dev/null > +++ linux-pm/Documentation/driver-api/pm/cpuidle.rst > @@ -0,0 +1,282 @@ > +.. |struct cpuidle_governor| replace:: :c:type:`struct cpuidle_governor <cpuidle_governor>` > +.. |struct cpuidle_device| replace:: :c:type:`struct cpuidle_device <cpuidle_device>` > +.. |struct cpuidle_driver| replace:: :c:type:`struct cpuidle_driver <cpuidle_driver>` > +.. |struct cpuidle_state| replace:: :c:type:`struct cpuidle_state <cpuidle_state>` > + > +======================== > +CPU Idle Time Management > +======================== > + > +:: > + > + Copyright (c) 2019 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > + > + > +CPU Idle Time Management Subsystem > +================================== > + > +Every time one of the logical CPUs in the system (the entities that appear to > +fetch and execute instructions: hardware threads, if present, or processor > +cores) is idle after an interrupt or equivalent wakeup event, which means that > +there are no tasks to run on it except for the special "idle" task associated > +with it, there is an opportunity to save energy for the processor that it > +belongs to. That can be done by making the idle logical CPU stop fetching > +instructions from memory and putting some of the processor's functional units > +depended on by it into an idle state in which they will draw less power. > + > +However, there may be multiple different idle states that can be used in such a > +situation in principle, so it may be necessary to find the most suitable one > +(from the kernel perspective) and ask the processor to use (or "enter") that > +particular idle state. That is the role of the CPU idle time management > +subsystem in the kernel, called ``CPUIdle``. > + > +The design of ``CPUIdle`` is modular and based on the code duplication avoidance > +principle, so the generic code that in principle need not depend on the hardware > +or platform design details in it is separate from the code that interacts with > +the hardware. It generally is divided into three categories of functional > +units: *governors* responsible for selecting idle states to ask the processor > +to enter, *drivers* that pass the governors' decisions on to the hardware and > +the *core* providing a common framework for them. > + > + > +CPU Idle Time Governors > +======================= > + > +A CPU idle time (``CPUIdle``) governor is a bundle of policy code invoked when > +one of the logical CPUs in the system turns out to be idle. Its role is to > +select an idle state to ask the processor to enter in order to save some energy. > + > +``CPUIdle`` governors are generic and each of them can be used on any hardware > +platform that the Linux kernel can run on. For this reason, data structures > +operated on by them cannot depend on any hardware architecture or platform > +design details as well. > + > +The governor itself is represented by a |struct cpuidle_governor| object > +containing four callback pointers, :c:member:`enable`, :c:member:`disable`, > +:c:member:`select`, :c:member:`reflect`, a :c:member:`rating` field described > +below, and a name (string) used for identifying it. > + > +For the governor to be available at all, that object needs to be registered > +with the ``CPUIdle`` core by calling :c:func:`cpuidle_register_governor()` with > +a pointer to it passed as the argument. If successful, that causes the core to > +add the governor to the global list of available governors and, if it is the > +only one in the list (that is, the list was empty before) or the value of its > +:c:member:`rating` field is greater than the value of that field for the > +governor currently in use, or the name of the new governor was passed to the > +kernel as the value of the ``cpuidle.governor=`` command line parameter, the new > +governor will be used from that point on (there can be only one ``CPUIdle`` > +governor in use at a time). Also, if ``cpuidle_sysfs_switch`` is passed to the > +kernel in the command line, user space can choose the ``CPUIdle`` governor to > +use at run time via ``sysfs``. > + > +Once registered, ``CPUIdle`` governors cannot be unregistered, so it is not > +practical to put them into loadable kernel modules. > + > +The interface between ``CPUIdle`` governors and the core consists of four > +callbacks: > + > +:c:member:`enable` > + :: > + > + int (*enable) (struct cpuidle_driver *drv, struct cpuidle_device *dev); > + > + The role of this callback is to prepare the governor for handling the > + (logical) CPU represented by the |struct cpuidle_device| object pointed > + to by the ``dev`` argument. The |struct cpuidle_driver| object pointed > + to by the ``drv`` argument represents the ``CPUIdle`` driver to be used > + with that CPU (among other things, it should contain the list of > + |struct cpuidle_state| objects representing idle states that the > + processor holding the given CPU can be asked to enter). > + > + It may fail, in which case it is expected to return a negative error > + code, and that causes the kernel to run the architecture-specific > + default code for idle CPUs on the CPU in question instead of ``CPUIdle`` > + until the ``->enable()`` governor callback is invoked for that CPU > + again. > + > +:c:member:`disable` > + :: > + > + void (*disable) (struct cpuidle_driver *drv, struct cpuidle_device *dev); > + > + Called to make the governor stop handling the (logical) CPU represented > + by the |struct cpuidle_device| object pointed to by the ``dev`` > + argument. > + > + It is expected to reverse any changes made by the ``->enable()`` > + callback when it was last invoked for the target CPU, free all memory > + allocated by that callback and so on. > + > +:c:member:`select` > + :: > + > + int (*select) (struct cpuidle_driver *drv, struct cpuidle_device *dev, > + bool *stop_tick); > + > + Called to select an idle state for the processor holding the (logical) > + CPU represented by the |struct cpuidle_device| object pointed to by the > + ``dev`` argument. > + > + The list of idle states to take into consideration is represented by the > + :c:member:`states` array of |struct cpuidle_state| objects held by the > + |struct cpuidle_driver| object pointed to by the ``drv`` argument (which > + represents the ``CPUIdle`` driver to be used with the CPU at hand). The > + value returned by this callback is interpreted as an index into that > + array (unless it is a negative error code). > + > + The ``stop_tick`` argument is used to indicate whether or not to stop > + the scheduler tick before asking the processor to enter the selected > + idle state. When the ``bool`` variable pointed to by it (which is set > + to ``true`` before invoking this callback) is cleared to ``false``, the > + processor will be asked to enter the selected idle state without > + stopping the scheduler tick on the given CPU (if the tick has been > + stopped on that CPU already, however, it will not be restarted before > + asking the processor to enter the idle state). > + > + This callback is mandatory (i.e. the :c:member:`select` callback pointer > + in |struct cpuidle_governor| must not be ``NULL`` for the registration > + of the governor to succeed). > + > +:c:member:`reflect` > + :: > + > + void (*reflect) (struct cpuidle_device *dev, int index); > + > + Called to allow the governor to evaluate the accuracy of the idle state > + selection made by the ``->select()`` callback (when it was invoked last > + time) and possibly use the result of that to improve the accuracy of > + idle state selections in the future. > + > +In addition, ``CPUIdle`` governors are required to take power management > +quality of service (PM QoS) constraints on the processor wakeup latency into > +account when selecting idle states. In order to obtain the current effective > +PM QoS wakeup latency constraint for a given CPU, a ``CPUIdle`` governor is > +expected to pass the number of the CPU to > +:c:func:`cpuidle_governor_latency_req()`. Then, the governor's ``->select()`` > +callback must not return the index of an indle state whose > +:c:member:`exit_latency` value is greater than the number returned by that > +function. > + > + > +CPU Idle Time Management Drivers > +================================ > + > +CPU idle time management (``CPUIdle``) drivers provide an interface between the > +other parts of ``CPUIdle`` and the hardware. > + > +First of all, a ``CPUIdle`` driver has to populate the :c:member:`states` array > +of |struct cpuidle_state| objects included in the |struct cpuidle_driver| object > +representing it. Going forward this array will represent the list of available > +idle states that the processor hardware can be asked to enter shared by all of > +the logical CPUs handled by the given driver. > + > +The entries in the :c:member:`states` array are expected to be sorted by the > +value of the :c:member:`target_residency` field in |struct cpuidle_state| in > +the ascending order (that is, index 0 should correspond to the idle state with > +the minimum value of :c:member:`target_residency`). [Since the > +:c:member:`target_residency` value is expected to reflect the "depth" of the > +idle state represented by the |struct cpuidle_state| object holding it, this > +sorting order should be the same as the ascending sorting order by the idle > +state "depth".] > + > +Three fields in |struct cpuidle_state| are used by the existing ``CPUIdle`` > +governors for computations related to idle state selection: > + > +:c:member:`target_residency` > + Minimum time to spend in this idle state including the time needed to > + enter it (which may be substantial) to save more energy than could > + be saved by staying in a shallower idle state for the same amount of > + time, in microseconds. > + > +:c:member:`exit_latency` > + Maximum time it will take a CPU asking the processor to enter this idle > + state to start executing the first instruction after a wakeup from it, > + in microseconds. > + > +:c:member:`flags` > + Flags representing idle state properties. Currently, governors only use > + the ``CPUIDLE_FLAG_POLLING`` flag which is set if the given object > + does not represent a real idle state, but an interface to a software > + "loop" that can be used in order to avoid asking the processor to enter > + any idle state at all. [There are other flags used by the ``CPUIdle`` > + core in special situations.] > + > +The :c:member:`enter` callback pointer in |struct cpuidle_state|, which must not > +be ``NULL``, points to the routine to execute in order to ask the processor to > +enter this particular idle state: > + > +:: > + > + void (*enter) (struct cpuidle_device *dev, struct cpuidle_driver *drv, > + int index); > + > +The first two arguments of it point to the |struct cpuidle_device| object > +representing the logical CPU running this callback and the > +|struct cpuidle_driver| object representing the driver itself, respectively, > +and the last one is an index of the |struct cpuidle_state| entry in the driver's > +:c:member:`states` array representing the idle state to ask the processor to > +enter. > + > +The analogous ``->enter_s2idle()`` callback in |struct cpuidle_state| is used > +only for implementing the suspend-to-idle system-wide power management feature. > +The difference between in and ``->enter()`` is that it must not re-enable > +interrupts at any point (even temporarily) or attempt to change the states of > +clock event devices, which the ``->enter()`` callback may do sometimes. > + > +Once the :c:member:`states` array has been populated, the number of valid > +entries in it has to be stored in the :c:member:`state_count` field of the > +|struct cpuidle_driver| object representing the driver. Moreover, if any > +entries in the :c:member:`states` array represent "coupled" idle states (that > +is, idle states that can only be asked for if multiple related logical CPUs are > +idle), the :c:member:`safe_state_index` field in |struct cpuidle_driver| needs > +to be the index of an idle state that is not "coupled" (that is, one that can be > +asked for if only one logical CPU is idle). > + > +In addition to that, if the given ``CPUIdle`` driver is only going to handle a > +subset of logical CPUs in the system, the :c:member:`cpumask` field in its > +|struct cpuidle_driver| object must point to the set (mask) of CPUs that will be > +handled by it. > + > +A ``CPUIdle`` driver can only be used after it has been registered. If there > +are no "coupled" idle state entries in the driver's :c:member:`states` array, > +that can be accomplished by passing the driver's |struct cpuidle_driver| object > +to :c:func:`cpuidle_register_driver()`. Otherwise, :c:func:`cpuidle_register()` > +should be used for this purpose. > + > +However, it also is necessary to register |struct cpuidle_device| objects for > +all of the logical CPUs to be handled by the given ``CPUIdle`` driver with the > +help of :c:func:`cpuidle_register_device()` after the driver has been registered > +and :c:func:`cpuidle_register_driver()`, unlike :c:func:`cpuidle_register()`, > +does not do that automatically. For this reason, the drivers that use > +:c:func:`cpuidle_register_driver()` to register themselves must also take care > +of registering the |struct cpuidle_device| objects as needed, so it is generally > +recommended to use :c:func:`cpuidle_register()` for ``CPUIdle`` driver > +registration in all cases. > + > +The registration of a |struct cpuidle_device| object causes the ``CPUIdle`` > +``sysfs`` interface to be created and the governor's ``->enable()`` callback to > +be invoked for the logical CPU represented by it, so it must take place after > +registering the driver that will handle the CPU in question. > + > +``CPUIdle`` drivers and |struct cpuidle_device| objects can be unregistered > +when they are not necessary any more which allows some resources associated with > +them to be released. Due to dependencies between them, all of the > +|struct cpuidle_device| objects representing CPUs handled by the given > +``CPUIdle`` driver must be unregistered, with the help of > +:c:func:`cpuidle_unregister_device()`, before calling > +:c:func:`cpuidle_unregister_driver()` to unregister the driver. Alternatively, > +:c:func:`cpuidle_unregister()` can be called to unregister a ``CPUIdle`` driver > +along with all of the |struct cpuidle_device| objects representing CPUs handled > +by it. > + > +``CPUIdle`` drivers can respond to runtime system configuration changes that > +lead to modifications of the list of available processor idle states (which can > +happen, for example, when the system's power source is switched from AC to > +battery or the other way around). Upon a notification of such a change, > +a ``CPUIdle`` driver is expected to call :c:func:`cpuidle_pause_and_lock()` to > +turn ``CPUIdle`` off temporarily and then :c:func:`cpuidle_disable_device()` for > +all of the |struct cpuidle_device| objects representing CPUs affected by that > +change. Next, it can update its :c:member:`states` array in accordance with > +the new configuration of the system, call :c:func:`cpuidle_enable_device()` for > +all of the relevant |struct cpuidle_device| objects and invoke > +:c:func:`cpuidle_resume_and_unlock()` to allow ``CPUIdle`` to be used again. > Index: linux-pm/Documentation/cpuidle/driver.txt > =================================================================== > --- linux-pm.orig/Documentation/cpuidle/driver.txt > +++ /dev/null > @@ -1,37 +0,0 @@ > - > - > - Supporting multiple CPU idle levels in kernel > - > - cpuidle drivers > - > - > - > - > -cpuidle driver hooks into the cpuidle infrastructure and handles the > -architecture/platform dependent part of CPU idle states. Driver > -provides the platform idle state detection capability and also > -has mechanisms in place to support actual entry-exit into CPU idle states. > - > -cpuidle driver initializes the cpuidle_device structure for each CPU device > -and registers with cpuidle using cpuidle_register_device. > - > -If all the idle states are the same, the wrapper function cpuidle_register > -could be used instead. > - > -It can also support the dynamic changes (like battery <-> AC), by using > -cpuidle_pause_and_lock, cpuidle_disable_device and cpuidle_enable_device, > -cpuidle_resume_and_unlock. > - > -Interfaces: > -extern int cpuidle_register(struct cpuidle_driver *drv, > - const struct cpumask *const coupled_cpus); > -extern int cpuidle_unregister(struct cpuidle_driver *drv); > -extern int cpuidle_register_driver(struct cpuidle_driver *drv); > -extern void cpuidle_unregister_driver(struct cpuidle_driver *drv); > -extern int cpuidle_register_device(struct cpuidle_device *dev); > -extern void cpuidle_unregister_device(struct cpuidle_device *dev); > - > -extern void cpuidle_pause_and_lock(void); > -extern void cpuidle_resume_and_unlock(void); > -extern int cpuidle_enable_device(struct cpuidle_device *dev); > -extern void cpuidle_disable_device(struct cpuidle_device *dev); > Index: linux-pm/Documentation/cpuidle/governor.txt > =================================================================== > --- linux-pm.orig/Documentation/cpuidle/governor.txt > +++ /dev/null > @@ -1,28 +0,0 @@ > - > - > - > - Supporting multiple CPU idle levels in kernel > - > - cpuidle governors > - > - > - > - > -cpuidle governor is policy routine that decides what idle state to enter at > -any given time. cpuidle core uses different callbacks to the governor. > - > -* enable() to enable governor for a particular device > -* disable() to disable governor for a particular device > -* select() to select an idle state to enter > -* reflect() called after returning from the idle state, which can be used > - by the governor for some record keeping. > - > -More than one governor can be registered at the same time and > -users can switch between drivers using /sysfs interface (when enabled). > -More than one governor part is supported for developers to easily experiment > -with different governors. By default, most optimal governor based on your > -kernel configuration and platform will be selected by cpuidle. > - > -Interfaces: > -extern int cpuidle_register_governor(struct cpuidle_governor *gov); > -struct cpuidle_governor > Index: linux-pm/MAINTAINERS > =================================================================== > --- linux-pm.orig/MAINTAINERS > +++ linux-pm/MAINTAINERS > @@ -4016,6 +4016,7 @@ S: Maintained > T: git git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git > B: https://bugzilla.kernel.org > F: Documentation/admin-guide/pm/cpuidle.rst > +F: Documentation/driver-api/pm/cpuidle.rst > F: drivers/cpuidle/* > F: include/linux/cpuidle.h > >
