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+CPU PM domains
+==============
+
+Newer CPUs are grouped in SoCs as clusters. A cluster in addition to the CPUs
+may have caches, floating point units and other architecture specific power
+controller that share resources when any of the CPUs are active. When the CPUs
+are in idle, some of these cluster components may also idle. A cluster may
+also be nested inside another cluster that provides common coherency
+interfaces to share data between the clusters. The organization of such
+clusters and CPU may be descibed in DT, since they are SoC specific.
+
+CPUIdle framework enables the CPUs to determine the sleep time and enter low
+power state to save power during periods of idle. CPUs in a cluster may enter
+and exit idle state independently of each other. During the time when all the
+CPUs are in idle state, the cluster may safely put some of the shared
+resources in their idle state. The time between the last CPU to enter idle and
+the first CPU to wake up is the time available for the cluster to enter its
+idle state.
+
+When SoCs power down the CPU during cpuidle, they generally have supplemental
+hardware that can handshake with the CPU with a signal that indicates that the
+CPU has stopped execution. The hardware is also responsible for warm booting
+the CPU on receiving an interrupt. In a cluster architecture, common resources
+that are shared by a cluster may also be powered down by an external
+microcontroller or a processor. The microcontroller may be programmed in
+advance to put the hardware blocks in a low power state, when the last active
+CPU sends the idle signal. When the signal is received, the microcontroller
+may trigger the hardware blocks to enter their low power state. When an
+interrupt to wakeup the processor is received, the microcontroller is
+responsible for bringing up the hardware blocks to its active state, before
+waking up the CPU. The timelines for such operations should be in the
+acceptable range for for CPU idle to get power benefits.
+
+CPU PM Domain Setup
+-------------------
+
+PM domains are represented in the DT as domain consumers and providers. A
+device may have a domain provider and a domain provider may support multiple
+domain consumers. Domains like clusters, may also be nested inside one
+another. A domain that has no active consumer, may be powered off and any
+resuming consumer would trigger the domain back to active. Parent domains may
+be powered off when the child domains are powered off. The CPU cluster can be
+fashioned as a PM domain. When the CPU devices are powered off, the PM domain
+may be powered off.
+
+Device idle is reference counted by runtime PM. When there is no active need
+for the device, runtime PM invokes callbacks to suspend the parent domain.
+Generic PM domain (genpd) handles the hierarchy of devices, domains and the
+reference counting of objects leading to last man down and first man up in the
+domain. The CPU domains helper functions defines PM domains for each CPU
+cluster and attaches the CPU devices to the respective PM domains.
+
+Platform drivers may use the following API to register their CPU PM domains.
+
+of_setup_cpu_pd() -
+Provides a single step registration of the CPU PM domain and attach CPUs to
+the genpd. Platform drivers may additionally register callbacks for power_on
+and power_off operations for the PM domain.
+
+of_setup_cpu_pd_single() -
+Define PM domain for a single CPU and attach the CPU to its domain.
+
+
+CPU PM Domain governor
+----------------------
+
+CPUs have a unique ability to determine their next wakeup. CPUs may wake up
+for known timer interrupts and unknown interrupts from idle. Prediction
+algorithms and heuristic based algorithms like the Menu governor for cpuidle
+can determine the next wakeup of the CPU. However, determining the wakeup
+across a group of CPUs is a tough problem to solve.
+
+A simplistic approach would be to resort to known wakeups of the CPUs in
+determining the next wakeup of any CPU in the cluster. The CPU PM domain
+governor does just that. By looking into the tick device of the CPUs, the
+governor can determine the sleep time between the last CPU and the first
+scheduled wakeup of any CPU in that domain. This combined with the PM QoS
+requirement for CPU_DMA_LATENCY can be used to determine the deepest possible
+idle state of the CPU domain.
A generic CPU PM domain functionality is provided by drivers/base/power/cpu_domains.c. This document describes the generic usecase of CPU's PM domains, the setup of such domains and a CPU specific genpd governor. Signed-off-by: Lina Iyer <lina.iyer@linaro.org> --- Documentation/power/cpu_domains.txt | 79 +++++++++++++++++++++++++++++++++++++ 1 file changed, 79 insertions(+) create mode 100644 Documentation/power/cpu_domains.txt