60 lines
2.6 KiB
Plaintext
60 lines
2.6 KiB
Plaintext
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PM quality of Service interface.
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This interface provides a kernel and user mode interface for registering
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performance expectations by drivers, subsystems and user space applications on
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one of the parameters.
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Currently we have {cpu_dma_latency, network_latency, network_throughput} as the
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initial set of pm_qos parameters.
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The infrastructure exposes multiple misc device nodes one per implemented
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parameter. The set of parameters implement is defined by pm_qos_power_init()
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and pm_qos_params.h. This is done because having the available parameters
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being runtime configurable or changeable from a driver was seen as too easy to
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abuse.
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For each parameter a list of performance requirements is maintained along with
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an aggregated target value. The aggregated target value is updated with
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changes to the requirement list or elements of the list. Typically the
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aggregated target value is simply the max or min of the requirement values held
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in the parameter list elements.
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From kernel mode the use of this interface is simple:
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pm_qos_add_requirement(param_id, name, target_value):
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Will insert a named element in the list for that identified PM_QOS parameter
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with the target value. Upon change to this list the new target is recomputed
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and any registered notifiers are called only if the target value is now
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different.
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pm_qos_update_requirement(param_id, name, new_target_value):
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Will search the list identified by the param_id for the named list element and
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then update its target value, calling the notification tree if the aggregated
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target is changed. with that name is already registered.
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pm_qos_remove_requirement(param_id, name):
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Will search the identified list for the named element and remove it, after
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removal it will update the aggregate target and call the notification tree if
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the target was changed as a result of removing the named requirement.
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From user mode:
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Only processes can register a pm_qos requirement. To provide for automatic
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cleanup for process the interface requires the process to register its
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parameter requirements in the following way:
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To register the default pm_qos target for the specific parameter, the process
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must open one of /dev/[cpu_dma_latency, network_latency, network_throughput]
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As long as the device node is held open that process has a registered
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requirement on the parameter. The name of the requirement is "process_<PID>"
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derived from the current->pid from within the open system call.
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To change the requested target value the process needs to write a s32 value to
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the open device node. This translates to a pm_qos_update_requirement call.
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To remove the user mode request for a target value simply close the device
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node.
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