67 lines
2.7 KiB
Plaintext
67 lines
2.7 KiB
Plaintext
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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Each parameters have defined units:
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* latency: usec
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* timeout: usec
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* throughput: kbs (kilo bit / sec)
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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 requests 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 request list or elements of the list. Typically the
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aggregated target value is simply the max or min of the request 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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handle = pm_qos_add_request(param_class, target_value):
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Will insert an element into the list for that identified PM_QOS class with the
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target value. Upon change to this list the new target is recomputed and any
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registered notifiers are called only if the target value is now different.
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Clients of pm_qos need to save the returned handle.
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void pm_qos_update_request(handle, new_target_value):
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Will update the list element pointed to by the handle with the new target value
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and recompute the new aggregated target, calling the notification tree if the
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target is changed.
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void pm_qos_remove_request(handle):
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Will remove the element. After removal it will update the aggregate target and
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call the notification tree if the target was changed as a result of removing
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the request.
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From user mode:
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Only processes can register a pm_qos request. To provide for automatic
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cleanup of a process, the interface requires the process to register its
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parameter requests 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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request on the parameter.
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To change the requested target value the process needs to write an s32 value to
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the open device node. Alternatively the user mode program could write a hex
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string for the value using 10 char long format e.g. "0x12345678". This
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translates to a pm_qos_update_request 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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