149 lines
5.9 KiB
Plaintext
149 lines
5.9 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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Two different PM QoS frameworks are available:
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1. PM QoS classes for cpu_dma_latency, network_latency, network_throughput.
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2. the per-device PM QoS framework provides the API to manage the per-device latency
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constraints.
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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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1. PM QoS framework
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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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Note: the aggregated target value is implemented as an atomic variable so that
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reading the aggregated value does not require any locking mechanism.
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From kernel mode the use of this interface is simple:
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void pm_qos_add_request(handle, 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 for future use in other
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pm_qos API functions.
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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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int pm_qos_request(param_class):
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Returns the aggregated value for a given PM QoS class.
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int pm_qos_request_active(handle):
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Returns if the request is still active, i.e. it has not been removed from a
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PM QoS class constraints list.
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int pm_qos_add_notifier(param_class, notifier):
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Adds a notification callback function to the PM QoS class. The callback is
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called when the aggregated value for the PM QoS class is changed.
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int pm_qos_remove_notifier(int param_class, notifier):
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Removes the notification callback function for the PM QoS class.
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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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2. PM QoS per-device latency framework
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For each device 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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Note: the aggregated target value is implemented as an atomic variable so that
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reading the aggregated value does not require any locking mechanism.
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From kernel mode the use of this interface is the following:
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int dev_pm_qos_add_request(device, handle, value):
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Will insert an element into the list for that identified device 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 dev_pm_qos need to save the handle for future use in other
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dev_pm_qos API functions.
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int dev_pm_qos_update_request(handle, new_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 trees if the
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target is changed.
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int dev_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 trees if the target was changed as a result of removing
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the request.
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s32 dev_pm_qos_read_value(device):
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Returns the aggregated value for a given device's constraints list.
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Notification mechanisms:
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The per-device PM QoS framework has 2 different and distinct notification trees:
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a per-device notification tree and a global notification tree.
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int dev_pm_qos_add_notifier(device, notifier):
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Adds a notification callback function for the device.
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The callback is called when the aggregated value of the device constraints list
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is changed.
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int dev_pm_qos_remove_notifier(device, notifier):
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Removes the notification callback function for the device.
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int dev_pm_qos_add_global_notifier(notifier):
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Adds a notification callback function in the global notification tree of the
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framework.
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The callback is called when the aggregated value for any device is changed.
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int dev_pm_qos_remove_global_notifier(notifier):
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Removes the notification callback function from the global notification tree
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of the framework.
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From user mode:
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No API for user space access to the per-device latency constraints is provided
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yet - still under discussion.
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