219 lines
9.5 KiB
ReStructuredText
219 lines
9.5 KiB
ReStructuredText
===============================
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PM Quality Of Service Interface
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===============================
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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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* CPU latency QoS.
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* The per-device PM QoS framework provides the API to manage the
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per-device latency constraints and PM QoS flags.
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The latency unit used in the PM QoS framework is the microsecond (usec).
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1. PM QoS framework
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===================
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A global list of CPU latency QoS requests is maintained along with an aggregated
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(effective) target value. The aggregated target value is updated with changes
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to the request list or elements of the list. For CPU latency QoS, the
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aggregated target value is simply the min of the request values held in the list
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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 space the use of this interface is simple:
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void cpu_latency_qos_add_request(handle, target_value):
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Will insert an element into the CPU latency QoS list with the target value.
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Upon change to this list the new target is recomputed and any registered
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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 cpu_latency_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
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value and recompute the new aggregated target, calling the notification tree
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if the target is changed.
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void cpu_latency_qos_remove_request(handle):
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Will remove the element. After removal it will update the aggregate target
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and call the notification tree if the target was changed as a result of
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removing the request.
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int cpu_latency_qos_limit():
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Returns the aggregated value for the CPU latency QoS.
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int cpu_latency_qos_request_active(handle):
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Returns if the request is still active, i.e. it has not been removed from the
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CPU latency QoS list.
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int cpu_latency_qos_add_notifier(notifier):
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Adds a notification callback function to the CPU latency QoS. The callback is
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called when the aggregated value for the CPU latency QoS is changed.
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int cpu_latency_qos_remove_notifier(notifier):
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Removes the notification callback function from the CPU latency QoS.
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From user space:
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The infrastructure exposes one device node, /dev/cpu_dma_latency, for the CPU
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latency QoS.
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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 as follows.
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To register the default PM QoS target for the CPU latency QoS, the process must
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open /dev/cpu_dma_latency.
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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, it can write a hex string for the value
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using the 10 char long format e.g. "0x12345678". This translates to a
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cpu_latency_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 and flags framework
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================================================
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For each device, there are three lists of PM QoS requests. Two of them are
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maintained along with the aggregated targets of resume latency and active
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state latency tolerance (in microseconds) and the third one is for PM QoS flags.
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Values are updated in response to changes of the request list.
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The target values of resume latency and active state latency tolerance are
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simply the minimum of the request values held in the parameter list elements.
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The PM QoS flags aggregate value is a gather (bitwise OR) of all list elements'
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values. One device PM QoS flag is defined currently: PM_QOS_FLAG_NO_POWER_OFF.
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Note: The aggregated target values are implemented in such a way that reading
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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, type, 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
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value and recompute the new aggregated target, calling the notification
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trees if the 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
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and call the notification trees if the target was changed as a result of
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removing the request.
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s32 dev_pm_qos_read_value(device, type):
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Returns the aggregated value for a given device's constraints list.
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enum pm_qos_flags_status dev_pm_qos_flags(device, mask)
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Check PM QoS flags of the given device against the given mask of flags.
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The meaning of the return values is as follows:
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PM_QOS_FLAGS_ALL:
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All flags from the mask are set
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PM_QOS_FLAGS_SOME:
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Some flags from the mask are set
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PM_QOS_FLAGS_NONE:
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No flags from the mask are set
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PM_QOS_FLAGS_UNDEFINED:
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The device's PM QoS structure has not been initialized
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or the list of requests is empty.
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int dev_pm_qos_add_ancestor_request(dev, handle, type, value)
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Add a PM QoS request for the first direct ancestor of the given device whose
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power.ignore_children flag is unset (for DEV_PM_QOS_RESUME_LATENCY requests)
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or whose power.set_latency_tolerance callback pointer is not NULL (for
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DEV_PM_QOS_LATENCY_TOLERANCE requests).
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int dev_pm_qos_expose_latency_limit(device, value)
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Add a request to the device's PM QoS list of resume latency constraints and
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create a sysfs attribute pm_qos_resume_latency_us under the device's power
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directory allowing user space to manipulate that request.
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void dev_pm_qos_hide_latency_limit(device)
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Drop the request added by dev_pm_qos_expose_latency_limit() from the device's
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PM QoS list of resume latency constraints and remove sysfs attribute
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pm_qos_resume_latency_us from the device's power directory.
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int dev_pm_qos_expose_flags(device, value)
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Add a request to the device's PM QoS list of flags and create sysfs attribute
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pm_qos_no_power_off under the device's power directory allowing user space to
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change the value of the PM_QOS_FLAG_NO_POWER_OFF flag.
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void dev_pm_qos_hide_flags(device)
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Drop the request added by dev_pm_qos_expose_flags() from the device's PM QoS
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list of flags and remove sysfs attribute pm_qos_no_power_off from the device's
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power directory.
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Notification mechanisms:
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The per-device PM QoS framework has a per-device notification tree.
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int dev_pm_qos_add_notifier(device, notifier, type):
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Adds a notification callback function for the device for a particular request
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type.
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The callback is called when the aggregated value of the device constraints
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list is changed.
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int dev_pm_qos_remove_notifier(device, notifier, type):
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Removes the notification callback function for the device.
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Active state latency tolerance
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This device PM QoS type is used to support systems in which hardware may switch
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to energy-saving operation modes on the fly. In those systems, if the operation
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mode chosen by the hardware attempts to save energy in an overly aggressive way,
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it may cause excess latencies to be visible to software, causing it to miss
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certain protocol requirements or target frame or sample rates etc.
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If there is a latency tolerance control mechanism for a given device available
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to software, the .set_latency_tolerance callback in that device's dev_pm_info
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structure should be populated. The routine pointed to by it is should implement
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whatever is necessary to transfer the effective requirement value to the
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hardware.
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Whenever the effective latency tolerance changes for the device, its
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.set_latency_tolerance() callback will be executed and the effective value will
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be passed to it. If that value is negative, which means that the list of
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latency tolerance requirements for the device is empty, the callback is expected
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to switch the underlying hardware latency tolerance control mechanism to an
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autonomous mode if available. If that value is PM_QOS_LATENCY_ANY, in turn, and
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the hardware supports a special "no requirement" setting, the callback is
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expected to use it. That allows software to prevent the hardware from
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automatically updating the device's latency tolerance in response to its power
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state changes (e.g. during transitions from D3cold to D0), which generally may
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be done in the autonomous latency tolerance control mode.
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If .set_latency_tolerance() is present for the device, sysfs attribute
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pm_qos_latency_tolerance_us will be present in the devivce's power directory.
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Then, user space can use that attribute to specify its latency tolerance
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requirement for the device, if any. Writing "any" to it means "no requirement,
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but do not let the hardware control latency tolerance" and writing "auto" to it
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allows the hardware to be switched to the autonomous mode if there are no other
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requirements from the kernel side in the device's list.
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Kernel code can use the functions described above along with the
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DEV_PM_QOS_LATENCY_TOLERANCE device PM QoS type to add, remove and update
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latency tolerance requirements for devices.
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