diff options
Diffstat (limited to 'Documentation/power')
| -rw-r--r-- | Documentation/power/pm_qos_interface.txt | 59 |
1 files changed, 50 insertions, 9 deletions
diff --git a/Documentation/power/pm_qos_interface.txt b/Documentation/power/pm_qos_interface.txt index 22cb8f51182a..ed743bbad87c 100644 --- a/Documentation/power/pm_qos_interface.txt +++ b/Documentation/power/pm_qos_interface.txt @@ -88,17 +88,19 @@ node. 2. PM QoS per-device latency and flags framework -For each device, there are two lists of PM QoS requests. One is maintained -along with the aggregated target of resume latency value and the other is for -PM QoS flags. Values are updated in response to changes of the request list. +For each device, there are three lists of PM QoS requests. Two of them are +maintained along with the aggregated targets of resume latency and active +state latency tolerance (in microseconds) and the third one is for PM QoS flags. +Values are updated in response to changes of the request list. -Target resume latency value is simply the minimum of the request values held in -the parameter list elements. The PM QoS flags aggregate value is a gather -(bitwise OR) of all list elements' values. Two device PM QoS flags are defined -currently: PM_QOS_FLAG_NO_POWER_OFF and PM_QOS_FLAG_REMOTE_WAKEUP. +The target values of resume latency and active state latency tolerance are +simply the minimum of the request values held in the parameter list elements. +The PM QoS flags aggregate value is a gather (bitwise OR) of all list elements' +values. Two device PM QoS flags are defined currently: PM_QOS_FLAG_NO_POWER_OFF +and PM_QOS_FLAG_REMOTE_WAKEUP. -Note: the aggregated target value is implemented in such a way that reading the -aggregated value does not require any locking mechanism. +Note: The aggregated target values are implemented in such a way that reading +the aggregated value does not require any locking mechanism. From kernel mode the use of this interface is the following: @@ -177,3 +179,42 @@ The callback is called when the aggregated value for any device is changed int dev_pm_qos_remove_global_notifier(notifier): Removes the notification callback function from the global notification tree of the framework. + + +Active state latency tolerance + +This device PM QoS type is used to support systems in which hardware may switch +to energy-saving operation modes on the fly. In those systems, if the operation +mode chosen by the hardware attempts to save energy in an overly aggressive way, +it may cause excess latencies to be visible to software, causing it to miss +certain protocol requirements or target frame or sample rates etc. + +If there is a latency tolerance control mechanism for a given device available +to software, the .set_latency_tolerance callback in that device's dev_pm_info +structure should be populated. The routine pointed to by it is should implement +whatever is necessary to transfer the effective requirement value to the +hardware. + +Whenever the effective latency tolerance changes for the device, its +.set_latency_tolerance() callback will be executed and the effective value will +be passed to it. If that value is negative, which means that the list of +latency tolerance requirements for the device is empty, the callback is expected +to switch the underlying hardware latency tolerance control mechanism to an +autonomous mode if available. If that value is PM_QOS_LATENCY_ANY, in turn, and +the hardware supports a special "no requirement" setting, the callback is +expected to use it. That allows software to prevent the hardware from +automatically updating the device's latency tolerance in response to its power +state changes (e.g. during transitions from D3cold to D0), which generally may +be done in the autonomous latency tolerance control mode. + +If .set_latency_tolerance() is present for the device, sysfs attribute +pm_qos_latency_tolerance_us will be present in the devivce's power directory. +Then, user space can use that attribute to specify its latency tolerance +requirement for the device, if any. Writing "any" to it means "no requirement, +but do not let the hardware control latency tolerance" and writing "auto" to it +allows the hardware to be switched to the autonomous mode if there are no other +requirements from the kernel side in the device's list. + +Kernel code can use the functions described above along with the +DEV_PM_QOS_LATENCY_TOLERANCE device PM QoS type to add, remove and update +latency tolerance requirements for devices. |