213 строки
6.4 KiB
ReStructuredText
213 строки
6.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
==========================================
|
|
Dynamic Thermal Power Management framework
|
|
==========================================
|
|
|
|
On the embedded world, the complexity of the SoC leads to an
|
|
increasing number of hotspots which need to be monitored and mitigated
|
|
as a whole in order to prevent the temperature to go above the
|
|
normative and legally stated 'skin temperature'.
|
|
|
|
Another aspect is to sustain the performance for a given power budget,
|
|
for example virtual reality where the user can feel dizziness if the
|
|
performance is capped while a big CPU is processing something else. Or
|
|
reduce the battery charging because the dissipated power is too high
|
|
compared with the power consumed by other devices.
|
|
|
|
The user space is the most adequate place to dynamically act on the
|
|
different devices by limiting their power given an application
|
|
profile: it has the knowledge of the platform.
|
|
|
|
The Dynamic Thermal Power Management (DTPM) is a technique acting on
|
|
the device power by limiting and/or balancing a power budget among
|
|
different devices.
|
|
|
|
The DTPM framework provides an unified interface to act on the
|
|
device power.
|
|
|
|
Overview
|
|
========
|
|
|
|
The DTPM framework relies on the powercap framework to create the
|
|
powercap entries in the sysfs directory and implement the backend
|
|
driver to do the connection with the power manageable device.
|
|
|
|
The DTPM is a tree representation describing the power constraints
|
|
shared between devices, not their physical positions.
|
|
|
|
The nodes of the tree are a virtual description aggregating the power
|
|
characteristics of the children nodes and their power limitations.
|
|
|
|
The leaves of the tree are the real power manageable devices.
|
|
|
|
For instance::
|
|
|
|
SoC
|
|
|
|
|
`-- pkg
|
|
|
|
|
|-- pd0 (cpu0-3)
|
|
|
|
|
`-- pd1 (cpu4-5)
|
|
|
|
The pkg power will be the sum of pd0 and pd1 power numbers::
|
|
|
|
SoC (400mW - 3100mW)
|
|
|
|
|
`-- pkg (400mW - 3100mW)
|
|
|
|
|
|-- pd0 (100mW - 700mW)
|
|
|
|
|
`-- pd1 (300mW - 2400mW)
|
|
|
|
When the nodes are inserted in the tree, their power characteristics are propagated to the parents::
|
|
|
|
SoC (600mW - 5900mW)
|
|
|
|
|
|-- pkg (400mW - 3100mW)
|
|
| |
|
|
| |-- pd0 (100mW - 700mW)
|
|
| |
|
|
| `-- pd1 (300mW - 2400mW)
|
|
|
|
|
`-- pd2 (200mW - 2800mW)
|
|
|
|
Each node have a weight on a 2^10 basis reflecting the percentage of power consumption along the siblings::
|
|
|
|
SoC (w=1024)
|
|
|
|
|
|-- pkg (w=538)
|
|
| |
|
|
| |-- pd0 (w=231)
|
|
| |
|
|
| `-- pd1 (w=794)
|
|
|
|
|
`-- pd2 (w=486)
|
|
|
|
Note the sum of weights at the same level are equal to 1024.
|
|
|
|
When a power limitation is applied to a node, then it is distributed along the children given their weights. For example, if we set a power limitation of 3200mW at the 'SoC' root node, the resulting tree will be::
|
|
|
|
SoC (w=1024) <--- power_limit = 3200mW
|
|
|
|
|
|-- pkg (w=538) --> power_limit = 1681mW
|
|
| |
|
|
| |-- pd0 (w=231) --> power_limit = 378mW
|
|
| |
|
|
| `-- pd1 (w=794) --> power_limit = 1303mW
|
|
|
|
|
`-- pd2 (w=486) --> power_limit = 1519mW
|
|
|
|
|
|
Flat description
|
|
----------------
|
|
|
|
A root node is created and it is the parent of all the nodes. This
|
|
description is the simplest one and it is supposed to give to user
|
|
space a flat representation of all the devices supporting the power
|
|
limitation without any power limitation distribution.
|
|
|
|
Hierarchical description
|
|
------------------------
|
|
|
|
The different devices supporting the power limitation are represented
|
|
hierarchically. There is one root node, all intermediate nodes are
|
|
grouping the child nodes which can be intermediate nodes also or real
|
|
devices.
|
|
|
|
The intermediate nodes aggregate the power information and allows to
|
|
set the power limit given the weight of the nodes.
|
|
|
|
User space API
|
|
==============
|
|
|
|
As stated in the overview, the DTPM framework is built on top of the
|
|
powercap framework. Thus the sysfs interface is the same, please refer
|
|
to the powercap documentation for further details.
|
|
|
|
* power_uw: Instantaneous power consumption. If the node is an
|
|
intermediate node, then the power consumption will be the sum of all
|
|
children power consumption.
|
|
|
|
* max_power_range_uw: The power range resulting of the maximum power
|
|
minus the minimum power.
|
|
|
|
* name: The name of the node. This is implementation dependent. Even
|
|
if it is not recommended for the user space, several nodes can have
|
|
the same name.
|
|
|
|
* constraint_X_name: The name of the constraint.
|
|
|
|
* constraint_X_max_power_uw: The maximum power limit to be applicable
|
|
to the node.
|
|
|
|
* constraint_X_power_limit_uw: The power limit to be applied to the
|
|
node. If the value contained in constraint_X_max_power_uw is set,
|
|
the constraint will be removed.
|
|
|
|
* constraint_X_time_window_us: The meaning of this file will depend
|
|
on the constraint number.
|
|
|
|
Constraints
|
|
-----------
|
|
|
|
* Constraint 0: The power limitation is immediately applied, without
|
|
limitation in time.
|
|
|
|
Kernel API
|
|
==========
|
|
|
|
Overview
|
|
--------
|
|
|
|
The DTPM framework has no power limiting backend support. It is
|
|
generic and provides a set of API to let the different drivers to
|
|
implement the backend part for the power limitation and create the
|
|
power constraints tree.
|
|
|
|
It is up to the platform to provide the initialization function to
|
|
allocate and link the different nodes of the tree.
|
|
|
|
A special macro has the role of declaring a node and the corresponding
|
|
initialization function via a description structure. This one contains
|
|
an optional parent field allowing to hook different devices to an
|
|
already existing tree at boot time.
|
|
|
|
For instance::
|
|
|
|
struct dtpm_descr my_descr = {
|
|
.name = "my_name",
|
|
.init = my_init_func,
|
|
};
|
|
|
|
DTPM_DECLARE(my_descr);
|
|
|
|
The nodes of the DTPM tree are described with dtpm structure. The
|
|
steps to add a new power limitable device is done in three steps:
|
|
|
|
* Allocate the dtpm node
|
|
* Set the power number of the dtpm node
|
|
* Register the dtpm node
|
|
|
|
The registration of the dtpm node is done with the powercap
|
|
ops. Basically, it must implements the callbacks to get and set the
|
|
power and the limit.
|
|
|
|
Alternatively, if the node to be inserted is an intermediate one, then
|
|
a simple function to insert it as a future parent is available.
|
|
|
|
If a device has its power characteristics changing, then the tree must
|
|
be updated with the new power numbers and weights.
|
|
|
|
Nomenclature
|
|
------------
|
|
|
|
* dtpm_alloc() : Allocate and initialize a dtpm structure
|
|
|
|
* dtpm_register() : Add the dtpm node to the tree
|
|
|
|
* dtpm_unregister() : Remove the dtpm node from the tree
|
|
|
|
* dtpm_update_power() : Update the power characteristics of the dtpm node
|