[v2,2/4] Documentation/powercap/dtpm: Add documentation for dtpm

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+==========================================
+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 userspace 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.
+
+===========
+1. 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
+
+====================
+1.1 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
+userspace a flat representation of all the devices supporting the
+power limitation without any power limitation distribution.
+
+============================
+1.2 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.
+
+================
+2. Userspace 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 dependant. Even
+   if it is not recommended for the userspace, 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.
+
+===============
+2.1 Constraints
+===============
+
+ * Constraint 0: The power limitation is immediately applied, without
+   limitation in time.
+
+=============
+3. Kernel API
+=============
+
+============
+3.1 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 a 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.
+
+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.
+
+================
+3.2 Nomenclature
+================
+
+ * dtpm_alloc() : Allocate and initialize a dtpm structure
+
+ * dtpm_register() : Add the dtpm node to the tree
+
+ * dtpm_register_parent() : Add an intermediate node
+
+ * dtpm_unregister() : Remove the dtpm node from the tree
+
+ * dtpm_update_power() : Update the power characteristics of the dtpm node