Re: [PATCH 2/4] Documentation/powercap/dtpm: Add documentation for dtpm

[Ram Chandrasekar <rkumbako@codeaurora.org>]

On 10/6/2020 6:20 AM, Daniel Lezcano wrote:
> The dynamic thermal and power management is a technique to dynamically
> adjust the power consumption of different devices in order to ensure a
> global thermal constraint.
> 
> An userspace daemon is usually monitoring the temperature and the
> power to take immediate action on the device.
> 
> The DTPM framework provides an unified API to userspace to act on the
> power.
> 
> Document this framework.
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
> ---
>   Documentation/power/powercap/dtpm.rst | 222 ++++++++++++++++++++++++++
>   1 file changed, 222 insertions(+)
>   create mode 100644 Documentation/power/powercap/dtpm.rst
> 
> diff --git a/Documentation/power/powercap/dtpm.rst b/Documentation/power/powercap/dtpm.rst
> new file mode 100644
> index 000000000000..ce11cf183994
> --- /dev/null
> +++ b/Documentation/power/powercap/dtpm.rst
> @@ -0,0 +1,222 @@
> +==========================================
> +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.

How is power_limit_uW different from max_power_uW?

> +
> + * 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

create a the -> create a

> +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

implements -> implement

> +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
>