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https://github.com/openbmc/phosphor-pid-control

OpenBMC PID-based Thermal Control Daemon
https://github.com/openbmc/phosphor-pid-control

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OpenBMC PID-based Thermal Control Daemon

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README 

        
# phosphor-pid-control



This is a daemon running within the OpenBMC environment. It uses a well-defined

configuration file to control the temperature of the tray components to keep

them within operating conditions. It may require coordination with host-side

tooling and OpenBMC.



## Overview



The BMC will run a daemon that controls the fans by pre-defined zones. The

application will use thermal control, such that each defined zone is kept within

a range and adjusted based on thermal information provided from locally readable

sensors as well as host-provided information over an IPMI OEM command.



A system (or tray) will be broken out into one or more zones, specified via

configuration files or dbus. Each zone will contain at least one fan and at

least one temperature sensor and some device margins. The sensor data can be

provided via sysfs, dbus, or through IPMI. In either case, default margins

should be provided in case of failure or other unknown situation.



The system will run a control loop for each zone with the attempt to maintain

the temperature within that zone within the margin for the devices specified.



## Configuring



[How to configure phosphor-pid-control](configure.md)



## Detailed Design



The software will run as a multi-threaded daemon that runs a control loop for

each zone, and has a master thread which listens for dbus messages. Each zone

will require at least one fan that it exclusively controls, however, zones can

share temperature sensors.



![Swampd Architecture](swampd_diagram.png "Swampd Architecture")



In this figure the communications channels between swampd and ipmid and

phosphor-hwmon are laid out.



### Zone Specification



A configuration file will need to exist for each board.



Each zone must have at least one fan that it exclusively controls. Each zone

must have at least one temperature sensor, but they may be shared.



The internal thermometers specified can be read via sysfs or dbus.



### Chassis Delta



Due to data center requirements, the delta between the outgoing air temperature

and the environmental air temperature must be no greater than 15C.



### IPMI Access to Phosphor-pid-control



[OEM-IPMI Definitions](ipmi.md)



#### Set Sensor Value



Tools needs to update the thermal controller with information not necessarily

available to the BMC. This will comprise of a list of temperature (or margin?)

sensors that are updated by the set sensor command. Because they don't represent

real sensors in the system, the set sensor handler can simply broadcast the

update as a properties update on dbus when it receives the command over IPMI.



#### Set Fan PWM



A tool can override a specific fan's PWM when we implement the set sensor IPMI

command pathway.



#### Get Fan Tach



A tool can read fan_tach through the normal IPMI interface presently exported

for sensors.



### Sensor Update Loop



The plan is to listen for fan_tach updates for each fan in a background thread.

This will receive an update from phosphor-hwmon each time it updates any sensor

it cares about.



By default phosphor-hwmon reads each sensor in turn and then sleeps for 1

second. We'll be updating phosphor-hwmon to sleep for a shorter period -- how

short though is still TBD. We'll also be updating phosphor-hwmon to support pwm

as a target.



### Thermal Control Loops



Each zone will require a control loop that monitors the associated thermals and

controls the fan(s). The EC PID loop is designed to hit the fans 10 times per

second to drive them to the desired value and read the sensors once per second.

We'll be receiving sensor updates with such regularly, however, at present it

takes ~0.13s to read all 8 fans. Which can't be read constantly without bringing

the system to its knees -- in that all CPU cycles would be spent reading the

fans. TBD on how frequently we'll be reading the fan sensors and the impact this

will have.



### Main Thread



The main thread will manage the other threads, and process the initial

configuration files. It will also register a dbus handler for the OEM message.



### Enabling Logging & Tuning



By default, swampd won't log information. To enable logging pass "-l" on the

command line with a parameter that is the folder into which to write the logs.



The log files will be named `{folderpath}/zone_{zoneid}.log`.



To enable tuning, pass "-t" on the command line.



See [Logging & Tuning](tuning.md) for more information.



## Code Layout



The code is broken out into modules as follows:



- `dbus` - Any read or write interface that uses dbus primarily.

- `experiments` - Small execution paths that allow for fan examination including

  how quickly fans respond to changes.

- `ipmi` - Manual control for any zone is handled by receiving an IPMI message.

  This holds the ipmid provider for receiving those messages and sending them

  onto swampd.

- `notimpl` - These are read-only and write-only interface implementations that

  can be dropped into a pluggable sensor to make it complete.

- `pid` - This contains all the PID associated code, including the zone

  definition, controller definition, and the PID computational code.

- `scripts` - This contains the scripts that convert YAML into C++.

- `sensors` - This contains a couple of sensor types including the pluggable

  sensor's definition. It also holds the sensor manager.

- `sysfs` - This contains code that reads from or writes to sysfs.

- `threads` - Most of swampd's threads run in this method where there's just a

  dbus bus that we manage.



## Example System Configurations



### Two Margin Sensors Into Three Fans (Non-Step PID)



A single zone system where multiple margin thermal sensors are fed into one PID

that generates the output RPM for a set of fans controlled by one PID.



margin sensors as input to thermal pid



```text

fleeting0+---->+-------+    +-------+     Thermal PID sampled

               |  min()+--->+  PID  |     slower rate.

fleeting1+---->+-------+    +---+---+

                                |

                                |

                                | RPM setpoint

                    Current RPM v

                             +--+-----+

  The Fan PID        fan0+--->        |  New PWM  +-->fan0

  samples at a               |        |           |

  faster rate        fan1+--->  PID   +---------->--->fan1

  speeding up the            |        |           |

  fans.              fan2+--->        |           +-->fan2

                       ^     +--------+                +

                       |                               |

                       +-------------------------------+

                              RPM updated by PWM.

```