Servod FAQ

How do I turn off servod?


  • servodutil -p $PORT stop
  • <unplug the servo device>
  • <Ctrl-C> also still works

the longer read

There are currently two mechanisms to help with servod management: ServoScratch (through servodutil) and ServoDeviceWatchdog.

ServoScratch leaves information about each servod instance around (e.g. port, pid). This information can be used to turn off servod.

ServoDeviceWatchdog regularly polls to ensure all devices (i.e. servos) that the system started with are still attached. If not, it will gracefully terminate servod.

Where is the code logic for the control |control|?


Look for the control name in data/ to see its configuration. Look for the python module inside drv/ named the same as the controls |drv| param. In there look for either set()/get() method, or if |subtype| defined in params for _Set_|subtype|/_Get_|subtype|.

the longer read

As you might have read in the file, servo control routing uses the |drv| and |cmd| and |subtype| arguments inside a control's params to determine where to look for the control.

The |drv| param describes what python module from the drv/ directory houses the controls drv class. The class name is obtained by converting the drv name into camel case.

If no subtype is defined then the drv class' (or a parent class') get() or set() method will be invoked.

If a subtype is defined, then a method called _Get_|subtype| or _Set_|subtype| will be called.

The |cmd| param in params defines if a param is set or get. If it is missing, then the params will be used for both set and get.

What is the interface object passed into these drivers?


It‘s a servo_interface or the Servod instance (if interface==“servo”). The control’s config defines what interface to use. That number is the index into the interface list for the servo device you're using (v2, micro, ccd, etc).

Common interfaces are:

  • 2: DUT i2c for INAs
  • 8: AP console
  • 9: cr50 console
  • 10: EC console

How do I reroute/overwrite a control for a board?


If you want control |control_name| to route to |real_control_name| for a specific board or servo device, you can creat a control with a tag which will remap the control defined in the tag to the control defined in the name. See this example.

  <doc>using real_control_name for control_name</doc>

This ensures that whenever control_name is called, real_control_name gets actually executed.

If you want to overwrite |control_name| for a board, just write a control named the same in the board overlay and make sure that clobber_ok="" inside its params. This ensures it will clobber a previous definition of the control if it exists.

Board overlays are some of the last configs to be pulled in, so you redefine there.

How do I add a new control?


Add your control to the an existing configuration file in data/ or create a new one. Use an existing driver in drv/ or create a new one. In your control be sure to set |drv| and |subtype| so that the method & driver you intend to call get called.

the longer read

Reading the README will help a lot with this.

Adding a new control involves adding a new configuration outlining what the control does and adding driver support to execute the control.

The first decision to make is if the new functionality should extend an existing configuration or not. Creating a new configuration means having to ensure it's included in all the right places, which might be more appropiate for niche controls.

The next decision to make is whether to extend an existing driver, or implement a new driver. When implementing a new driver, ensure that its added to drv/ as otherwise servod won't be able to find it.

Once a driver with the appropiate method and the control exist, you can test out how you want to manage information. Hard-coding aspects into the driver makes configuration files simpler as the params attribute needs fewer entries. However it makes the driver less flexible, as that control's params now cannot be overwritten on a per-board/servo device basis.

How do I share data between methods in a driver?


Use class variables and dictionaries inside your driver to ensure that data can be shared between different methods.

the longer read

As you might have read in the README each servod control gets its own instance of the driver class, with its parameters passed to that instance's constructor. So in order to share data across methods (to cache results, or build out more complex functionality) the simplest way is to use class variables inside the driver to share the information.

Keep in mind that while class variables in python are accessible through self.|varname|, assigning anything new to self.|varname| does not assign to the class variable but rather creates a new instance variable that masks the class variable. Either explictly call the class variable, or use a mutable object as your class variable (list, dict).

How do I get started with servo scripting/automation?


You can either import servo.client if hdctools is installed when executing these automations (a moblab, a chroot etc) and then just use the Sclient class to create a proxy and send controls/use servod. Example code using sclient.

Alternatively, you can create your own XMLRPC proxy and execute the exposed rpc functions (the Servod class' methods) through that proxy. Example code using proxy.