typecd is a system daemon for tracking the state of various USB Type C ports and connected peripherals on a Chromebook. It interfaces with the Linux Type C connector class framework to obtain notifications about Type C events, and to pull updated information about ports and port-partners state.
The general structure of the classes is best illustrated by a few diagrams:
Daemon
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UdevMonitor ---typec- udev- events---> PortManager ------> ECUtil SessionManagerProxy
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session_manager events
All communication and event notification between the Linux Kernel Type C connector class framework and typecd occurs through the udev mechanism. This class watches for any events of the typec subsystem. Other classes (in this case, PortManager) can register Observers with UdevMonitor. When any notification occurs, UdevMonitor calls the relevant function callback from the registered Observers.
This class has basic parsing to determine which Observer should be called (is this a port/partner/cable notification? is it add/remove/change?)
This class maintains a representation of all the state of a typec port exposed by the Type C connector class via sysfs. The primary entity for PortManager is a Port.
PortManager(UdevMonitor::Observer)
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Port0 Port1 .... PortN
PortManager sub-classes UdevMonitor::Observer, and registers itself to receive typec event notifications. In turn, it routes the notifications to the relevant object (Port, Partner, Cable) that the notification affects.
This class represents a physical Type C connector on the host, along with components that are connected to it. Each Port has a sysfs path associated with it of the form /sys/class/typec/portX where all the data (including relevant PD information) is exposed by the kernel. On udev events this sysfs directory is read to update the Port's in-memory state. A Port can be detailed as follows:
Port
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(sysfs path info) Partner Cable
This class represents a device/peripheral connected to a Port. There can only be 1 partner for each Port. Each Partner has a sysfs path associated with it of the form /sys/class/typec/portX-partner where all the data (including relevant PD information) is exposed by the kernel. On udev events this sysfs directory is read to update the Partner's in-memory state.
This class also stores a list of Alternate Modes which are supported by the partner. Each Alternate mode is given an index according to the index ascribed to it by the kernel.
Partner
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(sysfs path info) PD Identity info AltMode0 AltMode1 ... AltModeN
There are getters and setters to access the PD identity information (for example, {Get,Set}ProductVDO()). There are also functions to retrieve information associated with partner altmodes, like getting a pointer to an altmode (GetAltMode()).
This class represents a cable that connects a Port to a Partner. There can only be 1 cable for each Port. Each Cable has a sysfs path associated with it of the form /sys/class/typec/portX-cable where the PD identity data is exposed by the kernel.
This class also stores a list of Alternate Modes which are supported by the cable. Each Alternate mode is given an index according to the index ascribed to it by the kernel. At present only SOP' cable plug alt modes are supported. Even though each cable plug (i.e SOP' and SOP'') has its own device and sysfs path (of the form /sys/class/typec/portX-plug.{0|1}), since the Chrome OS Embedded Controller (EC) only enumerates SOP' alt modes, we don‘t create a separate class and instead just list the Alternate Modes of SOP’ as belonging to the associated Cable.
When UdevMonitor receives an add event for a SOP' plug device, the Cable code searches through the corresponding sysfs file and adds all the alternate Modes associated with that file. It also reacts to individual SOP' plug altmode device add udev events and registers those, in case they weren‘t already registered during SOP’ plug registration.
Cable
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(sysfs path info) PD Identity info SOP' AltMode0 SOP' AltMode1 ... SOP' AltModeN
There are getters and setters to access the PD identity information (for example, {Get,Set}ProductVDO()). There are also functions to retrieve information associated with partner altmodes, like getting a pointer to an altmode (GetAltMode()).
Since there is no consistent sysfs interface to trigger alternate (or USB4) mode entry/exit, typecd uses the EC to accomplish this. PortManager possesses a pointer to an object implementing this interface. In production code, this interface is implemented by CrosECUtil, which communicates with the EC via debugd by means of D-Bus IPC. debugd in turn uses ectool to send the relevant command to the EC.
The debugd API used by CrosECUtil is protected by D-Bus policy files that only allow users of type typecd to call it.
PortManager | | CrosECUtil |---------------------> (implements ECUtil) | | | (D-Bus) | | |------------> debugd | | | (ectool) | | \|/ Chrome OS EC
For unit tests, a mock implementation of the interface is used (MockECUtil) and its behaviour can be controlled based on what is being tested.
On devices where AP-driven mode entry is supported, the alternate mode which a Type C peripheral will enter is dictated by the current session state (logged in, locked, etc.). To receive the state updates, typecd registers a listener for session_manager session event D-Bus signals using SessionManagerProxy. When these signals are received, PortManager is notified. PortManager then updates its internal state variable (which tracks session state), and depending on the session event, performs an alternate mode switch (by exiting the current mode and then running the mode entry sequence again).
PortManager (implements SessionManagerObserverInterface) /|\ | | |-------------> SessionManagerProxy /|\ | | (D-Bus) | | |---------- session_manager
For unit tests, where it's difficult to emulate the asynchronous session_manager events, we emulate the same behaviour by calling the PortManager's SessionManagerObserverInterface functions.