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 port and port-partner state.
The general structure of the classes is best illustrated by a few diagrams:
Daemon
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UdevMonitor ---typec- udev- events---> PortManager
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
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()).