The Vehicle Hardware Abstraction Layer (VHAL) interface defines the properties OEMs can implement and contains property metadata (for example, whether the property is an int and which change modes are allowed). The VHAL interface is based on accessing (read, write, subscribe) a property, which is an abstraction for a specific function.
The VHAL uses the following interfaces:
vehicle_prop_config_t const *(*list_properties)(..., int* num_properties)
List configuration of all properties supported by the VHAL. Only supported properties are used by vehicle network service.
(*get)(..., vehicle_prop_value_t *data)
Read the current value of the property. For zoned property, each zone may have different value.
(*set)(..., const vehicle_prop_value_t *data)
Write a value to property. Result of write is defined per property.
(*subscribe)(..., int32_t prop, float sample_rate, int32_t zones)
- Start monitoring a property value change. For zoned property, subscription applies to requested zones. Zones = 0 is used to request all zones supported.
- VHAL should call separate callback when the property's value changes (=on change) or in const interval (=continuous type).
(*release_memory_from_get)(struct vehicle_hw_device* device, vehicle_prop_value_t *data)
Release memory allocated from get call.
The VHAL uses the following callback interfaces:
(*vehicle_event_callback_fn)(const vehicle_prop_value_t *event_data)
Notifies vehicle property's value change. Should be done only for subscribed properties.
(*vehicle_error_callback_fn)(int32_t error_code, int32_t property, int32_t operation)
Return global VHAL level error or error per property. Global error causes the HAL to restart, which can lead to restarting other components (including applications).
Properties can be read-only, write-only (used to pass information to VHAL
level), or read and write (support of most properties is optional). Each
property is uniquely identified by an int32 key and has a predefined type
A zoned property may have more than one value, based on the number of zones supported by the property.
The VHAL defines multiple area types:
This property is a singleton and does not have multiple areas.
Area based on windows, uses
Area based on mirrors, uses
Area based on seats, uses
Area based on doors, uses
Area based on wheels, uses
Each zoned property must use pre-defined area type. Each area type has a set of bit flags defined in an enum for the area type. For example, the SEAT area defines VehicleAreaSeat enums:
ROW_1_LEFT = 0x0001
ROW_1_CENTER = 0x0002
ROW_1_RIGHT = 0x0004
ROW_2_LEFT = 0x0010
ROW_2_CENTER = 0x0020
ROW_2_RIGHT = 0x0040
ROW_3_LEFT = 0x0100
Zoned properties are addressed via Area IDs. Each zoned property may
support one or more Area IDs. An Area ID is composed of one or more flags
from its respective enum. For example, a property using
VehicleAreaSeat might use the following Area IDs:
ROW_1_LEFT | ROW_1_RIGHT
The Area ID applies to both front seats.
Only applies to rear left seat.
Only applies to rear right seat.
Every property value comes with a
This indicates the current status for the property:
Property is available and the value is valid.
Property value is currently unavailable. This is used for transiently disabled features for a supported property.
Something is wrong with this property.
Configuring a property
vehicle_prop_config_t to provide configuration information
for each property. Information includes:
access(r, w, rw)
changeMode(represents how property is monitored: on change vs continuous)
areaConfigs(areaId, min, and max values)
configArray(additional configuration parameters)
configString(additional information passed as a string)
prop(Property ID, int)
Handling zone properties
A zoned property is equivalent to a collection of multiple properties where each sub property is accessible by specified Area ID value.
getcall for zoned property always includes the Area ID in the request, so only the current value for the requested Area ID is returned. If the property is a global, then Area ID is 0.
setcall for zoned property always includes the Area ID in the request, so only the requested Area ID is changed.
subscribecall will generate events for all Area IDs for the property.
During initialization, the value for the property may not be available yet as
the matching vehicle network message has not yet been received. In such cases,
get call should return
-EAGAIN. Some properties
(such as HVAC) have separate on/off power property. Calling
such a property (when powered off) should return a
status rather than returning an error.
Example: get HVAC Temperature
set call is an asynchronous operation involving event
notification after a requested change is made. In a typical operation, a
set call leads to making a change request across vehicle network.
When the change is performed by the electronic control unit (ECU) owning the
property, the updated value is returned through vehicle network and the vehicle
HAL sends an updated value as an event to vehicle network service (VNS).
set calls may require initial data to be ready but during
initialization, such data may not be available yet. In such cases, the
set call should return
-EAGAIN. Some properties with
separate power on /off should return
-ESHUTDOWN when the property
is powered off and set cannot be done.
set is made effective,
get does not
necessarily return the same value as what is set.
Example: set HVAC Temperature
Handling custom properties
To support partner-specific needs, the VHAL allows custom properties that are restricted to system apps. Use the following guidelines when working with custom properties:
- Property ID should be generated using the following fields:
VENDORgroup is used only for custom properties.
Select an appropriate Area Type.
Select the proper data type. BYTES type allows passing raw data, so this is enough in most cases. Sending big data frequently through custom properties can slow down the whole vehicle network access, so be careful when you add a big payload.
- Property ID
Choose a four nibble ID for the custom property.
- Access via
CarPropertyManager(for Java components) or via Vehicle Network Service API (for native). Do not modify other car APIs as it can lead to compatibility issues in the future.
Handling HVAC properties
You can use the VHAL to control HVAC by setting HVAC-related properties. Most HVAC properties are zoned properties, but a few are non-zoned (global) properties. Example properties defined include:
Set temperature per zone.
Control recirculation per zone).
For a full list of HVAC properties, search for
There are additional rules for mapping a zoned HVAC property to Area IDs when
the HVAC property uses
VehicleAreaSeat. Every available seat in
the car must be part of an Area ID in the Area ID array.
Example 1: A car has two front seats
and three back seats
(ROW_2_LEFT, ROW_2_CENTER, ROW_2_RIGHT).
There are two temperature control units: driver side and passenger side.
- A valid mapping set of Area IDs for
ROW_1_LEFT | ROW_2_LEFT
ROW_1_RIGHT | ROW_2_CENTER | ROW_2_RIGHT
- An alternative mapping for the same hardware configuration is:
ROW_1_LEFT | ROW_2_LEFT | ROW_2_CENTER
ROW_1_RIGHT | ROW_2_RIGHT
Example 2: A car has three seat rows with two seats in the front row
(ROW_1_LEFT, ROW_1_RIGHT) and three seats in the second
(ROW_2_LEFT, ROW_2_CENTER, ROW_2_RIGHT) and third rows
(ROW_3_LEFT, ROW_3_CENTER, ROW_3_RIGHT). There are three
temperature control units: driver side, passenger side, and rear. A
reasonable way to map
HVAC_TEMPERATURE_SET to Area IDs is a
three element array:
ROW_2_LEFT | ROW_2_CENTER | ROW_2_RIGHT | ROW_3_LEFT | ROW_3_CENTER | ROW_3_RIGHT
Handling sensor properties
VHAL sensor properties represent real sensor data or policy information such as driving status. Some sensor information (such as driving status and day/night mode) is accessible by any app without restriction as the data is mandatory to build a safe vehicle application. Other sensor information (such as vehicle speed) is more sensitive and requires specific permissions that users can manage.
Supported sensor properties include:
Should support. Determines day/night mode of display.
Gear selected by driver vs. actual gear.
Vehicle speed. Protected with permission.
Current odometer reading. Protected with permission.
Current fuel level in %.
Fuel level is low or not (boolean).
Vehicle Map Service
The Vehicle Map Service (VMS) provides a mechanism to exchange map data between clients through a pub/sub interface to support common vehicle features such as advanced driver assistance (ADAS). Clients may include vehicle systems interfacing through the VMS property in the VHAL or privileged Android applications. Data shared on VMS are intended to be limited to map data for use by vehicle systems and supporting apps.
VMS is intended for use only in Android Automotive implementations; AOSP does not contain default clients that publish or subscribe to VMS.
For the VMS property in the VHAL, the message types and data structures are described in VHAL 2.0 in the VmsMessageType enum, which lists the types of supported VMS messages. This enum is used as the first integer in the vehicle property integers array and determines how the rest of the message is decoded.