The vehicle HAL 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 vehicle HAL interface is based on accessing (read, write, subscribe) a property, which is an abstraction for a specific function.
The vehicle HAL uses the following interfaces:
vehicle_prop_config_t const *(*list_properties)(..., int* num_properties)
List configuration of all properties supported by the vehicle HAL. 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.
- Vehicle HAL 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 vehicle HAL 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 vehicle HAL level error or error per property. Global error causes HAL restart, which can lead to restarting other components (including applications).
Properties can be read-only, write-only (used to pass information to vehicle
HAL 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
FLOAT(and array), string, bytes.
- Zoned type has zone in addition to value.
The vehicle HAL defines three zone types:
Zone based on rows.
Zone based on seats.
Zone based on windows.
Each zoned property should use pre-defined zone type. If necessary, you can use a custom zone type for each property (for details, see Handling custom properties).
Configuring a property
vehicle_prop_config_t to provide configuration information
for each property. Information includes:
access(r, w, rw)
change_mode(represents how property is monitored: on change vs continuous)
min_value(int32, float, int64),
max_value(int32, float, int64)
prop(Property ID, int)
zone_flags(represents supported zones as bit flags)
In addition, some properties have specific configuration flags to represent capability.
Handling zone properties
A zoned property is equivalent to a collection of multiple properties where each sub property is accessible by specified zone value.
getcall for zoned property always includes zone in request, so only the current value for the requested zone should be returned.
setcall for zoned property always includes zone in request, so only the requested zone should be changed.
subscribecall includes flags of all zones subscribed. Events from un-subscribed zones should not be reported.
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 special value
rather than returning an error.
In addition, some properties (such as HVAC temperature) can have a value to indicate it is in max power mode rather than in specific temperature value. In such cases, use special values to represent such state.
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. The exception is a property
with change mode of
VEHICLE_PROP_CHANGE_MODE_ON_SET. This property
notifies change only when it is set by external component outside Android (for
example, clock properties such as
Example: set HVAC Temperature
Handling custom properties
To support partner-specific needs, the vehicle HAL allows custom properties that are restricted to system apps. Use the following guidelines when working with custom properties:
- Key should be in [
VEHICLE_PROPERTY_CUSTOM_START, VEHICLE_PROPERTY_CUSTOM_END] range. Other ranges are reserved for future extension; using such ranges can cause conflicts in future Android releases.
- Use only defined
value_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.
- Add access policy into
vendor_vns_policy.xml(otherwise, all access will be rejected).
- Access via
VendorExtensionManager(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 vehicle HAL 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 full list of HVAC properties, search for
Handling sensor properties
Vehicle HAL 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. Represents allowed operations in the current driving state. This information is used to block unsafe applications while driving.
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 Mapping Service (VMS)
Android 8.1 introduces support for the Vehicle Mapping Service (VMS), a new vehicle property intended for use only in Android Automotive implementations. VMS can be used for in-vehicle mapping services that support common vehicle features such as advanced driver assistance (ADAS).
The VMS property is a complex data type that expresses map data exchanged between an Android Automotive implementation and the underlying vehicle hardware responsible for managing onboard mapping data.
AOSP does not contain a default implementation of VMS, which currently exists only as the property and related enumeration. Message types and data structures are described in Vehicle HAL 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.