devices/automotive/properties.html

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<p>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.</p>

<h2 id=interfaces>HAL interfaces</h2>
<p>The vehicle HAL uses the following interfaces:</p>
<ul>
<li><code>vehicle_prop_config_t const *(*list_properties)(..., int*
num_properties)</code>
<br>List configuration of all properties supported by the vehicle HAL. Only
supported properties are used by vehicle network service.
</li>
<li><code>(*get)(..., vehicle_prop_value_t *data)</code>
<br>Read the current value of the property. For zoned property, each zone may
have different value.</li>
<li><code>(*set)(..., const vehicle_prop_value_t *data)</code>
<br>Write a value to property. Result of write is defined per property.</li>
<li><code>(*subscribe)(..., int32_t prop, float sample_rate, int32_t
zones)</code>
<ul>
<li>Start monitoring a property value change. For zoned property, subscription
applies to requested zones. Zones = 0 is used to request all zones supported.
</li>
<li>Vehicle HAL should call separate callback when the property's value changes
(=on change) or in const interval (=continuous type).</li>
</ul></li>
<li><code>(*release_memory_from_get)(struct vehicle_hw_device* device,
vehicle_prop_value_t *data)</code>
<br>Release memory allocated from get call.</li>
</ul>

<p>The vehicle HAL uses the following callback interfaces:</p>
<ul>
<li><code>(*vehicle_event_callback_fn)(const vehicle_prop_value_t
*event_data)</code>
<br>Notifies vehicle property's value change. Should be done only for
subscribed properties.</li>
<li><code>(*vehicle_error_callback_fn)(int32_t error_code, int32_t property,
int32_t operation)</code>
<br>Return global vehicle HAL level error or error per property. Global error
causes HAL restart, which can lead to restarting other components (including
applications).</li>
</ul>

<h2 id=properties>Vehicle properties</h2>
<p>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
(<code>value_type</code>):</p>

<ul>
<li><code>INT32</code> (and array), <code>INT64</code>, <code>BOOLEAN</code>,
<code>FLOAT</code> (and array), string, bytes.</li>
<li>Zoned type has zone in addition to value.</li>
</ul>

<h2 id-=zone_type>Zone types</h2>
<p>The vehicle HAL defines three zone types:</p>
<ul>
<li><code>vehicle_zone</code>
<br>Zone based on rows.</li>
<li><code>vehicle_seat</code>
<br>Zone based on seats.</li>
<li><code>vehicle_window</code>
<br>Zone based on windows.</li>
</ul>
<p>Each zoned property should use pre-defined zone type. If necessary, you can
use a custom zone type for each property (for details, see
<a href=#prop_custom>Handling custom properties</a>).</p>

<h2 id=prop_config>Configuring a property</h2>
<p>Use <code>vehicle_prop_config_t</code> to provide configuration information
for each property. Information includes:</p>
<ul>
<li><code>access</code> (r, w, rw)</li>
<li><code>change_mode</code> (represents how property is monitored: on change vs
continuous)</li>
<li><code>min_value</code> (int32, float, int64), <code>max_value</code> (int32,
float, int64)</li>
<li><code>min_sample_rate</code>, <code>max_sample_rate</code></li>
<li><code>permission_model</code></li>
<li><code>prop</code> (Property ID, int)</li>
<li><code>value_type</code></li>
<li><code>zone_flags</code> (represents supported zones as bit flags)</li>
</ul>
<p>In addition, some properties have specific configuration flags to represent
capability.</p>

<h2 id=zone_prop>Handling zone properties</h2>
<p>A zoned property is equivalent to a collection of multiple properties where
each sub property is accessible by specified zone value.</p>
<ul>
<li><code>get</code> call for zoned property always includes zone in request, so
only the current value for the requested zone should be returned.</li>
<li><code>set</code> call for zoned property always includes zone in request, so
only the requested zone should be changed.</li>
<li><code>subscribe</code> call includes flags of all zones subscribed. Events
from un-subscribed zones should not be reported.</li>
</ul>

<h3 id=get>Get calls</h3>
<p>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,
the <code>get</code> call should return <code>-EAGAIN</code>. Some properties
(such as HVAC) have separate on/off power property. Calling <code>get</code> for
such a property (when powered off) should return a special value
<code>(VEHICLE_INT_OUT_OF_RANGE_OFF/VEHICLE_FLOAT_OUT_OF_RANGE_OFF)</code>
rather than returning an error.</p>
<p>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.</p>
<ul>
<li>VEHICLE_INT_OUT_OF_RANGE_MAX/MIN</li>
<li>VEHICLE_FLOAT_OUT_OF_RANGE_MAX/MIN</li>
</ul>

<p>Example: get HVAC Temperature</p>
<img src="../images/vehicle_hvac_get.png" alt="Vehicle HAL get HVAC example">
<p class="img-caption"><strong>Figure 1</strong>. Get HVAC temperature (CS =
CarService, VNS = VehicleNetworkService, VHAL = Vehicle HAL)</p>

<h3 id=set>Set calls</h3>
<p>A <code>set</code> call is an asynchronous operation involving event
notification after a requested change is made. In a typical operation, a
<code>set</code> 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).</p>
<p>Some <code>set</code> calls may require initial data to be ready but during
initialization, such data may not be available yet. In such cases, the
<code>set</code> call should return <code>-EAGAIN</code>. Some properties with
separate power on /off should return <code>-ESHUTDOWN</code> when the property
is powered off and set cannot be done.</p>
<p>Until <code>set</code> is made effective, <code>get</code> does not
necessarily return the same value as what is set. The exception is a property
with change mode of <code>VEHICLE_PROP_CHANGE_MODE_ON_SET.</code> This property
notifies change only when it is set by external component outside Android (for
example, clock properties such as <code>VEHICLE_PROPERTY_UNIX_TIME</code>).</p>

<p>Example: set HVAC Temperature</p>
<img src="../images/vehicle_hvac_set.png" alt="Vehicle HAL set HVAC example">
<p class="img-caption"><strong>Figure 2</strong>. Set HVAC temperature (CD =
CarService, VNS = VehicleNetworkService, VHAL = Vehicle HAL)</p>

<h2 id=prop_custom>Handling custom properties</h2>
<p>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:</p>
<ul>
<li>Key should be in [<code>VEHICLE_PROPERTY_CUSTOM_START,
VEHICLE_PROPERTY_CUSTOM_END</code>] range. Other ranges are reserved for future
extension; using such ranges can cause conflicts in future Android releases.</li>
<li>Use only defined <code>value_type</code>. 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.</li>
<li>Add access policy into <code>vendor_vns_policy.xml</code> (otherwise, all
access will be rejected).</li>
<li>Access via <code>VendorExtensionManager</code> (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.</li>
</ul>

<h2 id=prop_hvac>Handling HVAC properties</h2>
<p>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:</p>
<ul>
<li><code>VEHICLE_PROPERTY_HVAC_TEMPERATURE_SET</code>
<br>Set temperature per zone.</li>
<li><code>VEHICLE_PROPERTY_HVAC_RECIRC_ON</code>
<br>Control recirculation per zone).</li>
</ul>
<p>For full list of HVAC properties, search for
<code>VEHICLE_PROPERTY_HVAC_*</code> in <code>vehicle.h</code>.</p>

<h2 id=prop_sensor>Handling sensor properties</h2>
<p>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.</p>
<p>Supported sensor properties include:</p>
<ul>
<li><code>DRIVING_STATUS</code>
<br>Should support. Represents allowed operations in the current driving state.
This information is used to block unsafe applications while driving.</li>
<li><code>NIGHT_MODE</code>
<br>Should support. Determines day/night mode of display.</li>
<li><code>GEAR_SELECTION/CURRENT_GEAR</code>
<br>Gear selected by driver vs. actual gear.</li>
<li><code>VEHICLE_SPEED</code>
<br>Vehicle speed. Protected with permission.</li>
<li><code>ODOMETER</code>
<br>Current odometer reading. Protected with permission.
</li>
<li><code>FUEL_LEVEL</code>
<br>Current fuel level in %.</li>
<li><code>FUEL_LEVEL_LOW</code>
<br>Fuel level is low or not (boolean).</li>
</ul>

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