The HAL interface, declared in sensors.h, represents the interface between the Android framework and the hardware-specific software. A HAL implementation must define each function declared in sensors.h. The main functions are:
get_sensors_list - Returns the list of all sensors. activate - Starts or stops a sensor. batch - Sets a sensor’s parameters such as sampling frequency and maximum
reporting latency. setDelay - Used only in HAL version 1.0. Sets the sampling frequency for a
given sensor. flush - Flushes the FIFO of the specified sensor and reports a flush complete
event when this is done. poll - Returns available sensor events. The implementation must be thread safe and allow these functions to be called from different threads.
The interface also defines several types used by those functions. The main types are:
sensors_module_tsensors_poll_device_tsensor_tsensors_event_tIn addition to the sections below, see sensors.h for more information on those types.
int (*get_sensors_list)(struct sensors_module_t* module, struct sensor_t const** list);
Provides the list of sensors implemented by the HAL. See sensor_t for details on how the sensors are defined.
The order in which the sensors appear in the list is the order in which the sensors will be reported to the applications. Usually, the base sensors appear first, followed by the composite sensors.
If several sensors share the same sensor type and wake-up property, the first
one in the list is called the “default” sensor. It is the one returned by
getDefaultSensor(int sensorType, bool wakeUp).
This function returns the number of sensors in the list.
int (*activate)(struct sensors_poll_device_t *dev, int sensor_handle, int enabled);
Activates or deactivates a sensor.
sensor_handle is the handle of the sensor to activate/deactivate. A sensor’s
handle is defined by the handle field of its sensor_t structure.
enabled is set to 1 to enable or 0 to disable the sensor.
One-shot sensors deactivate themselves automatically upon receiving an event,
and they must still accept to be deactivated through a call to activate(...,
enabled=0).
Non-wake-up sensors never prevent the SoC from going into suspend mode; that is, the HAL shall not hold a partial wake-lock on behalf of applications.
Wake-up sensors, when delivering events continuously, can prevent the SoC from going into suspend mode, but if no event needs to be delivered, the partial wake-lock must be released.
If enabled is 1 and the sensor is already activated, this function is a no-op
and succeeds.
If enabled is 0 and the sensor is already deactivated, this function is a no-op
and succeeds.
This function returns 0 on success and a negative error number otherwise.
int (*batch)(
struct sensors_poll_device_1* dev,
int sensor_handle,
int flags,
int64_t sampling_period_ns,
int64_t max_report_latency_ns);
Sets a sensor’s parameters, including sampling frequency and maximum report latency. This function can be called while the sensor is activated, in which case it must not cause any sensor measurements to be lost: Transitioning from one sampling rate to the other cannot cause lost events, nor can transitioning from a high maximum report latency to a low maximum report latency.
sensor_handle is the handle of the sensor to configure.
flags is currently unused.
sampling_period_ns is the sampling period at which the sensor should run, in
nanoseconds. See sampling_period_ns for more details.
max_report_latency_ns is the maximum time by which events can be delayed before
being reported through the HAL, in nanoseconds. See the max_report_latency_ns paragraph for more details.
This function returns 0 on success and a negative error number otherwise.
What the sampling_period_ns parameter means depends on the specified sensor's
reporting mode:
sampling_period_ns is the sampling rate, meaning the rate at which
events are generated. sampling_period_ns limits the sampling rate of events, meaning
events are generated no faster than every sampling_period_ns nanoseconds. There
might be periods longer than sampling_period_ns where no event is generated if
the measured values do not change for long periods. See on-change reporting mode for more
details. sampling_period_ns is ignored. It has no effect. sampling_period_ns is used
for special sensors. See Reporting modes for more information
about the impact of sampling_period_ns in the different modes.
For continuous and on-change sensors,
sampling_period_ns is less than
sensor_t.minDelay, then the HAL implementation must silently
clamp it to max(sensor_t.minDelay, 1ms). Android
does not support the generation of events at more than 1000Hz. sampling_period_ns is greater than
sensor_t.maxDelay, then the HAL
implementation must silently truncate it to sensor_t.maxDelay. Physical sensors sometimes have limitations on the rates at which they can run and the accuracy of their clocks. To account for this, we allow the actual sampling frequency to differ from the requested frequency, as long as it satisfies the requirements in the table below.
If the requested frequency is |
Then the actual frequency must be |
|---|---|
below min frequency (<1/maxDelay) |
between 90% and 110% of the min frequency |
between min and max frequency |
between 90% and 220% of the requested frequency |
above max frequency (>1/minDelay) |
between 90% and 110% of the max frequency and below 1100Hz |
Note that this contract is valid only at the HAL level, where there is always a single client. At the SDK level, applications might get different rates, due to the multiplexing happening in the Framework. See Framework for more details.
max_report_latency_ns sets the maximum time in nanoseconds, by which events can
be delayed and stored in the hardware FIFO before being reported through the
HAL while the SoC is awake.
A value of zero signifies that the events must be reported as soon as they are measured, either skipping the FIFO altogether, or emptying the FIFO as soon as one event from this sensor is present in it.
For example, an accelerometer activated at 50Hz with max_report_latency_ns=0
will trigger interrupts 50 times per second when the SoC is awake.
When max_report_latency_ns>0, sensor events do not need to be reported as soon
as they are detected. They can be temporarily stored in the hardware FIFO and
reported in batches, as long as no event is delayed by more than
max_report_latency_ns nanoseconds. That is, all events since the previous batch
are recorded and returned at once. This reduces the amount of interrupts sent
to the SoC and allows the SoC to switch to a lower power mode (idle) while the
sensor is capturing and batching data.
Each event has a timestamp associated with it. Delaying the time at which an event is reported does not impact the event timestamp. The timestamp must be accurate and correspond to the time at which the event physically happened, not the time it is being reported.
Allowing sensor events to be stored temporarily in the hardware FIFO does not
modify the behavior of poll: events from different sensors can be interleaved,
and as usual, all events from the same sensor are time-ordered.
See Batching for more details on sensor batching, including behaviors in suspend mode and out of suspend mode.
int (*setDelay)(
struct sensors_poll_device_t *dev,
int sensor_handle,
int64_t sampling_period_ns);
After HAL version 1.0, this function is deprecated and is never called.
Instead, the batch function is called to set the
sampling_period_ns parameter.
In HAL version 1.0, setDelay was used instead of batch to set sampling_period_ns.
int (*flush)(struct sensors_poll_device_1* dev, int sensor_handle);
Add a flush complete event to the end of the hardware FIFO for the specified sensor and flushes the FIFO; those events are delivered as usual (i.e.: as if the maximum reporting latency had expired) and removed from the FIFO.
The flush happens asynchronously (i.e.: this function must return immediately). If the implementation uses a single FIFO for several sensors, that FIFO is flushed and the flush complete event is added only for the specified sensor.
If the specified sensor has no FIFO (no buffering possible), or if the FIFO,
was empty at the time of the call, flush must still succeed and send a flush
complete event for that sensor. This applies to all sensors other than one-shot
sensors.
When flush is called, even if a flush event is already in the FIFO for that
sensor, an additional one must be created and added to the end of the FIFO, and
the FIFO must be flushed. The number of flush calls must be
equal to the number of flush complete events created.
flush does not apply to one-shot
sensors; if sensor_handle refers to a one-shot sensor,
flush must return -EINVAL and not generate any
flush complete metadata event.
This function returns 0 on success, -EINVAL if the specified sensor is a
one-shot sensor or wasn’t enabled, and a negative error number otherwise.
int (*poll)(struct sensors_poll_device_t *dev, sensors_event_t* data, int count);
Returns an array of sensor data by filling the data argument. This function
must block until events are available. It will return the number of events read
on success, or a negative error number in case of an error.
The number of events returned in data must be less or equal to
the count argument. This function shall never return 0 (no event).
When the device boots, get_sensors_list is called.
When a sensor gets activated, the batch function will be called with the
requested parameters, followed by activate(..., enable=1).
Note that in HAL version 1_0, the order was the opposite: activate was called
first, followed by set_delay.
When the requested characteristics of a sensor are changing while it is
activated, the batch function is called.
flush can be called at any time, even on non-activated sensors (in which case
it must return -EINVAL)
When a sensor gets deactivated, activate(..., enable=0) will be called.
In parallel to those calls, the poll function will be called repeatedly to
request data. poll can be called even when no sensors are activated.
sensors_module_t is the type used to create the Android hardware module for the
sensors. The implementation of the HAL must define an object
HAL_MODULE_INFO_SYM of this type to expose the get_sensors_list function. See the definition
of sensors_module_t in sensors.h and the
definition of hw_module_t for more information.
sensors_poll_device_1_t contains the rest of the methods defined above:
activate, batch, flush and
poll. Its common field (of type hw_device_t)
defines the version number of the HAL.
sensor_t represents an Android sensor. Here are some of its important fields:
name: A user-visible string that represents the sensor. This string often contains the part name of the underlying sensor, the type of the sensor, and whether it is a wake-up sensor. For example, “LIS2HH12 Accelerometer”, “MAX21000 Uncalibrated Gyroscope”, “BMP280 Wake-up Barometer”, “MPU6515 Game Rotation Vector”
handle: The integer used to refer to the sensor when registering to it or generating events from it.
type: The type of the sensor. See the explanation of sensor
type in What are Android sensors? for more details, and see Sensor types for official sensor types. For
non-official sensor types, type must start with SENSOR_TYPE_DEVICE_PRIVATE_BASE
stringType: The type of the sensor as a string. When the sensor has an official
type, set to SENSOR_STRING_TYPE_*. When the sensor has a manufacturer specific
type, stringType must start with the manufacturer reverse domain name. For
example, a sensor (say a unicorn detector) defined by the
Cool-product team at Fictional-Company could use
stringType=”com.fictional_company.cool_product.unicorn_detector”.
The stringType is used to uniquely identify non-official sensors types. See sensors.h for more
information on types and string types.
requiredPermission: A string representing the permission that applications must
possess to see the sensor, register to it and receive its data. An empty string
means applications do not require any permission to access this sensor. Some
sensor types like the heart rate
monitor have a mandatory requiredPermission. All sensors
providing sensitive user information (such as the heart rate) must be protected by a permission.
flags: Flags for this sensor, defining the sensor’s reporting mode and whether
the sensor is a wake-up sensor or not. For example, a one-shot wake-up sensor
will have flags = SENSOR_FLAG_ONE_SHOT_MODE | SENSOR_FLAG_WAKE_UP. The bits of
the flag that are not used in the current HAL version must be left equal to 0.
maxRange: The maximum value the sensor can report, in the same unit as the
reported values. The sensor must be able to report values without saturating
within [-maxRange; maxRange]. Note that this means the total range of the
sensor in the generic sense is 2*maxRange. When the sensor reports values over
several axes, the range applies to each axis. For example, a “+/- 2g”
accelerometer will report maxRange = 2*9.81 = 2g.
resolution: The smallest difference in value that the sensor can measure.
Usually computed based on maxRange and the number of bits in the measurement.
power: The power cost of enabling the sensor, in milliAmps. This is nearly
always more that the power consumption reported in the datasheet of the
underlying sensor. See Base
sensors != physical sensors for more details and see Power measurement process for details on
how to measure the power consumption of a sensor. If the
sensor’s power consumption depends on whether the device is moving, the power
consumption while moving is the one reported in the power field.
minDelay: For continuous sensors, the sampling period, in microseconds,
corresponding to the fastest rate the sensor supports. See sampling_period_ns for details on how this value is used. Beware that minDelay is expressed in
microseconds while sampling_period_ns is in nanoseconds. For on-change and
special reporting mode sensors, unless otherwise specified, minDelay must be 0.
For one-shot sensors, it must be -1.
maxDelay: For continuous and on-change sensors, the sampling period, in
microseconds, corresponding to the slowest rate the sensor supports. See sampling_period_ns for details on how this value is used. Beware that maxDelay is expressed in
microseconds while sampling_period_ns is in nanoseconds. For special and
one-shot sensors, maxDelay must be 0.
fifoReservedEventCount: The number of events reserved for this sensor in the
hardware FIFO. If there is a dedicated FIFO for this sensor, then
fifoReservedEventCount is the size of this dedicated FIFO. If the FIFO is
shared with other sensors, fifoReservedEventCount is the size of the part of
the FIFO that is reserved for that sensor. On most shared-FIFO systems, and on
systems that do not have a hardware FIFO this value is 0.
fifoMaxEventCount: The maximum number of events that could be stored in the
FIFOs for this sensor. This is always greater or equal to
fifoReservedEventCount. This value is used to estimate how quickly the FIFO
will get full when registering to the sensor at a specific rate, supposing no
other sensors are activated. On systems that do not have a hardware FIFO,
fifoMaxEventCount is 0. See Batching for more details.
For sensors with an official sensor type, some of the fields are overwritten by
the framework. For example, accelerometer sensors are forced to
have a continuous reporting mode, and heart rate monitors are forced to be
protected by the SENSOR_PERMISSION_BODY_SENSORS permission.
Sensor events generated by Android sensors and reported through the poll function are of type sensors_event_t. Here are some
important fields of sensors_event_t:
version: Must be sizeof(struct sensors_event_t)
sensor: The handle of the sensor that generated the event, as defined by
sensor_t.handle.
type: The sensor type of the sensor that generated the event, as defined by
sensor_t.type.
timestamp: The timestamp of the event in nanoseconds. This is the time the
event happened (a step was taken, or an accelerometer measurement was made),
not the time the event was reported. timestamp must be synchronized with the
elapsedRealtimeNano clock, and in the case of continuous sensors, the jitter
must be small. Timestamp filtering is sometimes necessary to satisfy the CDD
requirements, as using only the SoC interrupt time to set the timestamps
causes too high jitter, and using only the sensor chip time to set the
timestamps can cause de-synchronization from the
elapsedRealtimeNano clock, as the sensor clock drifts.
data and overlapping fields: The values measured by the sensor. The meaning and
units of those fields are specific to each sensor type. See sensors.h and the
definition of the different Sensor types for a
description of the data fields. For some sensors, the accuracy of the
readings is also reported as part of the data, through a status field. This
field is only piped through for those select sensor types, appearing at the SDK
layer as an accuracy value. For those sensors, the fact that the status field
must be set is mentioned in their sensor type definition.
Metadata events have the same type as normal sensor events:
sensors_event_meta_data_t = sensors_event_t. They are returned together with
other sensor events through poll. They possess the following fields:
version: Must be META_DATA_VERSION
type: Must be SENSOR_TYPE_META_DATA
sensor, reserved, and timestamp: Must be 0
meta_data.what: Contains the metadata type for this event. There is currently a
single valid metadata type: META_DATA_FLUSH_COMPLETE.
META_DATA_FLUSH_COMPLETE events represent the completion of the flush of a
sensor FIFO. When meta_data.what=META_DATA_FLUSH_COMPLETE, meta_data.sensor
must be set to the handle of the sensor that has been flushed. They are
generated when and only when flush is called on a sensor. See the section on
the flush function for more information.