Embedded Android : System Development - Part II (HAL)

Android System Development
Day-2
By Team Emertxe

Table of Content
● Introduction to HAL
● Android HAL (sensors)
– Android Architecture for Sensors
– Sensors Overview
– Android Sensor Framework
– Adding support for a new sensor
● Enabling IIO for sensors in kernel
● Verifying with test application for Sensor
– Compiling sensor HAL
● Adding permissions for device

Why Android HAL?
● Linux is designed to handle only systems calls from
application
● Android frameworks communicates with the underlying
hardware through Java APIs not by system calls
● Android HAL bridges the gap between Android app
framework and Linux device driver interface
● The HAL allows you to implement functionality without
affecting or modifying higher level system
Java Apps Linux Device Driver
HAL

What is Android HAL?
● Provides API’s through which Android service can place a
request to device
● Uses functions provided by Linux system to service the
request from android framework
● A C/C++ layer with purely vendor specific implementation
● Packaged into modules (.so) file & loaded by Android
system at appropriate time
“The hardware abstraction layer (HAL) defines a
standard interface for hardware vendors to implement
and allows Android to be agnostic about lower-level
driver implementations”
“The hardware abstraction layer (HAL) defines a
standard interface for hardware vendors to implement
and allows Android to be agnostic about lower-level
driver implementations”

Android System
(Architecture)
Android HAL
Android System Services
Binder IPC Proxies
Application Framework
Linux Kernel
Audio HAL Camera HAL
Other HALs
Other HALs
Other HALs

Android HAL
(Architecture)
Android HAL
Audio HAL DRM HAL BT HAL Camera HAL
Sensor HAL Input HAL Media HAL TV HAL

Android HAL
(Architecture)
● Each hardware-specific HAL interface has properties
that are defined in
hardware/libhardware/include/hardware/hardware.h
● It guarantees that HALs have a predictable structure
● Above interface allows Android system to load correct
versions of your HAL modules consistently

Android HAL
(Architecture)
● HAL interface consists of two general components
✔ Module - Android HAL - automatically loaded by the
dynamic linker (sensor.rpi3.so)
✔ Device – Device Specific HAL (provides complete
abstraction and control over device vendor) –
appropriately loaded at run time
Module 1
Device 0
Device 1

Android HAL
(Module)
● A module, stored as a shared library (.so file), represents
packaged HAL implementation
● Contains metadata such as version, name, and author of
the module, which helps Android find and load it correctly
● The “hardware.h” header file defines struct hw_module_t
● This structure represents a module & contains information
such as module version, author and name
● API are in <aosp>/hardware/libhardware/include/hardware

Android HAL
(Module...)
● In addition, hw_module_t struct contains a pointer to
another struct, hw_module_methods_t, that contains a
pointer to an "open" function for the module
● “open” function is used to initiate communication with the
hardware
● Each “hardware-specific HAL” usually extends generic
hw_module_t struct with additional information for that
specific piece of hardware

Android HAL
(Module hw_module_t)
# Member Type Description
1 tag Integer HARDWARE_MODULE_TAG
2 module_api_version Interger Module interface version (Minor +
Major)
3 hal_api_version Integer Meant to version module, module
methods and device
4 id String Ex - “DHT11”
5 name String Ex - “Temperature Sensor”
6 author String Ex - “Emertxe”
7 methods Pointer Open method
8 dso Pointer Pointer to Dynamic Shared
Objects
9 Reserved Bytes Reserved 128 bytes for future
use

Android HAL
(Module...)
● For example in the camera HAL, the camera_module_t
struct contains a hw_module_t struct along with other
camera-specific function pointers
typedef struct camera_module {
hw_module_t common;
int (*get_number_of_cameras)(void);
int (*get_camera_info)(int camera_id, struct camera_info *info);
} camera_module_t;

Android HAL
(Naming a module)
● Use HAL_MODULE_INFO_SYM name while creating module
in your HAL
● Example : Audio module
struct audio_module HAL_MODULE_INFO_SYM = {
.common = {
.tag = HARDWARE_MODULE_TAG,
.module_api_version = AUDIO_MODULE_API_VERSION_0_1,
.hal_api_version = HARDWARE_HAL_API_VERSION,
.id = AUDIO_HARDWARE_MODULE_ID,
.name = "NVIDIA Tegra Audio HAL",
.author = "The Android Open Source Project",
.methods = &hal_module_methods,
},
};

Android HAL
(Device)
● A device abstracts the actual hardware of your product
● Example:
✔ An audio module can contain a primary audio device (ear-
jack), a USB audio device, or a Bluetooth A2DP audio
device
✔ Different printer models from same company
● A device is represented by the hw_device_t structure
● APIs are in
<aosp>/hardware/libhardware_legacy/include/hardware_legacy

HAL
(Module hw_device_t)
# Member Type Description
1 tag Integer HARDWARE_DEVICE_TAG
2 version Interger Device API version
3 module Pointer Reference to module hw_module_t
4 Padding Interger Reserved 48 bytes for future use
5 close Pointer To close function

Android HAL
(Device...)
● Like a module, each type of device defines a more-detailed
version of the generic hw_device_t that contains function
pointers for specific features of the hardware
● Example : the audio_hw_device_t struct type contains
function pointers to audio device operations
struct audio_hw_device {
struct hw_device_t common;
...
uint32_t (*get_supported_devices)(const struct audio_hw_device *dev);
...
};
typedef struct audio_hw_device audio_hw_device_t;

Android HAL
(Structure)
● LDD is a HAL for Linux, therefore, Android HAL looks similar
to a Linux device driver
● Most of the Vendor specific implementations can be done
in Android HAL (rather than the driver)
● Therefore, the license difference between driver (Open
source license GPL) and HAL (Apache License) will give
more level of abstraction to vendor
● The driver triggers hardware (say sensor) and deliver the
data back to HAL which is passed back to Android
application

Sensors Overview
(What?)
● A device that responds to a physical stimulus (as heat,
light, sound, pressure, magnetism, or a particular
motion) and transmits a resulting impulse (as for
measurement or operating a control)
– Merriam Webster Dictionary

Sensors Overview
(What?)
● A sensor is a device that detects and responds to some
type of input from the physical environment. The
specific input could be light, heat, motion, moisture,
pressure, or any one of a great number of other
environmental phenomena. The output is generally a
signal that is converted to human-readable display at
the sensor location or transmitted electronically over a
network for reading or further processing
● Examples – temperature, motion, light, gravity etc..
Temperature
sensor
Light
sensor

Sensors Overview
(Why?)
● Sensors are widely used in medical, automation, mining,
automobiles, aerospace, robotics, smartphones, houses,
farming and more...
● The data is collected, processed and results are used in
important decision making and actions
Sensors
● Smartphones
● Robotics
● Medical
● Automation
● Mining
● Aerospace
● Automobiles

Sensors overview
(Android Sensor Stack)
Android HAL
Framework
Sensor
HAL
sensors.h
sensors.cpp
H/W manufacturer
AOSP
Sensors
Sensor Hub
Drivers

Sensors Overview
(Application)
● App access sensors through APIs
● App shall register a listener to a sensor
● App specifies its preferred sampling frequency and its
latency requirements
● Example :
✔ Register with accelerometer
✔ Request events at 100Hz
✔ Events to be reported with 1-second latency
● The application will receive events from the
accelerometer at a rate of at least 100Hz, and possibly
delayed up to 1 second

Sensors Overview
(Framework)
● Framework links several applications to HAL
● HAL itself is single-client
● Requests are multiplexed by framework to enable
access to sensors by many apps
● When first app registers to a sensor, the framework
sends a request to the HAL to activate the sensor
● Framework sends updated requested parameters to HAL
for additional registration requests from other apps to
same sensor
● Framework deactivates the sensor on exit of last app to
avoid unwanted power consumption

Sensors Overview
(Framework)
● Final Sampling frequency - max of all requested
sampling frequencies
● Meaning, some applications will receive events at a
frequency higher than the one they requested
● Final maximum reporting latency – min of all requested
ones
● If one app requests one sensor with a maximum
reporting latency of 0, all applications will receive the
events from this sensor in continuous mode even if
some requested the sensor with a non-zero maximum
reporting latency

Sensors Overview
(Implications of multiplexing)
● No guarantee that events won’t arrive at a faster rate
● No mechanism to send data down from the apps to
sensors or their drivers
● This ensures one app cannot modify the behaviour of
sensors and breaking other apps

Sensors Overview
(The communication)
B
i
n
d
e
r
I
P
C C/C++ processJava process
J
N
I
N
a
t
i
v
e
* JNI is located in frameworks/base/core/jni/ directory
* Native framework is located in frameworks/native/
N
a
t
i
v
e

Sensors Overview
(Types)
Motion sensors
● These sensors measure acceleration forces and
rotational forces along three axes
● This category includes accelerometers, gravity sensors,
gyroscopes, and rotational vector sensors
Environmental sensors
● These sensors measure various environmental
parameters, such as ambient air temperature and
pressure, illumination, and humidity
● This category includes barometers, photometers, and
thermometers
Position sensors
● These sensors measure the physical position of a device
● This category includes orientation sensors and
magnetometers

Sensors Overview
(Implementation)
Hardware-based
● Physical components built into a handset or tablet
● Derive their data by directly measuring specific
environmental properties such as acceleration,
geomagnetic field strength, or angular change
Software-based
● Are not physical devices, although they mimic hardware-
based sensors
● Derive their data from one or more of the hardware-
based sensors and are sometimes called virtual sensors
or synthetic sensors
● The linear acceleration sensor and the gravity sensor
are examples of software-based sensors

Sensors Overview
(Types)
Sensor Type Description Common Uses
Accelerometer Hardware Measures the acceleration force in
m/s2
that is applied to a device on all
three physical axes (x, y, and z),
including the force of gravity
Motion detection
(shake, tilt, etc.)
Ambient
Temprature
Hardware Measures the ambient room
temperature in degrees Celsius (°C).
See note below
Monitoring air
temperatures
Gravity Software or
Hardware
Measures the force of gravity in
m/s2
three physical axes (x, y, z)
Motion detection
(shake, tilt, etc.)
Gyroscope Hardware Measures a device's rate of rotation in
rad/s around each of the three physical
axes (x, y, and z)
Rotation
detection (spin,
turn, etc.)

Sensors Overview
(Types)
Light Hardware Measures the ambient light level
(illumination) in lx
Controlling screen
brightness
Linear
Acceleration
Software or
Hardware
Measures the acceleration force in
m/s2
three physical axes (x, y, and z),
excluding the force of gravity
Monitoring
acceleration along
a single axis
Magnetic Hardware Measures the ambient geomagnetic field
for all three physical axes (x, y, z) in μT
Creating a
compass
Orientation Software Measures degrees of rotation that a
device makes around all three physical
axes (x, y, z). As of API level 3 you can
obtain the inclination matrix and
rotation matrix for a device by using the
gravity sensor and the geomagnetic
field sensor in conjunction with the
getRotationMatrix() method
Determining device
position

Sensors Overview
(Types)
Pressure Hardware Measures the ambient air pressure
in hPa or mbar
Monitoring air
pressure changes
Proximity Hardware Measures the proximity of an object
in cm relative to the view screen of
a device. This sensor is typically
used to determine whether a
handset is being held up to a
person's ear
Phone position
during a call
Humidirt Hardware Measures the relative ambient
humidity in percent (%)
Monitoring
dewpoint,
absolute, and
relative humidity
Rotation
Vector
Software or
Hardware
Measures the orientation of a device
by providing the three elements of
the device's rotation vector
Motion
detection and
rotation
detection

Sensors Overview
(Supported Sensors)
Sensor Supported Sensor Supported
Accelerometer Yes Light Yes
Ambient Temprature Yes Linear Acceleration Yes
Gravity Yes Magnetic Yes
Gyroscope Yes Orientation Yes
Pressure Yes Humidirt Yes
Proximity Yes Rotation Vector Yes

Sensor Framework
● The sensor framework is part of the
android.hardware package and includes the following
classes and interfaces
✔ SensorManager
✔ Sensor
✔ SensorEvent
✔ SensorEventListener

Sensor Framework
Android Sensor Framework can be used for -
● Determine which sensors are available on a device
● Determine an individual sensor's capabilities, such as
its maximum range, manufacturer, power
requirements and resolution
● Acquire raw sensor data and define the minimum rate
at which you acquire sensor data
● Register and unregister sensor event listeners that
monitor sensor changes

Sensor HAL
(Integration)
● Step 1 – Make sure sensor is enabled in kernel
● Step 2 – Ensure it is functioning in user space
● Step 3 – Create source files, write Android.mk
● Step 4 – Compile code & copy shared library to target
● Step 5 - Test your library with Java app

Sensor HAL
(Enable DHT11 in Kernel)
● make ARCH=arm menuconfig
● Follow instructions as shown in next slides
● Or add CONFIG_DHT11=y

Sensor HAL
(Enable IIO in Kernel)

Sensor HAL
(Boot Configuration)
1.$ ARCH=arm CROSS_COMPILE=arm-linux-androideabi- make
zImage -j4
2. $ ARCH=arm CROSS_COMPILE=arm-linux-androideabi- make
dtbs -j4
● Copy “/arch/arm/boot/dts/overlays/dht11.dtbo” to
/boot/overlays folder
● Add following line in /boot/config.txt
✔ dtoverlay=dht11,gpiopin=4

● $ vim /device/brcm/rpi3/ueventd.rpi3.rc
● Add following line in the file
– /dev/iio:device0 0660 system system
● $ rm out/target/product/rpi3/ramdisk.img
● $ make -j2 bootimage
● New ramdisk will be generated in out folder
Sensor HAL
(Hands-on : Create ramdisk)

Sensor HAL
(Industrial IO deriver)
● Device path : /dev/iio:device0
● Device File path : /sys/bus/iio/devices/iio:device0
● Temperature input file – in_temp_input
● Humidity input file - in_humidityrelative_input

Sensor HAL – Hands-on
(Hands-on : test utility)
● Write Android.mk file for testing IIO DHT11 sensor
● Compile and generate testiio executable and run on
target

Sensor HAL
(Testing stand alone DHT11)
● Use testiio utility program
● Compile and use test-nusensors utility program
(hardware/libhardware/tests/nusensors/nusensors.cpp)
● Use strace to debug (su mode)
By default system FS is read only (RO). Remount file
system as RW to copy test utility in /system/bin
● $ adb shell
● $ su
● # mount -o rw, remount /system

Sensor HAL
(Hands-on : integrating 3rd
party HAL)
● Integrating open source sensor HAL
✔ Copy files in hardware/rpi/
✔ Write Android.mk
✔ Compile files and copy output in /system/lib/hw
✔ Use test-nusensors to test native library
✔ Use Android Studio to test at app level

Sensor HAL
(Sensor Module – interface functions)
# Type Name Description
1 struct common Type of hw_module_t
2 pointer get_sensor_list Function pointer
3 pointer set_operation_mode Function pointer
● Members of struct sensors_module_t are as follows
*See details in /hardware/libhardware/include/hardware/sensor.h

Sensor HAL
(Sensor Module)
1 struct common Type of hw_device_t
2 pointer activate Function pointer
3 pointer setDelay Function pointer
4 pointer poll Function pointer
● Interface for sensor that can be polled
● Used with API SENSOR_DEVICE_API_VERSION_0_1
● Members of struct sensors_poll_device_t are as follows

Sensor HAL
(Sensor Module)
1 struct common Type of hw_device_t
2 pointer activate Function pointer
3 pointer setDelay Function pointer
4 pointer poll Function pointer
5 pointer batch Function pointer
6 pointer flush Function pointer
● Interface for sensor that can be polled
● Used with API SENSOR_DEVICE_API_VERSION_1_0
● Members of struct sensors_poll_device_1_t

Sensor HAL
(Sensor Interface functions)
# function Description
1 get_sensor_list (list) Called at boot up to return
implemented sensors by HAL
2 activate (sensor, enable) To activate/deactivate sensor
3 batch (sensor, flags,
sampling period, max
report latency)
Sets a sensor’s parameters, including
sampling frequency and maximum
report latency.
4 setDelay (sensor,
sampling delay)
Deprecated, used by HAL v.01
5 flush (sensor) Flush hardware FIFO and send flush
complete event
6 poll ( ) Returns number of events or error.
The function shall never return 0.

Sensor HAL
(Sensor Event Mapping)
1 Integer version Set to sizeof sensors_event_t
2 Integer sensor Sensor handle (identifier)
3 Integer type Sensor Type
4 Integer reserved Reserved for future use
5 Integer timestamp Time is nano-seconds
6 Union Sensor members Used based on type of sensor
7 Integer flags Reserved flags (set to zero)
8 Integer reserved1[3] For future use
● Data received from h/w sensor shall be mapped to
Android sensor event (sensors_event_t)
● Members of sensors_event_t are as follows

Sensor HAL
(Sequence of calls)
● Device boot up : get_sensors_list is called
● Sensor activation : batch function will be called with the requested
parameters, followed by activate(..., enable=1)
✔ In HAL version 0_1, the order was opposite: activate was called first,
followed by set_delay
● Activated : batch function is called when requested to change characteristics
of a sensor
● flush can be called at any time, even on non-activated sensors (in which case
it must return -EINVAL)
● To deactivate sensor, 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)

Sensor HAL
(Wakeup and non-wakeup sensor)
● Wake up sensor will wake up the AP when
– Their FIFO is full
– Batch time out expires
● Non-wake up sensors waits for AP to wake up
– They overwrite the FIFO when full
● Most of the sensors can be implemented as wakeup or
non-wake up sensor
● A sensor having both wakeup as well as non-wakeup
variant shall maintain separate FIFO for each

Sensor HAL
(Dynamic sensor)
● A dynamic sensor is runtime pluggable (can be added to and removed
from the system at runtime)
● Operations such as batch, activate and setDelay to dynamic sensor
shall be treated as no-operation and return successful
● Sensor event of SENSOR_TYPE_DYNAMIC_SENSOR_META type
shall be delivered, irrespective of activation status, at connection and
disconnection of dynamic sensor
● Dynamic sensor shall use unique handle until next boot (if handle ‘H’ is
used by sensor A then same handle can’t be assigned to same sensor
A or other sensors even after disconnection of A until reboot)
● UUID field is used for identifying sensor in addition to name, vendor,
version and type
● UUID shall be unique and persistent for individual sensor unit

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