Embedded Android : System Development - Part III (Audio / Video HAL)

Team Emertxe
Android System Development
Day-3

● Android Audio HAL
– Audio Architecture
– Audio HAL interface
– Audio Policy
– Audio HAL compilation & verification
– Overview of Tinyalsa
● Android Video HAL
– Camera Architecture
– Overview of camera HAL interface
– Overview of V4L2
– Enabling V4l2 in kernel
– Camera HAL compilation and verification
Table of Content

Audio HAL
(Overview)
● Connects higher-level, audio-specific framework APIs in
android.media to the underlying audio driver and hardware
● Defines standard interface that audio services call into and that you
must implement for your audio hardware to function correctly
● Interfaces are located in hardware/libhardware/include/hardware

Audio Architecture
*source : Android documentation

Audio Architecture
Application
Application framework
Native Framework
Audio HAL
Tiny Alsa
Java
C++
C
Kernel ALSA

Audio Architecture
(Native Framework)
Application framework
Native Framework
Audio HAL
Tiny Alsa
AudioTrack AudioRecord AudioEffect
Audio Flinger
AudioPolicy
Service

Audio HAL
(Implementation steps)
● Implement interfaces described in audio.h,
audio_effect.h files
● Create an audio policy configuration file
– Describe audio topology and package the HAL
implementation into a shared library
● Configure pre-processing effects such as automatic gain
control and noise suppression

Audio HAL
(Interfaces)
● Main functions -
– hardware/libhardware/include/hardware/audio.h
– audio_stream_t
– stream_callback_t
– audio_stream_out_t
– audio_stream_in_t
– audio_hw_device_t
● Effects (downmixing, echo, or noise suppression)
– hardware/libhardware/include/hardware/audio_effect.h
– audio_effect_library_t
– struct effect_interface_s

Audio HAL
(Interfaces)
● Reference implementation -
– system/media/audio/include/system/audio.h
– device/samsung/tuna/audio

Audio HAL
(Audio policy configuration)
● Naugat introduced new XML based audio policy
– New audio policy configuration file audio_policy_configuration.xml
– Location : /system/etc
– Include build option USE_XML_AUDIO_POLICY_CONF := 1 in device
makefile
● Old policy (audio_policy.conf) is deprecated
– Location : device/<company>/<device>/audio/audio_policy.conf
– Example : device/samsung/tuna/audio/audio_policy.conf

Audio HAL
(Audio policy – non-XML sample)
global_configuration {
attached_output_devices AUDIO_DEVICE_OUT_SPEAKER
default_output_device AUDIO_DEVICE_OUT_SPEAKER
}
audio_hw_modules {
primary {
outputs {
primary {
sampling_rates 44100
channel_masks AUDIO_CHANNEL_OUT_STEREO
formats AUDIO_FORMAT_PCM_16_BIT
devices AUDIO_DEVICE_OUT_SPEAKER
flags AUDIO_OUTPUT_FLAG_PRIMARY
}
}
}
}
*frameworks/av/services/audiopolicy/audio_policy.conf

Audio HAL
(Audio policy configuration)
● XML files from other folders can be included using Xinclude element
– Example : <xi:include href=”a2dp_audio_policy_configuration.xml”/>
● Audio policy files
– A2DP: a2dp_audio_policy_configuration.xml
– Reroute submix: rsubmix_audio_policy_configuration.xml
– USB: usb_audio_policy_configuration.xml

Audio HAL
(Audio policy – XML sample)
<audioPolicyConfiguration version="1.0">
<globalConfiguration speaker_drc_enabled="true"/>
<module name="primary" halVersion="3.0">
<attachedDevices>
<item>Speaker</item>
<item>Built-In Mic</item>
</attachedDevices>
<defaultOutputDevice>Speaker</defaultOutputDevice>
</module>
<xi:include href="audio_policy_volumes.xml"/>
<xi:include href="default_volume_tables.xml"/>
</audioPolicyConfiguration>
*frameworks/av/services/audiopolicy/config/audio_policy_configuration.xml

Audio HAL
(Advantages of XML based policy)
● Audio profiles are now structured similar to HDMI Simple Audio Descriptors
and enable a different set of sampling rates/channel masks for each audio
format
● Explicit definitions of all possible connections between devices and streams
– Previously, an implicit rule made it possible to interconnect all devices attached to
the same HAL module, preventing audio policy from controlling connections
requested with audio patch APIs
– In XML format, topology description now defines connection limitations
● Support for includes avoids repeating standard A2DP, USB, or reroute submit
definitions
● Customizable volume curves
– Previously, volume tables were hard-coded
– In XML format, volume tables are described and can be customized

Audio HAL
(shared library)
● Create a device/<company>/<device>/audio directory to
contain your library's source files.
● Create an Android.mk file to build the shared library
● Make file shall contain
– LOCAL_MODULE := audio.primary.<device>
● Create device.mk (see rpi3.mk in device/brcm/rpi/ directory)

Audio HAL
(Preprocessing Effects)
● Default pre-processing effects applied for each AudioSource are specified
in the /system/etc/audio_effects.conf file
● To apply custom effects for every AudioSource, create a
/system/vendor/etc/audio_effects.conf file and specify the pre-processing
effects to turn on

Audio HAL
(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 = "Raspberry Pi Audio HW HAL",
.author = "The Android Open Source Project",
.methods = &hal_module_methods,
},
};
● Audio module shall be initialized as mentioned below
● Name shall be HAL_MODULE_INFO_SYM
● The hal_module_methods points to open function

Audio HAL
(hw_module_methods_t)
static struct hw_module_methods_t hal_module_methods = {
.open = adev_open,
};

Audio HAL
(tinyalsa)
● ALSA is an open source sound architecture for Linux
● TinyALSA is a small ALSA library aimed to be used in Android
● Source code is available in “external/tinyalsa” directory
● Utility executables
– tinyplay
– tinymix
– tinycap
– Tinypcminfo
● Shared library - libtinyalsa

TinyALSA vs ALSA
# Feature TinyALSA ALSA
1 Control Interface Partly support Full Support
2 Mixer Interface Partly support Full Support
3 PCM Interface Partly support Full Support
4 Raw MIDI Interface No Full Support
5 Sequencer Interface No Full Support
6 Timer Interface No Full Support

Audio HAL
(tinyalsa PCM APIs)
*Path: external/tinyalsa/include/tinyalsa/asoundlib.h
# API Description
1 pcm_open () Open PCM device
2 pcm_read () Read PCM audio data from PCM
3 pcm_write () Write data. This is start playback at first write.
4 pcm_close () Close driver
5 pcm_set_config () Set configuration parameters
6 pcm_get_config () Get configuration parameters
7 pcm_is_ready () Check if PCM driver is ready

Audio HAL
(tinyalsa Parameter APIs)
# API Description
1 pcm_params_get () Get parameters
2 pcm_params_get_mask () Get mask
3 pcm_params_get_min () Get min
4 pcm_params_set_min () Set Min
5 pcm_params_get_max () Get Max
6 pcm_params_set_max () Set Max
7 pcm_params_free () Free memory allocated for parameters
8 pcm_params_to_string () Human readable parameter string
9 pcm_params_format_test ()

Audio HAL
(tinyalsa MMAP APIs)
# API Description
1 pcm_mmap_write () Write to shared memory
2 pcm_mmap_read () Read from shared memory
3 pcm_mmap_begin () Request to access a portion of direct (mmap) area
4 pcm_mmap_commit () Completed the access to area requested (using
begin)
5 pcm_mmap_avail () Number of frames ready to be read (capture) /
written (playback)
8 pcm_prepare () Prepare the PCM substream to be triggerable
9 pcm_start () Start a PCM channel that doesn't transfer data
10 pcm_stop () Stop a PCM channel that doesn't transfer data
11 pcm_ioctl () ioctl function for PCM driver
12 pcm_wait () Interrupt driven API
13 pcm_get_poll_fd () Get poll file descriptor

Audio HAL
(tinyalsa Other APIs)
# API Description
1 pcm_get_error () Returns human readable reason (string) for the
last error
2 pcm_format_to_bits () Returns the sample size in bits for a PCM
format
3 pcm_get_buffer_size () buffer size that should be used for pcm_write
4 pcm_frames_to_bytes () buffer size that should be used for pcm_write
5 pcm_bytes_to_frames () buffer size that should be used for pcm_write
6 pcm_get_latency () Get the PCM latency in milliseconds
7 pcm_get_htimestamp () Returns available frames in pcm buffer to read
(in stream) or write (out stream)and
corresponding time stamp

Audio HAL
(Hands-on : configure audio)
● $ tinymix [Press Enter Key]
Mixer name: 'bcm2835 ALSA'
Number of controls: 6
ctl type num name value
0 INT 1 PCM Playback Volume 0
1 BOOL 1 PCM Playback Switch On
2 INT 1 PCM Playback Route 0
3 IEC958 1 IEC958 Playback Default unknown
4 IEC958 1 IEC958 Playback Con Mask unknown
5 IEC958 1 IEC958 Playback PCM Stream unknown

Audio HAL
(Hands-on : configure audio)
● Set volume
– $ tinymix 0 50
● Set route
– $ tinymix 2 1
● $ adb push <media-file-name> /data/
● Play media file
– $ tinyplay <media-file-name>.wav

Camera HAL
(Overview)
● Connects the higher level camera framework APIs in
android.hardware to underlying camera driver and hardware
● The camera HAL provides interfaces for use in implementing camera
pipeline components
● Refer to following files
– camera.h source file
– camera3.h source file
– camera_common.h source file

Camera Architecture
Camera API (v1/v2)
Native API (JNI) / Native Services
Native Camera Library
Camera Services
Camera HAL
Media Server Process
Java
C++
C

Camera Architecture
(APIs)
Camera API (v1/v2)
Android.hardware
.camera
Android.media
.MediaRecorder
Android.hardware
.Camera2
.CameraDevice
Android.hardware
.Camera2
.CameraManager
Android.hardware
.Camera2
.ICameraDeviceUser
Android.hardware
.Camera2
.ICameraService
ICameraService
CameraService
Camera Service
CameraDeviceClient
JNI Camera
Context
Stagefright
Native API Native Services

Camera Architecture
(Native Camera Library)
JNI Camera Context
ICameraClient
Camera
ICamera
CameraBase
ICameraService
Stagefright
CameraService
android.hardware
.Camera2
.CameraManager
android.hardware
.ICameraService

Camera Architecture
(Camera Service)
Camera
Camera Services
Camera HAL
Device v1
ICameraClient ICameraDevice
callbacks
ICameraService
Listner
CameraDevice
Client
Camera2ClientCameraClient CameraService
CameraHardware
Interface
Camera2Device Camera3Device
Camera HAL
Device v2
Camera HAL
Device v3
Camera HAL
Module
Camera APIsICamera

Camera Architecture
(Camera Services)
Camera Hardware
Interface
Camera HAL
Camera HAL
Device v1
Camera HAL
Device v2
Camera HAL
Device v3
Camera HAL
Module
Camera2Device Camera3Device Camera Service
V4L2 (Linux Kernel)

Camera HAL
(Implementation)
● HAL sits between camera driver and Android framework
● HAL interface is defined in :
– hardware/libhardware/include/hardware/camera.h
– hardware/libhardware/include/hardware/camera_common.h
● Camera_common.h defines camera_module, a standard structure to
obtain general information about the camera
● Declares a camera_device struct that in turn contains a
camera_device_ops struct with pointers to functions that implement
the HAL interface
● Camera parameters are defined in
frameworks/av/include/camera/CameraParameters.h

Camera HAL
(Creating shared library)
● Create a device/<company_name>/<device_name>/camera
directory to contain library's source files
● Create an Android.mk file to build the shared library
● Make file shall contain
– LOCAL_MODULE := camera.<device_name>
– LOCAL_MODULE_RELATIVE_PATH := hw
● Specify camera features by copying the necessary feature XML files
in frameworks/native/data/etc directory with device.mk
● See reference file “device/samsung/tuna/device.mk”

Camera HAL
(Creating shared library)
● Declare camera’s media codec, format, and resolution capabilities in
– device/<company_name>/<device_name>/media_profiles.xml
– device/<company_name>/<device_name>/media_codecs.xml XML files
● Add media_profiles.xml and media_codecs.xml in device.mk
● Add Camera app in PRODUCT_PACKAGES variable in device.mk to be part
of system image

Camera HAL
(v1, V2, V3)
● Supported camera HAL1 as many devices still rely on it
● Android camera service supports implementing both HALs (1 & 3)
● Useful when to support a less-capable front-facing camera with
camera HAL1
● Camera HAL2 is not supported as it was a temporary step on the
way to camera HAL3
● Single camera HAL module which lists multiple independent
camera devices (each may have own version number)
● Camera module 2 or newer required to support devices 2 or
newer

Camera3
● Re-designed to
– Increase the ability of app to control camera
– Be more efficient and maintainable
● Additional camera control enables
– Develop high-quality camera app
– Operate reliably across multiple products
– Use device-specific algorithms whenever possible & maximize
quality & performance
● Structures the operation modes into a single unified view

Camera3
● Single unified view results in better user
control for
– Focus and exposure
– Post-processing, such as noise reduction, contrast
and sharpening
● This simplified view makes it easier for app
developers to use the camera's various
functions

HAL operation
● Asynchronous requests for captures come from the framework
● HAL device must process requests in order
● For each request, produce output result metadata, & one or more
output image buffers
● First-in, first-out for requests & results, & for streams referenced
by subsequent requests
● Timestamps must be identical for all outputs from a given request,
so that the framework can match them together if needed
● All capture configuration and state [except for 3A (auto-exposure,
auto-white-balance, auto-focus) control routines] is encapsulated
in the requests and results

Framework operation
● Framework opens device
– camera_module_t common.open()→
● Framework checks version field & instantiates appropriate
handler for that version of camera hardware device
– camera3_device_t ops initialize()→ →
● Framework configure streams
– camera3_device_t ops configure_streams()→ →
● Framework allocate stream buffers
– camera3_device_t ops register_stream_buffers()→ →

Framework operation
● Framework requests default settings for some number of use cases with
calls to camera3_device_t ops construct_default_request_settings()→ →
● Framework construct a capture request and send it to HAL
– camera3_device_t ops process_capture_request()→ →
● HAL notifies framework of a started capture request
– camera3_callback_ops notify()→
● HAL notifies framework of a finished capture request
– camera3_callback_ops process_capture_result()→
● Framework calls close the device
– camera3_device_t common close()→ →

Camera Devices
● LEGACY : These devices expose capabilities to apps through the
Camera API2 interfaces that are approximately the same
capabilities as those exposed to apps through the Camera API1
interfaces. The legacy frameworks code conceptually translates
Camera API2 calls into Camera API1 calls; legacy devices do not
support Camera API2 features such as per-frame controls
● FULL : These devices support all of major capabilities of
Camera API2 and must use Camera HAL 3.2 or later and Android
5.0 or later
● LIMITED : These devices support some Camera API2 capabilities
(but not all) and must use Camera HAL 3.2 or later

Camera modes
● The camera 3 HAL device can implement one of two possible
operational modes
– Limited
– Full
● Full support is expected from new higher-end devices
● Limited mode has hardware requirements roughly in line with
those for a camera HAL device v1 implementation, and is
expected from older or inexpensive devices
● HAL must indicate its level of support with the
android.info.supportedHardwareLevel static metadata entry, with
0 indicating limited mode, and 1 indicating full mode support.

Camera HAL
(versions)
● Camera API1
– The app-level camera framework on Android 4.4 and earlier devices, exposed through
the android.hardware.Camera class
● Camera API2
– The app-level camera framework on Android 5.0 and later devices, exposed through
the android.hardware.camera2 package
● Camera HAL
– The camera module layer implemented by SoC vendors. The app-level public
frameworks are built on top of the camera HAL
● Camera HAL3.1
– Version of the camera device HAL released with Android 4.4
● Camera HAL3.2
– Version of the camera device HAL released with Android 5.0

Camera HAL
(Hands-on : enabling v4l2)
● $ make ARCH=arm menuconfig
● Select “Device Drivers”
● Select “Multimedia Support”
● Select “V4L platform devices”
● Select “Broadcom BM2835 MMAL camera interface driver”
● Save configuration and exit

Camera HAL
(Hands-on : enabling v4l2)
● $ ARCH=arm CROSS_COMPILE=arm-linux-androideabi- make zImage
-j4
● $ ARCH=arm CROSS_COMPILE=arm-linux-androideabi- make dtbs -j4
● Copy zImage and bcm2710-rpi-3-b.dtb in /boot folder (SD card)
● Copy vc4-kms-v3d.dtbo in /boot/overlay folder (SD card)

Enabling Camera
● Add following parameters in /boot/config.txt
– start_x=1
– gpu_mem=256 (128 or more)
● Ensure start_x.elf, fixup_x.dat files are
present in /boot folder in SD card

Camera Interface
(camera_device_ops_t)
● set_preview_window
● set_callbacks (notification and data)
● enable_msg_type
● disable_msg_type
● msg_type_enabled
● start_preview
● preview_enabled
● store_meta_data_in_buffers

Camera Interface
● start_recording
● stop_recording
● recording_enabled
● release_recording_frame
● auto_focus
● cancel_auto_focus
● take_picture
● cancel_picture

Camera Interface
● set_parameters
● get_parameters
● put_parameters (to return memory to HAL)
● send_command (to driver)
● release (hardware resources)
● dump (state of camera)

Camera Interface
(preview_stream_ops_t)
● deque_buffer
● enqueue_buffer
● cancel_buffer
● set_buffer_count
● set_buffers_geometry
● set_crop
● set_usage
● set_swap
● get_min_undequed_buffer_count
● lock_buffer
● set_timestamp

What is V4L2?
● An open source standard to capture real time
video for linux systems
● Second version of video for linux
● V4L2 is two layer driver system
● Top layer is videodev module
● Lower layer is collection of several driver modules
● V4L2 drivers are clients of videodev

V4L2
(Devices)
Device Name Minor Number Description
From To
/dev/video0 /dev/video63 0-63 Video Capturer Devices
/dev/radio0 /dev/radio63 64-127 AM/FM Radio Devices
/dev/vtx0 /dev/vtx31 192-223 Teletext Devices
/dev/vbi0 /dev/vbi15 224-239 VBI Devices

V4L2
(Video Device)
# Members Description
char name[32] Name of the device
int type Type of V4L2 device
int minor Device minor number
Pointer *fops File operations used
Function
pointer
void (*release)(struct
video_device *vfd)
Release function used by the
driver
Void pointer priv The private data

V4L2
(File operations)
# Funcrion Description
1 int (*open)() Called when a file descriptor is opened on the
device file (/dev/videoX)
2 int (*close)() Called on last close (release) of a file descriptor
3 int (*read)() Called to data (buffer) of size count (the number
of bytes of data requested)
4 int (*write)() Called to write data of size count (the number
of bytes of data to write)
5 int (*ioctl)() Called when the application calls ioctl()
6 int (*mmap)() Called when the application calls mmap()
7 int (*poll)() Called when the application calls select()

V4L2
(Property Negotiation)
● First the application asks for the possibles values
of some property
● Next chooses one of the possible values
● Next configures that value
● Finally checks right configuration of the value
● Important properties - Video Input, Video Output,
Norm (only for input devices), Modulator (only for
output devices), Input Channel, Window Size

V4L2
(Pixel format negotiation)
● Pixel format is how every pixel is stored in memory
● Application need to know this format to allow the
properly interpretation of that pixel
● There are two "families" of pixel formats RGB and
YUV
● Mostly, devices capture natively in YUV formats
● Video is converted to RGB formats for displaying in
the viewer

V4l2
(Buffers)
● V4L2_MMAP
– Memory mapping (allocated by the kernel)
● V4L2_USERPTR
– User memory (allocated by user app)
● DMABUF, read/write
– Direct Memory Access (GPU, OpenGL)
– Can handle fullHD (1080p) at 60FPS

V4L2
(Driver Compilation)
● Step 1 : Copy driver source files in following folder
– kernels/rpi/deriver/media/platform/<device>
● Step 2 : Edit Makefile to add following lines
– <driver_name>-obj := (add all file which required to build mofules with “.o”
extantion)
– obj-$(CONFIG_<unique word which easly identify and give information about
driver>) +=<driver_name>.o
● Step 3 : Add following line in Kconfig
– connfig <driver_name which show on menuconfig>
– tristate “<write the purpose of module>”
– default n (Must specify; other options y and m)
– --help--
<write brief description about module to help user selection while building the
kernel module>

V4L2
(Driver Compilation and loading)
● Step 4 : make ARCH=arm menuconfig
– Select driver with m option; save and exit
● Step 5 : Compile module from /kernel/rpi directory
– make ARCH=arm CROSSCOMPILE=<absolute path of compiler> -j4
modules
● Step 6 : copy <driver>.ko file in target board
– system/lib/modules/<kernel version>/kernel/media/v4l2-core
● Step 7 : Run following command
– insmod <driver_name>.ko (to load driver)
– rmmod <driver_name> (to remove driver)

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Embedded Android : System Development - Part III (Audio / Video HAL)

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