Real-Time Sensing on Android

Kernel:

    This layer enables raw H/W access. Android uses linux's input sub system (evdev) for sensor devices.

• The input sub system is an abstraction layer b/w i/p devices and i/p handlers.
• The i/p devices capture physical inputs and produces i/p events.
• The i/p events then go through the i/p core and are dispatched to any subscribed handlers, which in turn make them available through the standard UNIX file interface /dev/input/eventX.
• Thus, applications can access the raw sensors data as an i/p events by calling POSIX system calls.

HAL:

    It is a user space layer that interface with the kernel.

• It polls the i/p events from kernel (/dev/input/eventX) for reading sensor data and provides a unifying H/W interfaces for other user space process.
• This layer also hides vendor specific details.
• HAL is loaded into User-Space process as a shared library.

Sensor Service:

    It Uses HAL to access raw sensor data. This layer is in fact part of a system process that starts from system boot time.

Do 2 things-

• It re formats raw H/W sensor data using application friendly data    structure.
• It fuses reading from multiple H/W sensors to generate S/W sensors data.

Sensor manager:

    Is an Android library linked to each application at run time.

• It Provides registration and de registration calls for app implemented event handlers.
• Once app registers an event handler, sensor Manger's sensor thread reads sensor data from a sensor service.

Android and Kernel source download, building and flashing

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Android Source download:
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# Create bin directory in home if not present
mkdir ~/bin
# Set path
PATH=~/bin:$PATH

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#Download the Repo tool and ensure that it is executable:
curl https://storage.googleapis.com/git-repo-downloads/repo > ~/bin/repo
chmod a+x ~/bin/repo Or chmod 0777 ~/bin/repo

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# Make working directory. Create an empty directory to hold your working files
cd ~/
mkdir WORKING_DIRECTORY
cd WORKING_DIRECTORY

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# Set Http Post Buffer size
#You may find the default post buffer of your git client is too small to push large details to Gitblit.
Sometimes this cam be observed on your client as hanging during push.
Other times it can be observed by git erroring out with a message like : error: RPC failed; result=52, HTTP code = 0
Use :git config --global http.ppostBuffer 524288000
Places to add this (In any file you can add)
1. ~/.git/config
2. /etc/gitconfig
3. ~/.gitconfig
4. ~/.config/git/config

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# Run repo init to bring down the latest version of Repo with all its most recent bug fixes.
You must specify a URL for the manifest, which specifies where the various repositories included in the Android source will be placed within your working directory.
repo init -u https://android.googlesource.com/platform/manifest

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# To check out a branch other than "master", specify it with -b:
repo init -u https://android.googlesource.com/platform/manifest -b android-5.0.0_r6

# If want to configure by your name/ID use
repo init -u https://android.googlesource.com/platform/manifest -b android-5.0.0_r6 --config-name yatendra

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# To pull down the Android source tree to your working directory from the repositories as specified in the default manifest, run
repo sync

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# Using a Local mirror
mkdir -p /usr/local/aosp/mirror
cd /usr/local/aosp/mirror
repo init -u https://android.googlesource.com/mirror/manifest --mirror
repo sync

# Once the mirror is synced, new clients can be created from it. Note that it's important to specify an absolute path:
mkdir -p /usr/local/aosp/master
cd /usr/local/aosp/master
repo init -u /usr/local/aosp/mirror/platform/manifest.git
repo sync

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# Finally, to sync a client against the server, the mirror needs to be synced against the server, then the client against the mirror:

cd /usr/local/aosp/mirror
repo sync
cd /usr/local/aosp/master
repo sync

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# Note: Copy source as backup in Source directory

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# download factory binaries for Device (Without this android will not bring up)
https://developers.google.com/android/nexus/drivers
# It will list out all the device with android versions like -
Binaries for Nexus 5 (GSM/LTE) ("hammerhead")

    Android 4.4 (KRT16M)
    Android 4.4.2 (KOT49H)
    Android 4.4.3 (KTU84M)
    Android 4.4.4 (KTU84P)
    Android 4.4.4 Release 2 (KTU84Q)
    Android 5.0 (LRX21O)
    Android 5.1.1 (LMY48B)
    
# Select android version
# Download all the links related to Devices binaries
# Extract all the .tgz files and execute them in terminal
Press Enter and space till prompt has not come
>> Type "I ACCEPT" if you agree to the terms of the license:
Type "I ACCEPT" then Enter
# It will create the device folder which is needed to bring up android for particular device

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Kernel Source download:
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#Go to site https://android.googlesource.com/kernel/msm.git. Search for particular version "android-msm-hammerhead-3.4-lollipop-mr1". Click and Download
#set enviroment variables
export ARCH=arm
export SUBARCH=arm
export CROSS_COMPILE=arm-2010q1/bin/arm-none-linux-gnueabi-

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#Set Path for Kernel and Android
#Go to android source folder and export path
export ANDROID_PATH=$PWD
#Go to android Kernel folder and export path
export KERNEL_PATH=$PWD

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Building Sources:
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# JDK version should be Proper.
echo $JAVA_HOME
echo $ANDROID_JAVA_TOOLCHAIN

# Check the links manaully
ls -l /etc/alternatives/ja*
# If not set properly you can set JAVA Path as
export JAVA_HOME=/usr/lib/jvm/jdk1.7.0_72/
Or
Use update-alternatives for all ja* e.g update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.7.0_72/bin/java

After Setting java version just do lunch for android source

# Your source is downloaded and ready for build
Build Kernel:
# go to source directory
cd ${KERNEL_PATH}
make hammerhead_defconfig
make zImage-dtb

Copy zImage-dtb to particular device folder (Here lge/hammerhead-kernel for Nexus5)
cp ${KERNEL_PATH}/arch/arm/boot/zImage-dtb  ${ANDROID_PATH}/device/lge/hammerhead-kernel/

Build Android:
# go to source directory
cd ${ANDROID_PATH}
source build/envsetup.sh
lunch aosp_hammerhead-userdebug
> for Nexus 5
make -j4

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Flashing device:
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cd ${ANDROID_PRODUCT_OUT}
adb reboot bootloader
sudo fastboot flash boot boot.img
sudo fastboot flash system system.img
sudo fastboot flash userdata userdata.img
sudo fastboot reboot