Transcript Android on Beagleboard

Porting Android to
Beagleboard
Anish Dhanekula
Karl Arndt
What is Android?
• Linux-based Operating System for Mobile
Devices
• Developed by Android, Inc.
– Acquired by Google
• Open-Source, under Apache License
– Proprietary extensions developed by vendors
don’t have to be submitted back to opensource commnunity
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What is Android?
• Modified Linux Kernel
– Éclair (Android v2.0/2.1) based on 2.6.29
• Filesystem
– Uses the Bionic C Library instead of glibc
– Dalvik Java Virtual Machine
• Is it really Linux?
– Sort of… Definitely Linux kernel, but not a
traditional userspace
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What is Android?
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What’s a Beagleboard?
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What’s a Beagleboard?
• OMAP 3530
– TI SoC with ARM Cortex A8 and 64x DSP
• SDRAM and Flash on-package with
Package-on-Package (PoP)
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What’s a Beagleboard?
• Peripherals
– SD/MMC Socket for Storage
– DVI-D and S-Video
– Stereo In and Out
– USB
– Serial
• Bootloader
– U-Boot comes loaded on the PoP Flash
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Porting Process
• Get Android Source
– Kernel and Filesystem
• Prove it can work
• Modify Kernel files as necessary to work with
Beagleboard
• Compile
• Write to SD Card
• Try to Boot
• Doesn’t work – pray that your Beagleboard gets stolen
while you grab dinner
• Fix Kernel Boot Errors
• Repeat
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‘Git’ing Android Source
• Kernel – android.git.kernel.org
– OMAP tree
• Filesystem – source.android.com
– We used a prebuilt filesystem known to work
on Beagleboard in order to focus on the
kernel
• Git
– Source management tool
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Storage Format
• Kernel and Filesystem need to be stored
in a particular way on the SD Flash
• Two Partitions
– FAT32 Boot Partition
• Holds U-Boot update and compressed Kernel
Image (uImage)
– EXT3 Filesystem Partition
• Holds the complete root filesystem
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Early Boot Process
• OMAP PoP Flash comes with U-Boot, but
needs to be replaced on boot
– The PoP Flash U-Boot knows to attempt to
boot from the SD Flash first, and when it does
it finds the updated U-Boot
• Environment variables must be set to boot
properly
– setenv bootcmd ‘mmc init; fatload mmc 0 84000000 uImage; bootm 84000000’
– setenv bootargs 'mem=128M androidboot.console=ttyS2 console=tty0
console=ttyS2,115200n8 root=/dev/mmcblk0p2 rw init=/init
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Prove it can Work
• Rowboat
– Someone has already ported Android to Beagleboard
– Confirmed that this distribution works on our board
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Compiling the Kernel
• Since the kernel is being compiled on an x86
host for an ARM target, cross-compiling is
necessary
• The arm-eabi toolchain is used for this
– Included in the Android Filesystem Source
• make ARCH=arm CROSS_COMPILE=/prebuilt/linuxx86/toolchain/arm-eabi-4.4.0/bin/arm-eabi uImage –j4
• uImage is the compressed Kernel Image
• -j4 tells Make it can use 4 threads (two per CPU
on a dual-core machine
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Modifying Kernel Source
• Create the .config file
– make defconfig
– make ARCH=arm defconfig
– make ARCH=arm omap3_beagle_defconfig
• Edit .config to include all necessary
settings and modules
– make ARCH=arm menuconfig
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Modifying Kernel Source
• menuconfig
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Modifying Kernel Source
• .config
– Compilation parameters are chosen
• Particular implementation of an architecture
– Drivers are chosen
• Must determine the correct drivers for your
SoC/Board and include them here
• Drivers can be included as built-in or modular
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Modifying Kernel Source
• .config – selecting the right drivers
– Real Time Clock
• Real Time Clock initialization was failing on kernel
boot
• Found that the Beagle uses a TWL4030 I2C RTC
Controller
• Had to enable I2C, TWL Core, and TWL4030
Drivers
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Modifying Kernel Source
• .config – selecting the right drivers
– MMC/SD
• Filesystem is stored on SD card
• Found correct driver by sorting through Angstrom
Linux running on the Beagleboard
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Modifying Kernel Source
• Correct Drivers are chosen, but Kernel
won’t boot…
• Need to dig into actual Kernel Source and
Boot Process
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Modifying Kernel Source
• Kernel Boot Process
– Run code from /arch/arm/mach-omap2 to
initialize devices
– Run /init/main.c to mount the root filesystem
– Once the root filesystem is mounted, run /init
(or whatever is specified to U-Boot) as the first
process
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Modifying Kernel Source
• RTC is initialized now, but Kernel Boot still
hangs
– “Waiting for root device /dev/mmcblk0p2…”
– This message was found in /init/do_mounts.c
• Called in /init/main.c
• While loop continuously polling
– Device_probe_done()
– name_to_dev_t()
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Modifying Kernel Source
• One of the routines is not completing
– Traversed name_to_dev_t() with printk
• Seems to work fine
– Difficult to traverse name_to_dev_t()
• Decided to look through initial device initialization
code in /arch/arm/mach-omap2 and SD driver in
/drivers/mmc/host/omap_hsmmc.c
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Modifying Kernel Source
• /arch/arm/mach-omap2/boardomap3beagle
– Several __init statements
• omap3_beagle_init
– Calls omap3beagle_flash_init
– This is where the devices are initialized during
bootup, but we can’t find anything wrong…
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Next Steps
• Hard-coded device names
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Things we would do differently
• Spend less time modifying the .config file
and more time on the kernel source itself
• Maybe try Network File System (NFS) to
store the Filesystem
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References
• Linux Kernel in a Nutshell
– Greg Kroah-Hartman
• Linux from Scratch
– Gerard Beekmans
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code.google.com/p/beagleboard/wiki
code.google.com/p/rowboat/wiki
source.android.com
android.git.kernel.org
www.beagleboard.com
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Questions?
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