Building a kernel aside Replicant is faster to set up and faster to build since you do not need to fetch and use the huge Android build system.
Users wanting to add a driver to their kernel, or developers that want to work on kernel related areas can do that to speed up the development process. If the changes are integrated back into Replicant, they will automatically be built by the Android build system when building images.
Since you are not compiling any user space applications, you don't need the Android build system. The Linux kernel and bootloaders such as U-Boot can be built without the Android build system.
The Trisquel ARM version of gcc seem to work well. To install it run:
$ apt-get install gcc-arm-none-eabi
If you use distributions such as Parabola, this will probably not work because the arm-none-eabi-gcc is too recent for many device kernels. But there are efforts to make the kernel sources compatible with more recent compiler versions.
You can install Trisquel in a container to work around this.
This way, it will have very few CPU and memory overhead compared to a virtual machine.
It will also save disk space since you can just store the Trisquel rootfs in any directory.
Example with crespo under Trisquel¶
Getting the right parameters¶First download the following example image and its signature:
As usual, verify the signature after importing the release key:
$ gpg --armor --verify path/to/replicant-4.2-crespo.zip.asc path/to/replicant-4.2-crespo.zip
Make sure the check succeeds!
Then unpack the zip file:
$ mkdir replicant-4.2-crespo && cd replicant-4.2-crespo && unzip ../replicant-4.2-crespo.zipThat should have extracted a boot.img. We then should not forget to look at what format the boot.img is in:
$ file boot.img boot.img: Android bootimg, kernel (0x30008000), ramdisk (0x31000000), page size: 4096, cmdline (console=ttyFIQ0 no_console_suspend)
Here it says it's an "Android bootimg", so we need the following tools:
mkbootimgto pack an image
unpackbootimgto unpack an image
First, install these two tools.
Then extract the ramdisk, kernel image and parameters from the original
$ unpackbootimg -i boot.img -o boot Android magic found at: 0 BOARD_KERNEL_CMDLINE console=ttyFIQ0 no_console_suspend BOARD_KERNEL_BASE 30000000 BOARD_RAMDISK_OFFSET 01000000 BOARD_SECOND_OFFSET 00f00000 BOARD_TAGS_OFFSET 00000100 BOARD_PAGE_SIZE 4096 BOARD_SECOND_SIZE 0 BOARD_DT_SIZE 0
This will unpack the boot.img in the directory
We also check the kernel image format, since we will build that:
$ file boot/boot.img-zImage boot/boot.img-zImage: Linux kernel ARM boot executable zImage (little-endian)
If you want to be able to run "make menuconfig", install libncurses5-dev:
# apt-get install libncurses5-dev
Download the sources:
$ git clone https://git.replicant.us/replicant/kernel_samsung_crespo.git
Then in each console you build from, run:
$ export ARCH=arm $ export CROSS_COMPILE=arm-none-eabi-
Configure it for crespo:
$ make crespo_defconfig
If you want to configure it further:
$ make menuconfig
Then build a zImage:
$ make -j4 zImage
If the compilation succeeded, the image is at:
Many device-specific kernels often contains not very clean code. This is very common with high volume devices due to time to market constraints. Upstream Linux has a lot higher code quality standards, but having your patches merged there requires more time.
As a result, variations in the default kernel configuration for your device can result in build errors.
Compilation failures can also happen when you use another gcc version, like we do in this guide.
This happens frequently if you use a gcc that is more recent than your kernel.
The "not very clean" code also increases the probability of it.
If you manage to fix build failures for a kernel, feel free to submit patches.
We now create a new boot.img from the parameters and ramdisk we extracted from the default boot.img
$ mkbootimg --kernel path/to/arch/arm/boot/zImage --ramdisk boot/boot.img-ramdisk.gz --cmdline "console=ttyFIQ0 no_console_suspend" --base 0x30000000 --pagesize 4096 -o new-boot.img
Then we verify that it matches the default boot.img parameters:
$ unpackbootimg -i new-boot.img Android magic found at: 0 BOARD_KERNEL_CMDLINE console=ttyFIQ0 no_console_suspend BOARD_KERNEL_BASE 30000000 BOARD_RAMDISK_OFFSET 01000000 BOARD_SECOND_OFFSET 00f00000 BOARD_TAGS_OFFSET 00000100 BOARD_PAGE_SIZE 4096 BOARD_SECOND_SIZE 0 BOARD_DT_SIZE 0Here we check if the following parameters match:
- Kernel base addr (
- Ramdisk offset (
- Kernel tag offset (
- Page size (
- cmdline if you don't plan to change it (
Reboot the device to the bootloader, and run:
$ fastboot boot new-boot.img < waiting for device > downloading 'boot.img'... OKAY [ 0.435s] booting... OKAY [ 0.288s] finished. total time: 0.723s