[AT&T Samsung Galaxy Note II] garyd9:
gitignore

When we work with the kernel, we will generate a lot of temporary files that we don't want to put into the git system or upload to GitHub. To have "git" exclude those files from consideration, a file (or set of files) called ".gitignore" should be created that tells git what to ignore. A more complete explanation of the .gitignore file can be found here: https://help.github.com/articles/ignoring-files

Instead of having you create and edit several different ".gitignore" files, I've made a patch that you can apply directory to your kernel tree, using git, that will add those .gitignore files. The following commands, which you should copy to a terminal' window, will change into the kernel source directory, fetch the patch file (using wget), apply the patch (using git am) and then delete the patch file (as it's no longer needed.) If you have the kernel source in a different directory, you'll have to adjust one of the "cd" commands accordingly. After deleting the patch file, we push those changes to the github server. I'll explain the various git commands more in depth in a later section of this HOWTO.

cd
cd kernel-howto/linux_kernel_sgh-i317
wget https://raw.github.com/garyd9/kernel-howto-misc/master/add_gitignores.patch
git am < add_gitignores.patch
rm add_gitignores.patch
git push

Just a few more things...

The next item to mention before compiling the kernel is the kernel config file. Linux, as a kernel, can be used on literally thousands of different configurations. However, each platform might require a different kernel. How this works is that when the kernel is compiled, a kernel configuration is supplied that tells the compiling tools which drivers to include, how to configure them, what kind of processor the target platform has, etc. For now, we'll just use a default configuration file that Samsung provided with the kernel source. That configuration is called "t0att_04_defconfig" and (luckily for us) actually seems to be accurate. When building for another device, don't assume that the kernel configuration provided with the source is accurate... I've had at least one case when the configuration Samsung provided for their own device simply didn't work for that device. Later in this HOWTO, we'll be discussing the kernel configuration much more in depth, and even making changes to it.

Next, we need to talk about that prebuilt toolchain we downloaded earlier in the HOWTO. When compiling the kernel, we need to tell the system to use that very specific toolchain for doing the work. What we're actually doing is called "cross compiling", which means we're compiling for a different platform than the compiler itself is running on. In this case, we're running on an x86 (or x86_64) platform, but compiling for the "arm" platform. To tell the system which toolchain to use for cross compiling, we will use a command line parameter that defines "CROSS_COMPILE" to point to the proper toolchain files. Assuming you've been following this HOWTO (and if you haven't, you'll have to adjust this accordingly every place it occurs) that will look like this: "CROSS_COMPILE=$HOME/prebuilt/linux-x86/toolchain/arm-eabi-4.4.3/bin/arm-eabi-"

Finally, there's just one more thing before we can compile. The actual tool we use to kick off compiles is called "make." Make doesn't really know much about compiling software, but it's a great tool for figuring out when things need to get done and for doing them. By default, "make" reads a file called "Makefile." A Makefile describes how files might be dependent on each other, how to compile files and put things together. Because of the magic of a Makefile, you won't have to recompile the entire kernel each time you make a minor change. Instead, "make" will only recompile what's needed to include updates to the source.

Are you ready for actually compiling? Okay, go ahead and copy these commands. There are two commands here. The first one just prepares the kernel config file, and the second one compiles the kernel. As always, if you've made changes to paths (locations) of things, you will have to edit these commands.

cd
cd kernel-howto/linux_kernel_sgh-i317
make arch=arm t0att_04_defconfig CROSS_COMPILE=$HOME/prebuilt/linux-x86/toolchain/arm-eabi-4.4.3/bin/arm-eabi-
make CROSS_COMPILE=$HOME/prebuilt/linux-x86/toolchain/arm-eabi-4.4.3/bin/arm-eabi-

Depending on the speed of your machine, this can take a while to complete. You'll be witness to many different files getting compiled and linked together as it runs. When it's done, you'll see something like this in your terminal window:

OBJCOPY arch/arm/boot/zImage
Kernel: arch/arm/boot/zImage is ready
Building modules, stage 2.
MODPOST 7 modules
CC arch/arm/mvp/commkm/commkm.mod.o
LD [M] arch/arm/mvp/commkm/commkm.ko
CC arch/arm/mvp/mvpkm/mvpkm.mod.o
LD [M] arch/arm/mvp/mvpkm/mvpkm.ko
CC arch/arm/mvp/pvtcpkm/pvtcpkm.mod.o
LD [M] arch/arm/mvp/pvtcpkm/pvtcpkm.ko
CC drivers/interceptor/vpnclient.mod.o
LD [M] drivers/interceptor/vpnclient.ko
CC drivers/net/wireless/bcmdhd/dhd.mod.o
LD [M] drivers/net/wireless/bcmdhd/dhd.ko
CC drivers/net/wireless/btlock/btlock.mod.o
LD [M] drivers/net/wireless/btlock/btlock.ko
CC drivers/scsi/scsi_wait_scan.mod.o
LD [M] drivers/scsi/scsi_wait_scan.ko

Congrats - you've just compiled a linux kernel for Android.

There's only one problem: It's completely useless in it's current form. If you were to install that kernel on your phone, it simply wouldn't boot. What it really needs now is a ramdisk.

The next sections (plural) in this HOWTO will discuss what ramdisks are, how to extract one from an existing kernel, and how to package a ramdisk image with a kernel to make a flashable boot image. We also need to talk about partitions, block devices, making backups, using the 'dd' command, and a few other things.

You didn't think this was easy, did you?