Replicant 4.0 Porting Guide

This guide is a step-by-step explanation of the process of porting a new device to Replicant 4.0.

Overview

Porting a new device to Replicant is a long task, so make sure you're ready to go through all the steps mentioned below. While it's not technically hard (unless you have to write free software replacements yourself), the process itself takes time as many steps are involved:

Prerequisites

Before porting your device to Replicant, you must make sure it complies with the following requirements:

If your device fails to comply with one of these requirements, it won't be possible to port Replicant to it.
If you don't know about whether your device complies or not, you'll probably learn it along the way.

Discovering the phone's hardware and associated blobs

Finding the device's codenames

First of all, you'll have to find out the device's codename that was given by its manufacturer. Wikipedia usually has that information on the device's article. For instance, the codename for the European version of the Nexus S given by Samsung is i9023. This codename will help in the process of getting informations about the device.

Then, a second codename (that can turn out the be the same as the previous one) is given to the device at Android-level. If your device is supported by CyanogenMod, you can find it out from the CyanogenMod Wiki or on CyanogenMod download page. For instance, the Nexus S codename is: crespo.

Investigating the hardware

It is useful to have a general idea of what kind of hardware is present in the phone. From the Wikipedia and CyanogenMod pages about the device, it's already possible to know what System on a Chip (SoC) it uses and a couple other details.

To learn more details, you can consider looking for a teardown of the device (for instance on iFixit), that will reveal what chips are used on the device. Looking at the kernel defconfig for the device will also help a lot, you can also try to find the service manual for the device.

You can then compare that to the devices that are already supported in Replicant to get an idea of what will possibly work.

Finding out if the device checks the kernel's signature

One very important step is to find out if the device is Tivoized: that means that even though the manufacturer releases the kernel source code for the device, the bootloader checks the kernel signature and will refuse to start it if it's not properly signed by the manufacturer. In other words, if you build the kernel yourself, the device will refuse to run it since it's not signed by the manufacturer. Since the Linux kernel is released under the GPLv2, there are no specific dispositions to counter Tivoization, and so porting the device to Replicant is pointless as it will require a prebuilt and signed kernel from the manufacturer.

This is not an easy information to find out, but the developers involved in the CyanogenMod port will probably know that information. It's a good idea to just ask them.

Discovering the way of flashing the device

To install the future Replicant image on the device, you have to find out how the device can be flashed with a new operating system. The CyanogenMod Wiki has install guides for the supported devices and you'll probably find install guides for non-official CM ports as well. It is very important to understand the flashing procedure as it will have to be documented on the Replicant wiki.

There are basically two ways of flashing a new operating system:
  1. Through the bootloader: a program has to send the images to the phone in bootloader mode. Make sure that program is free if your device supports flashing via bootloader.
  2. With recovery: a recovery image has to be installed instead of the current kernel so that at next reboot, recovery permits the installation of another operating system. Make sure this doesn't involve rooting the phone using non-free software.

The non-free blobs

The key information to get before starting the port is the list of the non-free components that are required by CyanogenMod.
The easiest way to do this is to spot the device repository in CyanogenMod repos and look for the extract-files.sh or proprietary-blobs.txt file on the ics branch.
There is usually a link to the device repository from the CyanogenMod Wiki

For instance, the list of non-free components for the Nexus S is extract-files.sh

From that list, spot what is related to what hardware component (audio, camera, sensors, gps, modem, etc): that gives an idea of the amount of work required to add support for the phone.

Getting started with Replicant development

In order to prepare everything for the Replicant port:

Cloning the device files

Once your Replicant tree is ready, you can start adding the necessary repos for your device.
That means cloning the necessary repos in the right place. These repos are:

You can find the device-specific repo from the device's page on the CyanogenMod Wiki.
Make sure you check out the branches that match the CM 9.0 version (the branch may be called ics).

Once you have cloned the device-specific repo for your device and checked out the correct branch, refer to the cm.dependencies file to find what repos are left to clone.
Clone these repos in the correct locations and remove the prefix (e.g. android_device_samsung_crespo must be cloned in device/samsung/ and renamed to crespo).

If your cloned the kernel source for your device, it is likely that the kernel build is already integrated, so you can skip the next sections below.

In case of a missing kernel repository

If the kernel repo is nowhere to be found (make sure you've asked the CyanogenMod team), you'll need to get the kernel source directly from the vendor, especially if your device is supported by a 3rd party CyanogenMod fork.
Keep in mind that the Linux kernel is GPLv2, so vendors have the legal obligation to release the modified kernel sources as soon as they sell you the device.
That means the kernel sources will be available online. Here are some websites where such releases are done:

Once you have the kernel sources, read the instructions to find out which defconfig to use.

Since manufacturers usually don't release the git history along with the files, you'll need to recreate a git repo:

Now that you have a git repo, you can move it to the Replicant code tree, under the name: kernel/vendor/devices (e.g. kernel/samsung/aries).
Make sure to make the devices name match the devices in android_device_vendor_devices-common if the kernel is shared across these devices or to match the device in android_device_vendor_device.

In case of a prebuilt kernel

Some devices are still using a prebuilt kernel. Even though the CyanogenMod team is trying to avoid that, it remains in many repos.
For such devices, you will need to remove the prebuilt binaries and the instructions to copy the prebuilt kernel and its modules.

In the device repository (device/vendor/device) and common repository for your device (if any), remove the prebuilt kernel and modules (usually called kernel and module.ko (replace module with the name of a module) or a modules directory).
Remove the instructions to copy these prebuilts on the makefiles. Remove instructions such as:

PRODUCT_COPY_FILES += \
    $(LOCAL_KERNEL):kernel

LOCAL_KERNEL := $(LOCAL_PATH)/kernel

and anything regarding TARGET_PREBUILT_KERNEL as well as the instructions to copy the prebuilt modules.

The BoardConfig.mk (or BoardConfigCommon.mk in the common directory for your device) will most likely hold a line like:

TARGET_PREBUILT_KERNEL := device/samsung/p5/kernel

you must remove this line.

Now that the device repository has no prebuilt instructions, you can add the instructions to build the kernel. In the BoardConfig.mk file, add the following lines:

TARGET_KERNEL_SOURCE := kernel/samsung/p3
TARGET_KERNEL_CONFIG := samsung_p5_defconfig

and make sure to replace the location and defconfig by the correct values for your devices (being the location of the device kernel tree and the appropriate defconfig).

Building the correct kernel image format

There are different types of kernel images:

You need to find out which type of kernel image your device uses. Asking people who know about that is the best idea.

Android image

This is the easiest case to handle: just make sure the CONFIG_INITRAMFS_SOURCE option in the kernel defonfig is left blank or undefined:

CONFIG_INITRAMFS_SOURCE=""

zImage with built-in initramfs

Building a zImage with a built-in initramfs requires the following steps:
In the kernel defconfig, define the CONFIG_INITRAMFS_SOURCE option that way:

CONFIG_INITRAMFS_SOURCE="../../root"

Once this is done, duplicate the defconfig and add the _recovery prefix before the _defconfig ending (e.g. herring_recovery_defconfig), edit that file and replace CONFIG_INITRAMFS_SOURCE with:

CONFIG_INITRAMFS_SOURCE="../../recovery/root"

Back to the device repository, edit the BoardConfig.mk file and add the following line:

TARGET_KERNEL_RECOVERY_CONFIG := samsung_p5_recovery_defconfig

and make sure to replace the defconfig by the appropriate defconfig you just cloned (the one with the _recovery_defconfig ending).

Still in the device repository, create a bootimg.mk file containing the following:

LOCAL_PATH := $(call my-dir)

$(INSTALLED_BOOTIMAGE_TARGET): $(INSTALLED_KERNEL_TARGET)
    $(ACP) $(INSTALLED_KERNEL_TARGET) $@

$(INSTALLED_RECOVERYIMAGE_TARGET): $(INSTALLED_RECOVERY_KERNEL_TARGET)
    $(ACP) $(INSTALLED_RECOVERY_KERNEL_TARGET) $@

Edit the BoardConfig.mk file and add the following line:

BOARD_CUSTOM_BOOTIMG_MK := device/vendor/device/bootimg.mk

and make sure to replace device/vendor/device/ to the correct path to your device's repository.

uImage with built-in initramfs

Follow the previous instructions (zImage with built-in initramfs) and set the BOARD_USES_UBOOT variable in the BoardConfig.mk file:

BOARD_USES_UBOOT := true