3.2.1. Users Guide

Linux Kernel User’s Guide

• Users Guide

  • Overview

  • Getting the Kernel Source Code

  • Preparing to Build

    • Compiler

    • Cleaning the Kernel Sources

  • Configuring the Kernel

    • Using Default Configurations

    • Customizing the Configuration

  • Compiling the Sources

    • Compiling the Kernel

    • Compiling the Device Tree Binaries

    • Compiling the Kernel Modules

  • Creating the kernel fitImage for high security device / GP devices

    • Describing FIT source

    • Generating the fitImage

    • Build uboot again

  • Installing the Kernel

    • Installing the Kernel Image and Device Tree Binaries

    • Installing the Kernel Modules

3.2.1.1. Overview

This document will cover the basic steps for building the Linux kernel.

3.2.1.2. Getting the Kernel Source Code

The easiest way to get access to the kernel source code is by downloading and installing the Processor SDK Linux AM68A. You can download the latest Processor SDK Linux AM68A installer from AM68A-Linux-SDK-Download-page. Once installed, the kernel source code is included in the SDK’s board-support directory. For your convenience the sources also includes the kernel’s git repository including commit history. Alternatively, Kernel sources can directly be fetched from GIT.

3.2.1.3. Preparing to Build

It is important that when using the GCC toolchain provided with the SDK or stand alone from TI that you do NOT source the environment-setup file included with the toolchain when building the kernel. Doing so will cause the compilation of host side components within the kernel tree to fail.

Note

The following commands are intended to be run from the root of the kernel tree unless otherwise specified. The root of the kernel tree is the top-level directory and can be identified by looking for the “MAINTAINERS” file.

3.2.1.3.1. Compiler

Refer to Yocto-built SDK Toolchains section to use the toolchain packaged in the Processor SDK (recommended).

Refer to ARM toolchains to download and setup ARM toolchains, if the Processor SDK is not used.

In either of the above setups, the build commands in the next section will assume the below variables are set appropriately.

• CROSS_COMPILE_64

The current compiler supported for this release along with download location can be found in the release notes for the kernel release.

3.2.1.3.2. Cleaning the Kernel Sources

Prior to compiling the Linux kernel it is often a good idea to make sure that the kernel sources are clean and that there are no remnants left over from a previous build.

Note

The next step will delete any saved .config file in the kernel tree as well as the generated object files. If you have done a previous configuration and do not wish to lose your configuration file you should save a copy of the configuration file (.config) before proceeding.

The command to clean the kernel is:

make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" distclean

3.2.1.4. Configuring the Kernel

Before compiling the Linux kernel it needs to be configured to select what components will become part of the kernel image, which components will be build as dynamic modules, and which components will be left out all together. This is done using the Linux kernel configuration system.

It is often easiest to start with a base default configuration and then customize it for your use case if needed. Apply Linux kernel configurations with a command of the form:

make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" <defconfig>

3.2.1.4.1. Using Default Configurations

For this sdk, the defconfig found in arch/arm64/configs is used to create the prebuilt files. We recommend users to use this kernel configuration (or at least use it as a starting point).

For example, to apply the recommended kernel configuration for K3 devices, use:

make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" defconfig ti_arm64_prune.config

The config fragments found in <path-to-ti-linux-kernel>/kernel/configs can be used to trim/add features when building a kernel that targets only TI EVMs. Append a config fragment to the end of “make” command like above to add/remove features.

After the configuration step has run the full configuration file is saved to the root of the kernel tree as .config. Any further configuration changes are based on this file until it is cleaned up by doing a kernel clean as mentioned above.

3.2.1.4.2. Customizing the Configuration

When you want to customize the kernel configuration the easiest way is to use the built in kernel configuration systems. One popular configuration system is menuconfig. menuconfig is an ncurses based configuration utility.

To invoke the kernel configuration you simply use a command like:

make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" <config type>

i.e. for menuconfig the command would look like

make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" menuconfig

Once the configuration window is open you can then select which kernel components should be included in the build. Exiting the configuration will save your selections to a file in the root of the kernel tree called .config.

3.2.1.5. Compiling the Sources

3.2.1.5.1. Compiling the Kernel

By default U-boot expects to boot kernel Image, DTB, and DTOs found in root/boot of the SD card if using SD/MMC boot. The exception is for HS-SE (High Security - Security Enforced) devices where the FIT image (Flattened Image Tree) named fitImage will boot by default.

The FIT image includes the kernel Image, DTB, and DTOs. Booting with the FIT image could be enabled/disabled by setting/resetting u-boot environment variable boot_fit. If boot_fit is set to 1, then u-boot will boot the FIT image found in root/boot of the SD card.

Once the kernel has been configured it must be compiled to generate the bootable kernel Image as well as any dynamic kernel modules that were selected. To rebuild kernel Image to boot as is or for FIT image boot, use this command:

make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" Image

This will result in a kernel image file being created in the arch/arm64/boot/ directory called Image.

3.2.1.5.2. Compiling the Device Tree Binaries

Each TI evm has an unique device tree binary file required by the kernel. Therefore, you will need to build and install the correct dtb for the target device. TI device tree files are located in arch/arm64/boot/dts/ti. Below list various TI evms and the matching device tree file.

Boards	Device Tree File
AM62Ax SK	k3-am62a7-sk.dts
AM62x LP SK	k3-am62-lp-sk.dts
AM62SIP SK / AM62x SK	k3-am625-sk.dts
AM64x EVM	k3-am642-evm.dts
AM64x SK	k3-am642-sk.dts
AM65x EVM / AM65x IDK	k3-am654-base-board.dts, daughter cards use .dtso files
J721e EVM	k3-j721e-common-proc-board.dts
J721e SK	k3-j721e-sk.dts
J7200 EVM	k3-j7200-common-proc-board.dts
J721S2 EVM	k3-j721s2-common-proc-board.dts
AM68 SK	k3-am68-sk-base-board.dts
J784S4 EVM	k3-j784s4-evm.dts
AM69 SK	k3-am69-sk.dts
J722S EVM	k3-j722s-evm.dts

To build an individual device tree file find the name of the dts file for the board you are using and replace the .dts extension with .dtb. Then run the following command:

make DTC_FLAGS=-@ ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" ti/<dt filename>.dtb

The compiled device tree file with be located in arch/arm64/boot/dts/ti.

For example, the AM64x EVM device tree file is named k3-am642-evm.dts. To build the device tree binary you would run:

make DTC_FLAGS=-@ ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" ti/k3-am642-evm.dtb

Alternatively, you can build every device tree binary with command

make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" dtbs

3.2.1.5.3. Compiling the Kernel Modules

By default the majority of the Linux drivers used in the sdk are not integrated into the kernel image file (Image). These drivers are built as dynamic modules. The command to build these modules is:

make ARCH=arm64 CROSS_COMPILE="$CROSS_COMPILE_64" modules

This will result in .ko (kernel object) files being placed in the kernel tree. These .ko files are the dynamic kernel modules.

Note

If you make a change to the kernel which requires you to recompile the kernel, then you should also recompile the kernel modules and reinstall the kernel modules. Otherwise your kernel modules may refuse to load, which will result in a loss of functionality.

3.2.1.6. Creating the kernel fitImage for high security device / GP devices

SDKs have pre-built FIT images that contain the default Kernel and DTB files. But developers may want to deploy and test new Kernel and DTB without going through the standard build system. For the specific purpose, board specific fitImage.its will be present in the prebuilt-images directory.

Pre-requisites ( Already part of SDK installations ):

• Uboot build directory for ARMV8

• Linux Image and DTB

Note

GP/HS-FS devices will also enforce authentication if booting fitImage. To disable authentication enforcement, FIT_SIGNATURE_ENFORCE needs to be disabled in defconfig for the specific board during uboot build.

3.2.1.6.1. Describing FIT source

FIT Image is a packed structure containing binary blobs and configurations. The Kernel FIT Image that we have has Kernel Image, DTB and the DTBOs

kernel-1 {
        description = "Linux kernel";
        data = /incbin/("linux.bin");
        type = "kernel";
        arch = "arm64";
        os = "linux";
        compression = "gzip";
        load = <0x81000000>;
        entry = <0x81000000>;
        hash-1 {
                algo = "sha512";
        };
};
fdt-ti_k3-j721e-common-proc-board.dtb {
        description = "Flattened Device Tree blob";
        data = /incbin/("arch/arm64/boot/dts/ti/k3-j721e-common-proc-board.dtb");
        type = "flat_dt";
        arch = "arm64";
        compression = "none";
        load = <0x83000000>;
        hash-1 {
                algo = "sha512";
        };
};
fdt-ti_k3-j721e-evm-virt-mac-client.dtbo {
        description = "Flattened Device Tree blob";
        data = /incbin/("arch/arm64/boot/dts/ti/k3-j721e-evm-virt-mac-client.dtbo");
        type = "flat_dt";
        arch = "arm64";
        compression = "none";
        load = <0x83080000>;
        hash-1 {
                algo = "sha512";
        };
};

Change the path in data variables to point to the respective files in your local machine.

For e.g change “linux.bin” to “<path-to-tisdk>/board-support/prebuilt-images/Image”.

The new addition to the FIT from 8.6 to 9.0 is the FIT Signature.

conf-ti_k3-j721e-common-proc-board.dtb {
        description = "Linux kernel, FDT blob";
        fdt = "fdt-ti_k3-j721e-common-proc-board.dtb";
        kernel = "kernel-1";
        signature-1 {
                algo = "sha512,rsa4096";
                key-name-hint = "custMpk";
                sign-images = "kernel", "fdt";
        };
};

Specify all images you need the signature to authenticate as a part of sign-images. The key-name-hint needs to be changed if you are using some other key other than the TI dummy key that we are using for this example. It should be the name of the file containing the keys.

Note

Generating new set of keys:

$ mkdir keys
$ openssl genpkey -algorithm RSA -out keys/dev.key \
-pkeyopt rsa_keygen_bits:4096 -pkeyopt rsa_keygen_pubexp:65537
$ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt

3.2.1.6.2. Generating the fitImage

Note

For signing a secondary platform like SK boards, you’ll require additional steps

• Change the CONFIG_DEFAULT_DEVICE_TREE

  For e.g

  diff --git a/configs/j721e_evm_a72_defconfig b/configs/j721e_evm_a72_defconfig
  index a5c1df7e0054..6d0126d955ef 100644
  --- a/configs/j721e_evm_a72_defconfig
  +++ b/configs/j721e_evm_a72_defconfig
  @@ -13,7 +13,7 @@ CONFIG_CUSTOM_SYS_INIT_SP_ADDR=0x80480000
   CONFIG_ENV_SIZE=0x20000
   CONFIG_DM_GPIO=y
   CONFIG_SPL_DM_SPI=y
  -CONFIG_DEFAULT_DEVICE_TREE="k3-j721e-common-proc-board"
  +CONFIG_DEFAULT_DEVICE_TREE="k3-j721e-sk"
   CONFIG_SPL_TEXT_BASE=0x80080000
   CONFIG_DM_RESET=y
   CONFIG_SPL_MMC=y
  

• Change the binman nodes to package u-boot.dtb for the correct set of platform

  For e.g

  diff --git a/arch/arm/dts/k3-j721e-binman.dtsi b/arch/arm/dts/k3-j721e-binman.dtsi
  index 673be646b1e3..752fa805fe8d 100644
  --- a/arch/arm/dts/k3-j721e-binman.dtsi
  +++ b/arch/arm/dts/k3-j721e-binman.dtsi
  @@ -299,8 +299,8 @@
  #define SPL_J721E_SK_DTB "spl/dts/k3-j721e-sk.dtb"
  
  #define UBOOT_NODTB "u-boot-nodtb.bin"
  -#define J721E_EVM_DTB "u-boot.dtb"
  -#define J721E_SK_DTB "arch/arm/dts/k3-j721e-sk.dtb"
  +#define J721E_EVM_DTB "arch/arm/dts/k3-j721e-common-proc-board.dtb"
  +#define J721E_SK_DTB "u-boot.dtb"
  

This step will embed the public key in the u-boot.dtb file that was already built during the initial u-boot build.

mkimage -r -f fitImage.its -k $UBOOT_PATH/board/ti/keys -K $UBOOT_PATH/build/$ARMV8/dts/dt.dtb fitImage

Note

If you have another set of keys then change the -k argument to point to the folder where your keys are present, the build requires the presence of both .key and .crt file.

3.2.1.6.3. Build uboot again

The updated u-boot.dtb needs to be packed in u-boot.img for authentication so rebuild uboot ARMV8 without changing any parameters.

Refer to Simplified SDK Build Using Top-Level Makefile

3.2.1.7. Installing the Kernel

Once the Linux kernel, dtb files and modules have been compiled they must be installed. In the case of the kernel image this can be installed by copying the kernel image file to the location where it is going to be read from. The device tree binaries should also be copied to the same directory that the kernel image was copied to.

3.2.1.7.1. Installing the Kernel Image and Device Tree Binaries

cd <kernel sources dir>
sudo cp arch/arm64/boot/Image <rootfs path>/boot
sudo cp arch/arm64/boot/dts/ti/<dt file>.dtb <rootfs path>/boot

For example, if you wanted to copy the kernel image and AM64x EVM device tree file to the rootfs partition of a SD card you would enter the below commands:

cd <kernel sources dir>
sudo cp arch/arm64/boot/Image /media/rootfs/boot
sudo cp arch/arm64/boot/dts/ti/k3-am642-evm.dtb /media/rootfs/boot

Starting with U-boot 2013.10, the kernel and device tree binaries are read from the root file system’s boot directory when booting from MMC/EMMC. (NOT from the /boot/ partition on the MMC). This would mean you copy the kernel image and device tree binaries to /media/rootfs/boot instead of /media/boot.

3.2.1.7.2. Installing the Kernel Modules

To install the kernel modules you use another make command similar to the others, but with an additional parameter which give the base location where the modules should be installed. This command will create a directory tree from that location like lib/modules/<kernel version> which will contain the dynamic modules corresponding to this version of the kernel. The base location should usually be the root of your target file system. The general format of the command is:

sudo make ARCH=arm64  INSTALL_MOD_PATH=<path to root of file system> modules_install

For example if you are installing the modules on the rootfs partition of the SD card you would do:

sudo make ARCH=arm64 INSTALL_MOD_PATH=/media/rootfs modules_install

Note

Append INSTALL_MOD_STRIP=1 to the make modules_install command to reduce the size of the resulting installation