Linux Kernel Image
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The document discusses the construction of the Linux kernel image. It describes how the kernel code is organized and hardware-independent. It then explains the process of building the composite kernel image (vmlinux) by linking object files together. This involves using the linker to combine files like head.o, piggy.o and misc.o into a binary image. It also discusses the roles of the bootstrap loader and how it loads and decompresses the kernel image.
Linux Kernel Image
- 1. Construction of the Linux kernel image Embedded Linux Course 4-1
- 2. Linux Kernel Organization • Linux supports numerous architectures this means that it can be run on many types of processors, which include alpha, arm, i386, ia64, ppc, ppc64, and s390x. • Most of the source code is written in C and is hardware independent • A portion of the code is heavily hardware dependent and is written in a mix of C and assembly for the particular architecture. • The heavily machine-dependent portion is wrapped by a long list of system calls that serve as an interface. Embedded Linux Course
- 3. vmlinux image components Embedded Linux Course
- 4. make zImage V=1 arm-linux-ld -EL -p --no-undefined -X -o vmlinux -T arch/arm/kernel/vmlinux.lds arch/arm/kernel/head.o arch/arm/kernel/init_task.o init/built-in.o --start-group usr/built-in.o arch/arm/kernel/built-in.o arch/arm/mm/built-in. o arch/arm/common/built-in.o arch/arm/mach-s3c2410/ built-in.o arch/arm/nwfpe/built-in.o kernel/built-in.o mm/built-in.o fs/built-in.o ipc/built-in.o security/built-in.o crypto/built-in.o lib/lib.a arch/arm/lib/lib.a lib/built-in.o arch/arm/lib/built-in.o drivers/built-in.o sound/built-in.o net/built-in.o --end-group .tmp_kallsyms2.o 0xC0008000 Embedded Linux Course
- 5. Low-Level Architecture Objects Embedded Linux Course
- 7. Vmlinux (ELF object)objcopy Image (binary object) Embedded Linux Course gzip Asm wrapper around piggy.gz contains kernel image piggy.gz asm piggy.o Kernel proper Stipped binary kernel image compressed binary kernel misc.o head.o Bootable Kernel image Composite kernel image construction make zImage V=1
- 8. make zImage V=1 make -f scripts/Makefile.build obj=arch/arm/boot MACHINE=arch/arm/mach-s3c2410/ arch/arm/boot/zImage arm-linux-objcopy -O binary -R .note -R .comment -S vmlinux arch/arm/boot/Image Kernel: arch/arm/boot/Image is ready ------------------------------------------------------------------------------------------------------------- make -f scripts/Makefile.build obj=arch/arm/boot/compressed arch/arm/boot/compressed/vmlinux gzip -f -9 < arch/arm/boot/compressed/../Image > arch/arm/boot/compressed/piggy.gz arm-linux-gcc -Wp,-MD,arch/arm/boot/compressed/.piggy.o.d -nostdinc - isystem /usr/local/arm/3.4.1/lib/gcc/arm-linux/3.4.1/include -D__KERNEL__ - Iinclude -mlittle-endian -D__ASSEMBLY__ -mapcs-32 -mno-thumb-interwork - D__LINUX_ARM_ARCH__=4 -march=armv4 -mtune=arm9tdmi -msoft-float -c - o arch/arm/boot/compressed/piggy.o arch/arm/boot/compressed/piggy.S Embedded Linux Course
- 9. make zImage V=1 (cont.) arm-linux-ld -EL --defsym zreladdr=0x30008000 --defsym params_phys=0x30000100 -p --no-undefined -X /usr/local/arm/3.4.1/lib/gcc/arm-linux/3.4.1/libgcc.a -T arch/arm/boot/compressed/vmlinux.lds arch/arm/boot/compressed/head.o arch/arm/boot/compressed/piggy.o arch/arm/boot/compressed/misc.o -o arch/arm/boot/compressed/vmlinux arm-linux-objcopy -O binary -R .note -R .comment -S arch/arm/boot/compressed/vmlinux arch/arm/boot/zImage Kernel: arch/arm/boot/zImage is ready refers to: arch/arm/mach-s3c2410/Makefile.boot Embedded Linux Course
- 10. Bootstrap Loader • Not to be confused with a bootloader, many architectures use a bootstrap loader (or second-stage loader) to load the Linux kernel image into memory. • Some bootstrap loaders perform checksum verification of the kernel image, and most perform decompression and relocation of the kernel image. • The bootloader controls the board upon power-up and does not rely on the Linux kernel in any way Embedded Linux Course
- 11. Bootstrap Loader (cont.) • In contrast, the bootstrap loader's primary purpose in life is to act as the glue between a board-level bootloader and the Linux kernel • It is the bootstrap loader's responsibility to provide a proper context for the kernel to run in, as well as perform the necessary steps to decompress and relocate the kernel binary image • It is similar to the concept of a primary and secondary loader found in the PC architecture. Embedded Linux Course
- 12. Composite kernel image : vmlinux Piggy.o Binary Kernel image misc.o head.o arm-linux-ld -EL --defsym zreladdr=0x30008000 --defsym params_phys=0x30000100 -p --no-undefined -X /usr/local/arm/3.4.1/lib/gcc/arm-linux/3.4.1/libgcc.a -T arch/arm/boot/compressed/vmlinux.lds arch/arm/boot/compressed/head.o arch/arm/boot/compressed/piggy.o arch/arm/boot/compressed/misc.o -o arch/arm/boot/compressed/vmlinux Embedded Linux Course Bootstrap Loader
- 13. ARM boot control flow Uboot Embedded Linux Course head.o head.o main.o start start start_kernel Power On Boot loader Bootstrap loader Kernel vmlinux Kernel arch/arm/boot/compressed/head.S main.o arch/arm/kernel/head.S
- 14. Kernel Entry Point: head.o • arch/<ARCH>/kernel/head.S • The head.o module performs architecture- and often CPU-specific initialization in preparation for the main body of the kernel. CPU-specific tasks are kept as generic as possible across processor families • Machine-specific initialization is performed elsewhere Embedded Linux Course
- 15. head.o performs the following tasks • Checks for valid processor and architecture • Creates initial page table entries • Enables the processor's memory management unit (MMU) • Establishes limited error detection and reporting • Jumps to the start of the kernel proper, main.c Embedded Linux Course
- 16. Kernel Startup: main.c • Every architecture's head.o module has a similar construct for passing control to the kernel proper. – b start_kernel • start_kernel() located in .../init/main.c. • main.c does all the startup work for the Linux kernel, from initializing the first kernel thread all the way to mounting a root file system and executing the very first user space Linux application program. Embedded Linux Course
- 17. Further Study • See the architecture-dependent portions of the linux source code, for example, system initialization and bootstrapping, exception vector handling, address translation, and device I/O. • Cross-Referencing Linux – http://lxr.linux.no To create a device driver, the programmer must have a register-leve specification for a given piece of hardware Embedded Linux Course