linux/Documentation/riscv/boot-image-header.rst
Linus Torvalds 7c672abc12 It's a somewhat calmer cycle for docs this time, as the churn of the mass
RST conversion is happily mostly behind us.
 
  - A new document on reproducible builds.
 
  - We finally got around to zapping the documentation for hardware support
    that was removed in 2004; one doesn't want to rush these things.
 
  - The usual assortment of fixes, typo corrections, etc.
 
 You'll still find a handful of annoying conflicts against other trees,
 mostly tied to the last RST conversions; resolutions are straightforward
 and the linux-next ones are good.
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Merge tag 'docs-5.4' of git://git.lwn.net/linux

Pull documentation updates from Jonathan Corbet:
 "It's a somewhat calmer cycle for docs this time, as the churn of the
  mass RST conversion is happily mostly behind us.

   - A new document on reproducible builds.

   - We finally got around to zapping the documentation for hardware
     support that was removed in 2004; one doesn't want to rush these
     things.

   - The usual assortment of fixes, typo corrections, etc"

* tag 'docs-5.4' of git://git.lwn.net/linux: (67 commits)
  Documentation: kbuild: Add document about reproducible builds
  docs: printk-formats: Stop encouraging use of unnecessary %h[xudi] and %hh[xudi]
  Documentation: Add "earlycon=sbi" to the admin guide
  doc🔒 remove reference to clever use of read-write lock
  devices.txt: improve entry for comedi (char major 98)
  docs: mtd: Update spi nor reference driver
  doc: arm64: fix grammar dtb placed in no attributes region
  Documentation: sysrq: don't recommend 'S' 'U' before 'B'
  mailmap: Update email address for Quentin Perret
  docs: ftrace: clarify when tracing is disabled by the trace file
  docs: process: fix broken link
  Documentation/arm/samsung-s3c24xx: Remove stray U+FEFF character to fix title
  Documentation/arm/sa1100/assabet: Fix 'make assabet_defconfig' command
  Documentation/arm/sa1100: Remove some obsolete documentation
  docs/zh_CN: update Chinese howto.rst for latexdocs making
  Documentation: virt: Fix broken reference to virt tree's index
  docs: Fix typo on pull requests guide
  kernel-doc: Allow anonymous enum
  Documentation: sphinx: Don't parse socket() as identifier reference
  Documentation: sphinx: Add missing comma to list of strings
  ...
2019-09-17 16:22:26 -07:00

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ReStructuredText

=================================
Boot image header in RISC-V Linux
=================================
:Author: Atish Patra <atish.patra@wdc.com>
:Date: 20 May 2019
This document only describes the boot image header details for RISC-V Linux.
TODO:
Write a complete booting guide.
The following 64-byte header is present in decompressed Linux kernel image::
u32 code0; /* Executable code */
u32 code1; /* Executable code */
u64 text_offset; /* Image load offset, little endian */
u64 image_size; /* Effective Image size, little endian */
u64 flags; /* kernel flags, little endian */
u32 version; /* Version of this header */
u32 res1 = 0; /* Reserved */
u64 res2 = 0; /* Reserved */
u64 magic = 0x5643534952; /* Magic number, little endian, "RISCV" */
u32 magic2 = 0x56534905; /* Magic number 2, little endian, "RSC\x05" */
u32 res4; /* Reserved for PE COFF offset */
This header format is compliant with PE/COFF header and largely inspired from
ARM64 header. Thus, both ARM64 & RISC-V header can be combined into one common
header in future.
Notes
=====
- This header can also be reused to support EFI stub for RISC-V in future. EFI
specification needs PE/COFF image header in the beginning of the kernel image
in order to load it as an EFI application. In order to support EFI stub,
code0 should be replaced with "MZ" magic string and res5(at offset 0x3c) should
point to the rest of the PE/COFF header.
- version field indicate header version number
========== =============
Bits 0:15 Minor version
Bits 16:31 Major version
========== =============
This preserves compatibility across newer and older version of the header.
The current version is defined as 0.2.
- The "magic" field is deprecated as of version 0.2. In a future
release, it may be removed. This originally should have matched up
with the ARM64 header "magic" field, but unfortunately does not.
The "magic2" field replaces it, matching up with the ARM64 header.
- In current header, the flags field has only one field.
===== ====================================
Bit 0 Kernel endianness. 1 if BE, 0 if LE.
===== ====================================
- Image size is mandatory for boot loader to load kernel image. Booting will
fail otherwise.