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Diffstat (limited to 'Documentation/riscv/boot-image-header.txt')
| -rw-r--r-- | Documentation/riscv/boot-image-header.txt | 50 |
1 files changed, 50 insertions, 0 deletions
diff --git a/Documentation/riscv/boot-image-header.txt b/Documentation/riscv/boot-image-header.txt new file mode 100644 index 000000000000..1b73fea23b39 --- /dev/null +++ b/Documentation/riscv/boot-image-header.txt @@ -0,0 +1,50 @@ + 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. +The complete booting guide will be available at Documentation/riscv/booting.txt. + +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 res3; /* Reserved for additional RISC-V specific header */ + 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 0.1. + +- res3 is reserved for offset to any other additional fields. This makes the + header extendible in future. One example would be to accommodate ISA + extension for RISC-V in future. For current version, it is set to be zero. + +- In current header, the flag 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. |