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-rw-r--r--Documentation/arm64/booting.txt43
1 files changed, 35 insertions, 8 deletions
diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
index 37fc4f632176..85af34d55cee 100644
--- a/Documentation/arm64/booting.txt
+++ b/Documentation/arm64/booting.txt
@@ -72,27 +72,54 @@ The decompressed kernel image contains a 64-byte header as follows:
u32 code0; /* Executable code */
u32 code1; /* Executable code */
- u64 text_offset; /* Image load offset */
- u64 res0 = 0; /* reserved */
- u64 res1 = 0; /* reserved */
+ u64 text_offset; /* Image load offset, little endian */
+ u64 image_size; /* Effective Image size, little endian */
+ u64 flags; /* kernel flags, little endian */
u64 res2 = 0; /* reserved */
u64 res3 = 0; /* reserved */
u64 res4 = 0; /* reserved */
u32 magic = 0x644d5241; /* Magic number, little endian, "ARM\x64" */
- u32 res5 = 0; /* reserved */
+ u32 res5; /* reserved (used for PE COFF offset) */
Header notes:
+- As of v3.17, all fields are little endian unless stated otherwise.
+
- code0/code1 are responsible for branching to stext.
+
- when booting through EFI, code0/code1 are initially skipped.
res5 is an offset to the PE header and the PE header has the EFI
- entry point (efi_stub_entry). When the stub has done its work, it
+ entry point (efi_stub_entry). When the stub has done its work, it
jumps to code0 to resume the normal boot process.
-The image must be placed at the specified offset (currently 0x80000)
-from the start of the system RAM and called there. The start of the
-system RAM must be aligned to 2MB.
+- Prior to v3.17, the endianness of text_offset was not specified. In
+ these cases image_size is zero and text_offset is 0x80000 in the
+ endianness of the kernel. Where image_size is non-zero image_size is
+ little-endian and must be respected. Where image_size is zero,
+ text_offset can be assumed to be 0x80000.
+
+- The flags field (introduced in v3.17) is a little-endian 64-bit field
+ composed as follows:
+ Bit 0: Kernel endianness. 1 if BE, 0 if LE.
+ Bits 1-63: Reserved.
+
+- When image_size is zero, a bootloader should attempt to keep as much
+ memory as possible free for use by the kernel immediately after the
+ end of the kernel image. The amount of space required will vary
+ depending on selected features, and is effectively unbound.
+
+The Image must be placed text_offset bytes from a 2MB aligned base
+address near the start of usable system RAM and called there. Memory
+below that base address is currently unusable by Linux, and therefore it
+is strongly recommended that this location is the start of system RAM.
+At least image_size bytes from the start of the image must be free for
+use by the kernel.
+
+Any memory described to the kernel (even that below the 2MB aligned base
+address) which is not marked as reserved from the kernel e.g. with a
+memreserve region in the device tree) will be considered as available to
+the kernel.
Before jumping into the kernel, the following conditions must be met: