12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455 |
- .. SPDX-License-Identifier: GPL-2.0+
- Global data
- ===========
- Globally required fields are held in the global data structure. A pointer to the
- structure is available as symbol gd. The symbol is made available by the macro
- %DECLARE_GLOBAL_DATA_PTR.
- Register pointing to global data
- --------------------------------
- On most architectures the global data pointer is stored in a register.
- +------------+----------+
- | ARC | r25 |
- +------------+----------+
- | ARM 32bit | r9 |
- +------------+----------+
- | ARM 64bit | x18 |
- +------------+----------+
- | M68000 | d7 |
- +------------+----------+
- | MicroBlaze | r31 |
- +------------+----------+
- | NDS32 | r10 |
- +------------+----------+
- | Nios II | gp |
- +------------+----------+
- | PowerPC | r2 |
- +------------+----------+
- | RISC-V | gp (x3) |
- +------------+----------+
- | SuperH | r13 |
- +------------+----------+
- | x86 32bit | fs |
- +------------+----------+
- The sandbox, x86_64, and Xtensa are notable exceptions.
- Clang for ARM does not support assigning a global register. When using Clang
- gd is defined as an inline function using assembly code. This adds a few bytes
- to the code size.
- Binaries called by U-Boot are not aware of the register usage and will not
- conserve gd. UEFI binaries call the API provided by U-Boot and may return to
- U-Boot. The value of gd has to be saved every time U-Boot is left and restored
- whenever U-Boot is reentered. This is also relevant for the implementation of
- function tracing. For setting the value of gd function set_gd() can be used.
- Global data structure
- ---------------------
- .. kernel-doc:: include/asm-generic/global_data.h
- :internal:
|