mirror of
https://github.com/brain-hackers/u-boot-brain
synced 2024-08-21 01:18:52 +09:00
54 lines
1.6 KiB
ReStructuredText
54 lines
1.6 KiB
ReStructuredText
![]() |
.. 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 |
|
||
|
+------------+----------+
|
||
|
|
||
|
The sandbox, x86, 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:
|