mirror of
https://github.com/AsahiLinux/u-boot
synced 2024-12-16 16:23:14 +00:00
181cbd4017
The XTRN_DECLARE_GLOBAL_DATA_PTR declarations in ppc code are permanently
commented out, so there are no users for this macro:
#if 1
#define DECLARE_GLOBAL_DATA_PTR register volatile gd_t *gd asm ("r2")
#else /* We could use plain global data, but the resulting code is bigger */
#define XTRN_DECLARE_GLOBAL_DATA_PTR extern
#define DECLARE_GLOBAL_DATA_PTR XTRN_DECLARE_GLOBAL_DATA_PTR \
gd_t *gd
#endif
Remove all references to this macro, but add a documentation note regarding
the possibility of using plain global data for the GD pointer.
Signed-off-by: Ovidiu Panait <ovpanait@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
58 lines
1.9 KiB
ReStructuredText
58 lines
1.9 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 |
|
|
+------------+----------+
|
|
| Nios II | gp |
|
|
+------------+----------+
|
|
| PowerPC | r2 |
|
|
+------------+----------+
|
|
| RISC-V | gp (x3) |
|
|
+------------+----------+
|
|
| SuperH | r13 |
|
|
+------------+----------+
|
|
| x86 32bit | fs |
|
|
+------------+----------+
|
|
|
|
The sandbox, x86_64, and Xtensa are notable exceptions.
|
|
|
|
Current implementation uses a register for the GD pointer because this results
|
|
in smaller code. However, using plain global data for the GD pointer would be
|
|
possible too (and simpler, as it does not require the reservation of a specific
|
|
register for it), but the resulting code is bigger.
|
|
|
|
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:
|