ARM: implement ENTRY_FUNCTION_WITHSTACK

The point of ENTRY_FUNCTION is to write the entry point in C. Due to
lack of __naked on ARM64, the start of the entry point will have prologue
using stack and it's not possible to set up the stack safely without
branching into non-inline assembly[0]. On ARM32, where we got __naked, we
have the potential for a different problem: If BootROM sets up stack for
us and we branch to a naked function, which doesn't set up its own
stack, compiler may decide to spill local variables overwriting
instructions it had already run[1].

For code reuse between ARM and ARM64, it would be nice to use the same
entry point structure for both. Currently, the only way is to write it
in non-inline assembly using the ENTRY_PROC macro.

This introduces another way: the ARM64 barebox header has enough space
for 8 instructions of which 5 are unused (2 instructions compiler prologue
+ 1 instruction to jump after the header), we could place a stack setup
routine there to avoid having to write a separate assembly file.
For ARM32, we just call arm_setup_stack and branch out directly after,
freeing board porters of the burden of getting it right.

Add a new ENTRY_FUNCTION_WITHSTACK to realize this.

[0]: 76bced6fe146 ("ARM: document arm_setup_stack() pitfalls"),
[1]: b51b15ba1738 ("RISC-V: board-dt-2nd: move low level init into nonnaked function")

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.barebox.org/20220114084227.308804-1-a.fatoum@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/arm/lib/pbl.lds.S b/arch/arm/lib/pbl.lds.S
index 0a0fb8b5ac..e77b3220fc 100644
--- a/arch/arm/lib/pbl.lds.S
+++ b/arch/arm/lib/pbl.lds.S
@@ -31,6 +31,7 @@ SECTIONS
 	.text      :
 	{
 		_stext = .;
+		*(.text_head_prologue*)
 		*(.text_head_entry*)
 		__bare_init_start = .;
 		*(.text_bare_init*)