[PATCH v2] ARM: Prevent the compiler from using NEON registers

[Samuel Holland]

For ARMv8-A, NEON is standard, so the compiler can use it even when no
special target flags are provided. For example, it can use stores from
NEON registers to zero-initialize large structures. GCC 11 decides to
do this inside the DRAM init code for the Allwinner H6.

However, GCC 11 has a bug where it generates misaligned NEON register
stores even with -mstrict-align. Since the MMU is not enabled this early
in SPL, the misaligned store causes an exception and breaks booting.

Work around this issue by restricting the compiler to using GPRs only,
not vector registers. This prevents any future surprises relating to
NEON use as well.

Signed-off-by: Samuel Holland <samuel at sholland.org>
---

Changes in v2:
 - Updated commit message to describe problem more precisely

 arch/arm/config.mk | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/config.mk b/arch/arm/config.mk
index 16c63e12667..964c6b026ec 100644
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -25,6 +25,7 @@ endif
 
 PLATFORM_RELFLAGS += -fno-common -ffixed-r9
 PLATFORM_RELFLAGS += $(call cc-option, -msoft-float) \
+		     $(call cc-option,-mgeneral-regs-only) \
       $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
 
 # LLVM support
-- 
2.31.1