[OpenWrt-Devel] [RFC PATCH] mvebu: compile the kernel in Thumb-2 mode for ARMv7 targets

[Rui Salvaterra]

(Sending as RFC due to the note below.)

The Thumb-2 instruction set generates denser code, allowing for more efficient
use of the cache and consequently higher execution performance.

NOTE: This requires enabling a linker workaround to avoid the emission of
R_ARM_THM_JUMP11 relocations [1] in modules, which the kernel doesn't support.
Since this effectively implies -fno-optimize-sibling-calls [2], we're generating
suboptimal code. While compat modules load and run correctly without this
workaround, WireGuard fails to load with an unknown relocation 102 error.

[1] https://static.docs.arm.com/ihi0044/e/IHI0044E_aaelf.pdf (page 28)
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/arch/arm/Makefile?h=linux-5.4.y#n129

Signed-off-by: Rui Salvaterra <rsalvaterra at gmail.com>
---
 target/linux/mvebu/cortexa9/config-default | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 target/linux/mvebu/cortexa9/config-default

diff --git a/target/linux/mvebu/cortexa9/config-default b/target/linux/mvebu/cortexa9/config-default
new file mode 100644
index 0000000000..6aff77fda7
--- /dev/null
+++ b/target/linux/mvebu/cortexa9/config-default
@@ -0,0 +1,2 @@
+CONFIG_THUMB2_AVOID_R_ARM_THM_JUMP11=y
+CONFIG_THUMB2_KERNEL=y
-- 
2.27.0


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