[OpenWrt-Devel] [RFC PATCH v2] 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.

Vmlinux (uncompressed) size comparison for my personal configuration (Linux
5.4.46, compiled with gcc 9.3.0 and binutils 2.34):

Pure ARM:
24243392 bytes

Thumb-2:
22102716 bytes

NOTE: This requires enabling a linker bug 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 (and in-tree) 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-5.4 | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 target/linux/mvebu/cortexa9/config-5.4

diff --git a/target/linux/mvebu/cortexa9/config-5.4 b/target/linux/mvebu/cortexa9/config-5.4
new file mode 100644
index 0000000000..6aff77fda7
--- /dev/null
+++ b/target/linux/mvebu/cortexa9/config-5.4
@@ -0,0 +1,2 @@
+CONFIG_THUMB2_AVOID_R_ARM_THM_JUMP11=y
+CONFIG_THUMB2_KERNEL=y
-- 
2.27.0


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