[PATCH v2] kbuild: Enable armthumb BCJ filter for Thumb-2 kernel

[Jubin Zhong]

xz_wrap.sh use $SRCARCH to detect the BCJ filter. However, assigning
arm BCJ filter to Thumb-2 kernel is not optimal. In my case, about 5%
decrease of image size is observed with armthumb BCJ filter:

Test results:
  hardware:      QEMU emulator version 3.1.0
  config:        vexpress_defconfig with THUMB2_KERNEL & KERNEL_XZ on
  arm BCJ:       4029808
  armthumb BCJ:  3827280

Choose armthumb BCJ filter for Thumb-2 kernel to make smaller images.

Signed-off-by: Jubin Zhong <zhongjubin@xxxxxxxxxx>

----
v1:
https://lore.kernel.org/lkml/1637323647-19988-1-git-send-email-zhongjubin@xxxxxxxxxx/
---
 lib/decompress_unxz.c | 6 +++++-
 scripts/xz_wrap.sh    | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/lib/decompress_unxz.c b/lib/decompress_unxz.c
index 9f4262e..9d9cb48 100644
--- a/lib/decompress_unxz.c
+++ b/lib/decompress_unxz.c
@@ -129,7 +129,11 @@
 #	define XZ_DEC_POWERPC
 #endif
 #ifdef CONFIG_ARM
-#	define XZ_DEC_ARM
+#	ifdef CONFIG_THUMB2_KERNEL
+#		define XZ_DEC_ARMTHUMB
+#	else
+#		define XZ_DEC_ARM
+#	endif
 #endif
 #ifdef CONFIG_IA64
 #	define XZ_DEC_IA64
diff --git a/scripts/xz_wrap.sh b/scripts/xz_wrap.sh
index 76e9cbc..3824fdf 100755
--- a/scripts/xz_wrap.sh
+++ b/scripts/xz_wrap.sh
@@ -9,6 +9,8 @@
 # You can do whatever you want with this file.
 #
 
+. include/config/auto.conf
+
 BCJ=
 LZMA2OPTS=
 
@@ -20,4 +22,8 @@ case $SRCARCH in
 	sparc)          BCJ=--sparc ;;
 esac
 
+if [ -n "${CONFIG_THUMB2_KERNEL}" ]; then
+	BCJ=--armthumb
+fi
+
 exec $XZ --check=crc32 $BCJ --lzma2=$LZMA2OPTS,dict=32MiB
-- 
1.8.5.6