[PATCH] make debug symbols of kernel build relocatable

[Felix Moessbauer <felix.moessbauer@siemens.com>]

This patch adds support to inject custom KCFLAGS into
the linux-custom kernel build.
Using this infrastructure, we add the debug-prefix-map
flag to strip off the absolute prefix of the sources location.
This makes the debug information relocatable.

By that, we also fix low ccache hit rates in combination with ssbuild
due to the random builddir that sbuild uses.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
---
 meta/recipes-kernel/linux/files/debian/isar/build.tmpl | 2 +-
 meta/recipes-kernel/linux/files/debian/rules.tmpl      | 3 ++-
 meta/recipes-kernel/linux/linux-custom.inc             | 4 ++++
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/meta/recipes-kernel/linux/files/debian/isar/build.tmpl b/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
index b7749cec..8fe816b3 100644
--- a/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
+++ b/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
@@ -22,7 +22,7 @@ do_build() {
     set -x
 
     # Build the Linux kernel
-    ${MAKE} O=${KERNEL_BUILD_DIR} ${PARALLEL_MAKE} || exit ${?}
+    ${MAKE} O=${KERNEL_BUILD_DIR} ${PARALLEL_MAKE} KCFLAGS="${KCFLAGS}" || exit ${?}
 
     # Stop tracing
     set +x
diff --git a/meta/recipes-kernel/linux/files/debian/rules.tmpl b/meta/recipes-kernel/linux/files/debian/rules.tmpl
index a1166287..8063c495 100755
--- a/meta/recipes-kernel/linux/files/debian/rules.tmpl
+++ b/meta/recipes-kernel/linux/files/debian/rules.tmpl
@@ -13,7 +13,8 @@ isar_env=$(strip \
 	export DEB_HOST_GNU_TYPE='$(DEB_HOST_GNU_TYPE)' && \
 	export MAKE='$(MAKE)' && \
 	export O='${O}' && \
-	export S='${S}' \
+	export S='${S}' && \
+	export CURDIR='$(CURDIR)' \
 )
 
 %:
diff --git a/meta/recipes-kernel/linux/linux-custom.inc b/meta/recipes-kernel/linux/linux-custom.inc
index a6b1cdab..55bf0418 100644
--- a/meta/recipes-kernel/linux/linux-custom.inc
+++ b/meta/recipes-kernel/linux/linux-custom.inc
@@ -78,11 +78,15 @@ TEMPLATE_VARS += "                \
     LINUX_VERSION_EXTENSION       \
     KERNEL_NAME_PROVIDED          \
     KERNEL_CONFIG_FRAGMENTS       \
+    KCFLAGS                       \
 "
 
 inherit dpkg
 inherit template
 
+# Add custom cflags to the kernel build
+KCFLAGS ?= "-fdebug-prefix-map=${CURDIR}=."
+
 # Derive name of the kernel packages from the name of this recipe
 KERNEL_NAME_PROVIDED ?= "${@ d.getVar('PN', True).partition('linux-')[2]}"
 
-- 
2.30.2