[PATCH v4 6/6] perf docs: Document cross compilation

[Leo Yan]

Records the commands for cross compilation with two methods.

The first method relies on Multiarch. The second approach is to explicitly
specify the PKG_CONFIG variables, which is widely used in build system
(like Buildroot, Yocto, etc).

Co-developed-by: James Clark <james.clark@xxxxxxx>
Signed-off-by: James Clark <james.clark@xxxxxxx>
Signed-off-by: Leo Yan <leo.yan@xxxxxxx>
---
 tools/perf/Documentation/Build.txt | 37 ++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/tools/perf/Documentation/Build.txt b/tools/perf/Documentation/Build.txt
index 3766886c4bca..2237ceee74ba 100644
--- a/tools/perf/Documentation/Build.txt
+++ b/tools/perf/Documentation/Build.txt
@@ -71,3 +71,40 @@ supported by GCC. UBSan detects undefined behaviors of programs at runtime.
   $ UBSAN_OPTIONS=print_stacktrace=1 ./perf record -a
 
 If UBSan detects any problem at runtime, it outputs a “runtime error:” message.
+
+4) Cross compilation
+====================
+The perf tool can be cross compiled in below methods.
+
+As Multiarch is commonly supported in Linux distributions, we can install
+libraries for multiple architectures on the same system and then cross-compile
+Linux perf. For example, Aarch64 libraries and toolchains can be installed on
+an x86_64 machine, allowing us to compile perf for an Aarch64 target.
+
+Below is the command for building perf with dynamic linking:
+As Multiarch is commonly supported in Linux distros, therefore, we can
+install multiple architectures libs in the same system and then cross compile
+the Linux perf. For example, the Aarch64 libraries and toolchain can be
+installed on the x86_64 machine, based on it we can compile the perf for
+Aarch64 target.
+
+Below is the command for building the perf with dynamic linking.
+
+  $ cd /path/to/Linux
+  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -C tools/perf
+
+For static linking, the option `LDFLAGS="-static"` is required.
+
+  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
+    LDFLAGS="-static" -C tools/perf
+
+In the embedded system world, a use case is to explicitly specify the package
+configuration paths for cross building:
+
+  $ PKG_CONFIG_SYSROOT_DIR="/path/to/cross/build/sysroot" \
+    PKG_CONFIG_LIBDIR="/usr/lib/:/usr/local/lib" \
+    make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -C tools/perf
+
+In this case, the variable PKG_CONFIG_SYSROOT_DIR can be used alongside the
+variable PKG_CONFIG_LIBDIR or PKG_CONFIG_PATH to prepend the sysroot path to
+the library paths for cross compilation.
-- 
2.34.1