Makefile: Add build time and compiler info string

When we are doing opensbi development, we want to know the build time
and compiler info for debug purpose.

To enable this message, please add "BUILD_INFO=y", like:

```
make BUILD_INFO=y
```

NOTE: Using `BUILD_INFO=y` without specifying SOURCE_DATE_EPOCH will
violate "reproducible builds". So it's ONLY for development and debug
purpose, and should NOT be used in a product which follows "reproducible
builds".

Signed-off-by: Wei Fu <wefu@redhat.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

3 files changed, 63 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 16d9dca..8623c1c 100644
--- a/Makefile
+++ b/Makefile
@@ -150,6 +150,25 @@ endif
 # Check whether the linker supports creating PIEs
 OPENSBI_LD_PIE := $(shell $(CC) $(CLANG_TARGET) $(RELAX_FLAG) $(USE_LD_FLAG) -fPIE -nostdlib -Wl,-pie -x c /dev/null -o /dev/null >/dev/null 2>&1 && echo y || echo n)
 
+# Build Info:
+# OPENSBI_BUILD_TIME_STAMP -- the compilation time stamp
+# OPENSBI_BUILD_COMPILER_VERSION -- the compiler version info
+BUILD_INFO ?= n
+ifeq ($(BUILD_INFO),y)
+OPENSBI_BUILD_DATE_FMT = +%Y-%m-%d %H:%M:%S %z
+ifdef SOURCE_DATE_EPOCH
+	OPENSBI_BUILD_TIME_STAMP ?= $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" \
+		"$(OPENSBI_BUILD_DATE_FMT)" 2>/dev/null || \
+		date -u -r "$(SOURCE_DATE_EPOCH)" \
+		"$(OPENSBI_BUILD_DATE_FMT)" 2>/dev/null || \
+		date -u "$(OPENSBI_BUILD_DATE_FMT)")
+else
+	OPENSBI_BUILD_TIME_STAMP ?= $(shell date "$(OPENSBI_BUILD_DATE_FMT)")
+endif
+OPENSBI_BUILD_COMPILER_VERSION=$(shell $(CC) -v 2>&1 | grep ' version ' | \
+	sed 's/[[:space:]]*$$//')
+endif
+
 # Setup list of objects.mk files
 ifdef PLATFORM
 platform-object-mks=$(shell if [ -d $(platform_src_dir)/ ]; then find $(platform_src_dir) -iname "objects.mk" | sort -r; fi)
@@ -247,6 +266,10 @@ GENFLAGS	+=	-I$(include_dir)
 ifneq ($(OPENSBI_VERSION_GIT),)
 GENFLAGS	+=	-DOPENSBI_VERSION_GIT="\"$(OPENSBI_VERSION_GIT)\""
 endif
+ifeq ($(BUILD_INFO),y)
+GENFLAGS	+=	-DOPENSBI_BUILD_TIME_STAMP="\"$(OPENSBI_BUILD_TIME_STAMP)\""
+GENFLAGS	+=	-DOPENSBI_BUILD_COMPILER_VERSION="\"$(OPENSBI_BUILD_COMPILER_VERSION)\""
+endif
 GENFLAGS	+=	$(libsbiutils-genflags-y)
 GENFLAGS	+=	$(platform-genflags-y)
 GENFLAGS	+=	$(firmware-genflags-y)
@@ -398,6 +421,11 @@ $(build_dir)/%.dep: $(src_dir)/%.c
 $(build_dir)/%.o: $(src_dir)/%.c
 	$(call compile_cc,$@,$<)
 
+ifeq ($(BUILD_INFO),y)
+$(build_dir)/lib/sbi/sbi_init.o: $(libsbi_dir)/sbi_init.c FORCE
+	$(call compile_cc,$@,$<)
+endif
+
 $(build_dir)/%.dep: $(src_dir)/%.S
 	$(call compile_as_dep,$@,$<)
 
@@ -551,3 +579,6 @@ ifeq ($(install_root_dir),$(install_root_dir_default)/usr)
 	$(if $(V), @echo " RM        $(install_root_dir_default)")
 	$(CMD_PREFIX)rm -rf $(install_root_dir_default)
 endif
+
+.PHONY: FORCE
+FORCE:
diff --git a/README.md b/README.md
index 9fdf097..54fabe4 100644
--- a/README.md
+++ b/README.md
@@ -254,6 +254,29 @@ to produce broken binaries with missing relocations; it is therefore currently
 recommended that this combination be avoided or *FW_PIC=n* be used to disable
 building OpenSBI as a position-independent binary.
 
+Building with timestamp and compiler info
+-----------------------------------------
+
+When doing development, we may want to know the build time and compiler info
+for debug purpose. OpenSBI can also be built with timestamp and compiler info.
+To build with those info and print it out at boot time, we can just simply add
+`BUILD_INFO=y`, like:
+```
+make BUILD_INFO=y
+```
+
+But if you have used `BUILD_INFO=y`, and want to switch back to `BUILD_INFO=n`,
+you must do
+```
+make clean
+```
+before the next build.
+
+NOTE: Using `BUILD_INFO=y` without specifying SOURCE_DATE_EPOCH will violate
+[reproducible builds]. This definition is ONLY for development and debug
+purpose, and should NOT be used in a product which follows "reproducible
+builds".
+
 Contributing to OpenSBI
 -----------------------
 
@@ -336,3 +359,4 @@ make I=<install_directory> install_docs
 [Doxygen manual]: http://www.doxygen.nl/manual/index.html
 [Kendryte standalone SDK]: https://github.com/kendryte/kendryte-standalone-sdk
 [third party notices]: ThirdPartyNotices.md
+[reproducible builds]: https://reproducible-builds.org
diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
index 843659e..b1c7cf0 100644
--- a/lib/sbi/sbi_init.c
+++ b/lib/sbi/sbi_init.c
@@ -48,6 +48,14 @@ static void sbi_boot_print_banner(struct sbi_scratch *scratch)
 		   OPENSBI_VERSION_MINOR);
 #endif
 
+#ifdef OPENSBI_BUILD_TIME_STAMP
+	sbi_printf("Build time: %s\n", OPENSBI_BUILD_TIME_STAMP);
+#endif
+
+#ifdef OPENSBI_BUILD_COMPILER_VERSION
+	sbi_printf("Build compiler: %s\n", OPENSBI_BUILD_COMPILER_VERSION);
+#endif
+
 	sbi_printf(BANNER);
 }