[PATCH V2] Makefile: Add build time and compiler info string.

[tekkamanninja at gmail.com]

From: Wei Fu <wefu at redhat.com>

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: `BUILD_INFO=y` will violate "reproducible builds".
So it's ONLY for development and debug purpose, and should NEVER be used
in a real product.

Signed-off-by: Wei Fu <wefu at redhat.com>
---
 Makefile           | 22 ++++++++++++++++++++++
 README.md          | 23 +++++++++++++++++++++++
 lib/sbi/sbi_init.c |  8 ++++++++
 3 files changed, 53 insertions(+)

diff --git a/Makefile b/Makefile
index 16d9dca..a794c8d 100644
--- a/Makefile
+++ b/Makefile
@@ -150,6 +150,15 @@ 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 ($(BUILD_INFO),y)
+OPENSBI_BUILD_TIME_STAMP=$(shell date)
+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 $(platform_src_dir) -iname "objects.mk" | sort -r; fi)
@@ -247,6 +256,10 @@ GENFLAGS	+=	-I$(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 +411,12 @@ $(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
+	$(call compile_cc,$@,$<)
+	$(warning 'BUILD_INFO=y' violates 'reproducible builds', for DEBUG ONLY.)
+endif
+
 $(build_dir)/%.dep: $(src_dir)/%.S
 	$(call compile_as_dep,$@,$<)
 
@@ -551,3 +570,6 @@ 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..86faa74 100644
--- a/README.md
+++ b/README.md
@@ -254,6 +254,28 @@ 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` will violate [reproducible builds]. This definition
+is ONLY for development and debug purpose. It should NEVER be used in a real
+product.
+
 Contributing to OpenSBI
 -----------------------
 
@@ -336,3 +358,4 @@ 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
\ No newline at end of file
diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
index f0eb365..ef00d9f 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);
 }
 
-- 
2.25.4