Kbuild: make DWARF version a choice

Adds a default CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT which allows
the implicit default version of DWARF emitted by the toolchain to
progress over time.

Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice, making it
mutually exclusive with CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT. Users
may want to select this if they are using a newer toolchain, but have
consumers of the DWARF debug info that aren't yet ready for newer DWARF
versions' debug info.

Does so in a way that's forward compatible with existing
configs, and makes adding future versions more straightforward. This
patch does not change the current behavior or selection of DWARF
version for users upgrading to kernels with this patch.

GCC since ~4.8 has defaulted to DWARF v4 implicitly, and GCC 11 has
bumped this to v5.

Remove the Kconfig help text  about DWARF v4 being larger.  It's
empirically false for the latest toolchains for x86_64 defconfig, has no
point of reference (I suspect it was DWARF v2 but that's stil
empirically false), and debug info size is not a qualatative measure.

Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
Suggested-by: Fangrui Song <maskray@google.com>
Suggested-by: Jakub Jelinek <jakub@redhat.com>
Suggested-by: Mark Wielaard <mark@klomp.org>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

diff --git a/Makefile b/Makefile
index c1cac349ba4e362e4a69b4a73306a650facaf7f6..c567c4343880a5de3884beadbfaf22b4e16f9c0d 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -830,8 +830,9 @@ ifneq ($(LLVM_IAS),1)
 KBUILD_AFLAGS  += -Wa,-gdwarf-2
 endif
 
-ifdef CONFIG_DEBUG_INFO_DWARF4
-DEBUG_CFLAGS   += -gdwarf-4
+ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
+DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
 endif
 
 ifdef CONFIG_DEBUG_INFO_REDUCED

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 9cf4d12b81fbfdb37d0e0d3b0bf46beddd0ee36f..3555edcfd4ab666b6cc03412741a2cc7ff41d8e7 100644 (file)
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -256,13 +256,33 @@ config DEBUG_INFO_SPLIT
          to know about the .dwo files and include them.
          Incompatible with older versions of ccache.
 
+choice
+       prompt "DWARF version"
+       help
+         Which version of DWARF debug info to emit.
+
+config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
+       bool "Rely on the toolchain's implicit default DWARF version"
+       help
+         The implicit default version of DWARF debug info produced by a
+         toolchain changes over time.
+
+         This can break consumers of the debug info that haven't upgraded to
+         support newer revisions, and prevent testing newer versions, but
+         those should be less common scenarios.
+
+         If unsure, say Y.
+
 config DEBUG_INFO_DWARF4
-       bool "Generate dwarf4 debuginfo"
+       bool "Generate DWARF Version 4 debuginfo"
        help
-         Generate dwarf4 debug info. This requires recent versions
-         of gcc and gdb. It makes the debug information larger.
-         But it significantly improves the success of resolving
-         variables in gdb on optimized code.
+         Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
+
+         If you have consumers of DWARF debug info that are not ready for
+         newer revisions of DWARF, you may wish to choose this or have your
+         config select this.
+
+endchoice # "DWARF version"
 
 config DEBUG_INFO_BTF
        bool "Generate BTF typeinfo"