2 files changed, 42 insertions, 3 deletions

diff --git a/Documentation/process/changes.rst b/Documentation/process/changes.rst
index 560beaef5a7c..81cdb528ad46 100644
--- a/Documentation/process/changes.rst
+++ b/Documentation/process/changes.rst
@@ -32,6 +32,8 @@ you probably needn't concern yourself with isdn4k-utils.
 GNU C                  3.2              gcc --version
 GNU make               3.81             make --version
 binutils               2.20             ld -v
+flex                   2.5.35           flex --version
+bison                  2.0              bison --version
 util-linux             2.10o            fdformat --version
 module-init-tools      0.9.10           depmod -V
 e2fsprogs              1.41.4           e2fsck -V
@@ -79,6 +81,19 @@ The build system has, as of 4.13, switched to using thin archives (`ar T`)
 rather than incremental linking (`ld -r`) for built-in.o intermediate steps.
 This requires binutils 2.20 or newer.
 
+Flex
+----
+
+Since Linux 4.16, the build system generates lexical analyzers
+during build.  This requires flex 2.5.35 or later.
+
+
+Bison
+-----
+
+Since Linux 4.16, the build system generates parsers
+during build.  This requires bison 2.0 or later.
+
 Perl
 ----
 
@@ -333,6 +348,16 @@ Binutils
 
 - <https://www.kernel.org/pub/linux/devel/binutils/>
 
+Flex
+----
+
+- <https://github.com/westes/flex/releases>
+
+Bison
+-----
+
+- <ftp://ftp.gnu.org/gnu/bison/>
+
 OpenSSL
 -------
 
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index ee528e30f80f..0f9ef3fbbaf5 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -186,8 +186,6 @@ $(foreach m, $(notdir $1), \
         $(addprefix $(obj)/, $(foreach s, $3, $($(m:%$(strip $2)=%$(s)))))))
 endef
 
-ifdef REGENERATE_PARSERS
-
 # LEX
 # ---------------------------------------------------------------------------
 LEX_PREFIX = $(if $(LEX_PREFIX_${baseprereq}),$(LEX_PREFIX_${baseprereq}),yy)
@@ -195,9 +193,15 @@ LEX_PREFIX = $(if $(LEX_PREFIX_${baseprereq}),$(LEX_PREFIX_${baseprereq}),yy)
 quiet_cmd_flex = LEX     $@
       cmd_flex = $(LEX) -o$@ -L -P $(LEX_PREFIX) $<
 
+ifdef REGENERATE_PARSERS
 .PRECIOUS: $(src)/%.lex.c_shipped
 $(src)/%.lex.c_shipped: $(src)/%.l
         $(call cmd,flex)
+endif
+
+.PRECIOUS: $(obj)/%.lex.c
+$(filter %.lex.c,$(targets)): $(obj)/%.lex.c: $(src)/%.l FORCE
+        $(call if_changed,flex)
 
 # YACC
 # ---------------------------------------------------------------------------
@@ -206,19 +210,29 @@ YACC_PREFIX = $(if $(YACC_PREFIX_${baseprereq}),$(YACC_PREFIX_${baseprereq}),yy)
 quiet_cmd_bison = YACC    $@
       cmd_bison = $(YACC) -o$@ -t -l -p $(YACC_PREFIX) $<
 
+ifdef REGENERATE_PARSERS
 .PRECIOUS: $(src)/%.tab.c_shipped
 $(src)/%.tab.c_shipped: $(src)/%.y
         $(call cmd,bison)
+endif
+
+.PRECIOUS: $(obj)/%.tab.c
+$(filter %.tab.c,$(targets)): $(obj)/%.tab.c: $(src)/%.y FORCE
+        $(call if_changed,bison)
 
 quiet_cmd_bison_h = YACC    $@
       cmd_bison_h = bison -o/dev/null --defines=$@ -t -l -p $(YACC_PREFIX) $<
 
+ifdef REGENERATE_PARSERS
 .PRECIOUS: $(src)/%.tab.h_shipped
 $(src)/%.tab.h_shipped: $(src)/%.y
         $(call cmd,bison_h)
-
 endif
 
+.PRECIOUS: $(obj)/%.tab.h
+$(filter %.tab.h,$(targets)): $(obj)/%.tab.h: $(src)/%.y FORCE
+        $(call if_changed,bison_h)
+
 # Shipped files
 # ===========================================================================