diff --git a/share/mk/bsd.sys.mk b/share/mk/bsd.sys.mk
--- a/share/mk/bsd.sys.mk
+++ b/share/mk/bsd.sys.mk
@@ -60,7 +60,7 @@
 .endif # !NO_WCAST_ALIGN !NO_WCAST_ALIGN.${COMPILER_TYPE}
 .endif # WARNS >= 4
 .if ${WARNS} >= 6
-CWARNFLAGS+=	-Wchar-subscripts -Wnested-externs -Wredundant-decls\
+CWARNFLAGS+=	-Wchar-subscripts -Wnested-externs \
 		-Wold-style-definition
 .if !defined(NO_WMISSING_VARIABLE_DECLARATIONS)
 CWARNFLAGS.clang+=	-Wmissing-variable-declarations
@@ -163,7 +163,6 @@
 CWARNFLAGS+=	-Wno-error=empty-body			\
 		-Wno-error=maybe-uninitialized		\
 		-Wno-error=nonnull-compare		\
-		-Wno-error=redundant-decls		\
 		-Wno-error=shift-negative-value		\
 		-Wno-error=tautological-compare		\
 		-Wno-error=unused-const-variable
diff --git a/sys/conf/Makefile.powerpc b/sys/conf/Makefile.powerpc
--- a/sys/conf/Makefile.powerpc
+++ b/sys/conf/Makefile.powerpc
@@ -53,7 +53,6 @@
         -Wno-error=enum-conversion
 .elif "${COMPILER_TYPE}" == "gcc"
 DPAAWARNFLAGS += \
-	-Wno-error=redundant-decls \
 	-Wno-error=int-in-bool-context
 .endif
 
diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -3,7 +3,7 @@
 #
 # Warning flags for compiling the kernel and components of the kernel:
 #
-CWARNFLAGS?=	-Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes \
+CWARNFLAGS?=	-Wall -Wnested-externs -Wstrict-prototypes \
 		-Wmissing-prototypes -Wpointer-arith -Wcast-qual \
 		-Wundef -Wno-pointer-sign ${FORMAT_EXTENSIONS} \
 		-Wmissing-include-dirs -fdiagnostics-show-option \
diff --git a/sys/conf/kern.pre.mk b/sys/conf/kern.pre.mk
--- a/sys/conf/kern.pre.mk
+++ b/sys/conf/kern.pre.mk
@@ -228,7 +228,6 @@
 	-Wno-nested-externs \
 	-Wno-parentheses \
 	-Wno-pointer-arith \
-	-Wno-redundant-decls \
 	-Wno-strict-prototypes \
 	-Wno-switch \
 	-Wno-undef \
@@ -298,7 +297,7 @@
 # Infiniband C flags.  Correct include paths and omit errors that linux
 # does not honor.
 OFEDINCLUDES=	-I$S/ofed/include -I$S/ofed/include/uapi ${LINUXKPI_INCLUDES}
-OFEDNOERR=	-Wno-cast-qual -Wno-pointer-arith -Wno-redundant-decls
+OFEDNOERR=	-Wno-cast-qual -Wno-pointer-arith
 OFEDCFLAGS=	${CFLAGS:N-I*} -DCONFIG_INFINIBAND_USER_MEM \
 		${OFEDINCLUDES} ${CFLAGS:M-I*} ${OFEDNOERR}
 OFED_C_NOIMP=	${CC} -c -o ${.TARGET} ${OFEDCFLAGS} ${WERROR}
diff --git a/sys/conf/kmod.mk b/sys/conf/kmod.mk
--- a/sys/conf/kmod.mk
+++ b/sys/conf/kmod.mk
@@ -544,8 +544,7 @@
 	-I${SYSDIR}/cddl/contrib/opensolaris/uts/common \
 	-include ${ZINCDIR}/os/freebsd/spl/sys/ccompile.h
 OPENZFS_CWARNFLAGS= \
-	-Wno-nested-externs \
-	-Wno-redundant-decls
+	-Wno-nested-externs
 
 .include <bsd.dep.mk>
 .include <bsd.clang-analyze.mk>
diff --git a/sys/modules/ibcore/Makefile b/sys/modules/ibcore/Makefile
--- a/sys/modules/ibcore/Makefile
+++ b/sys/modules/ibcore/Makefile
@@ -56,4 +56,4 @@
 
 .include <bsd.kmod.mk>
 
-CWARNFLAGS+= -Wno-cast-qual -Wno-pointer-arith -Wno-redundant-decls
+CWARNFLAGS+= -Wno-cast-qual -Wno-pointer-arith
diff --git a/sys/powerpc/conf/QORIQ64 b/sys/powerpc/conf/QORIQ64
--- a/sys/powerpc/conf/QORIQ64
+++ b/sys/powerpc/conf/QORIQ64
@@ -14,7 +14,7 @@
 include 	"dpaa/config.dpaa"
 makeoptions	DEBUG=-g		#Build kernel with gdb(1) debug symbols
 makeoptions	WITH_CTF=1
-#makeoptions	WERROR="-Werror -Wno-format -Wno-redundant-decls"
+#makeoptions	WERROR="-Werror -Wno-format"
 
 options 	FPU_EMU
 
diff --git a/sys/powerpc/conf/dpaa/DPAA b/sys/powerpc/conf/dpaa/DPAA
--- a/sys/powerpc/conf/dpaa/DPAA
+++ b/sys/powerpc/conf/dpaa/DPAA
@@ -12,7 +12,7 @@
 
 machine		powerpc powerpc
 #makeoptions	DEBUG=-g		#Build kernel with gdb(1) debug symbols
-#makeoptions	WERROR="-Werror -Wno-format -Wno-redundant-decls"
+#makeoptions	WERROR="-Werror -Wno-format"
 makeoptions	NO_MODULES=yes
 
 # Platform support
diff --git a/sys/powerpc/conf/dpaa/config.dpaa b/sys/powerpc/conf/dpaa/config.dpaa
--- a/sys/powerpc/conf/dpaa/config.dpaa
+++ b/sys/powerpc/conf/dpaa/config.dpaa
@@ -6,7 +6,6 @@
 	-Wno-cast-qual -Wno-unused-function -Wno-init-self -fms-extensions \
 	-include $S/contrib/ncsw/build/dflags.h \
 	-Wno-error=missing-prototypes \
-	-Wno-redundant-decls \
 	-I$S/contrib/ncsw/build/ \
 	-I$S/contrib/ncsw/inc \
 	-I$S/contrib/ncsw/inc/cores \