[PATCH v2 1/4] ARC: allow to override default mcpu compiler flag

[Eugeniy Paltsev]

Kernel builds set their own default -mcpu for a given ISA build.
But that gets in the way of "custom" -mcpu flags from propagating
into kernel build.

This will also be used in next patches for HSDK-4xD board support which
uses a different -mcpu to effect dual issue scheduling.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@xxxxxxxxxxxx>
---
 arch/arc/Kconfig  |  9 +++++++++
 arch/arc/Makefile | 21 +++++++++++++++++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index ff306246d0f8..7034c217708f 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -377,6 +377,15 @@ config ARC_HAS_SWAPE
 	bool "Insn: SWAPE (endian-swap)"
 	default y
 
+config ARC_TUNE_MCPU
+	string "Override default -mcpu compiler flag"
+	default ""
+	help
+	  Override default -mcpu=xxx compiler flag (which is set depending on
+	  the ISA version) with the specified value.
+	  NOTE: If specified flag isn't supported by current compiler the
+	  ISA default value will be used as a fallback.
+
 if ISA_ARCV2
 
 config ARC_USE_UNALIGNED_MEM_ACCESS
diff --git a/arch/arc/Makefile b/arch/arc/Makefile
index 20e9ab6cc521..2b66e8264174 100644
--- a/arch/arc/Makefile
+++ b/arch/arc/Makefile
@@ -10,8 +10,25 @@ CROSS_COMPILE := $(call cc-cross-prefix, arc-linux- arceb-linux-)
 endif
 
 cflags-y	+= -fno-common -pipe -fno-builtin -mmedium-calls -D__linux__
-cflags-$(CONFIG_ISA_ARCOMPACT)	+= -mA7
-cflags-$(CONFIG_ISA_ARCV2)	+= -mcpu=hs38
+
+tune-mcpu-def-$(CONFIG_ISA_ARCOMPACT)	:= -mA7
+tune-mcpu-def-$(CONFIG_ISA_ARCV2)	:= -mcpu=hs38
+
+ifeq ($(CONFIG_ARC_TUNE_MCPU),"")
+cflags-y				+= $(tune-mcpu-def-y)
+else
+tune-mcpu				:= $(shell echo $(CONFIG_ARC_TUNE_MCPU))
+tune-mcpu-ok 				:= $(call cc-option-yn, $(tune-mcpu))
+ifeq ($(tune-mcpu-ok),y)
+cflags-y				+= $(tune-mcpu)
+else
+# The flag provided by 'CONFIG_ARC_TUNE_MCPU' option isn't known by this compiler
+# (probably the compiler is too old). Use ISA default mcpu flag instead as a safe option.
+$(warning ** WARNING ** CONFIG_ARC_TUNE_MCPU flag '$(tune-mcpu)' is unknown, fallback to '$(tune-mcpu-def-y)')
+cflags-y				+= $(tune-mcpu-def-y)
+endif
+endif
+
 
 ifdef CONFIG_ARC_CURR_IN_REG
 # For a global register defintion, make sure it gets passed to every file
-- 
2.21.3