[POWERPC] add Kconfig option for optimizing for cell

Since the PPE on cell is an in-order core, it suffers significantly
from wrong instruction scheduling.  This adds a Kconfig option that
enables passing -mtune=cell to gcc in order to generate object
code that runs well on cell.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/Makefile

@@ -92,6 +92,10 @@ else
 endif
 endif
 
+ifeq ($(CONFIG_TUNE_CELL),y)
+	CFLAGS += $(call cc-option,-mtune=cell)
+endif
+
 # No AltiVec instruction when building kernel
 CFLAGS += $(call cc-option,-mno-altivec)
 

arch/powerpc/platforms/Kconfig.cputype

@@ -71,6 +71,18 @@ config POWER4
 	depends on PPC64
 	def_bool y
 
+config TUNE_CELL
+	bool "Optimize for Cell Broadband Engine"
+	depends on PPC64
+	help
+	  Cause the compiler to optimize for the PPE of the Cell Broadband
+	  Engine. This will make the code run considerably faster on Cell
+	  but somewhat slower on other machines. This option only changes
+	  the scheduling of instructions, not the selection of instructions
+	  itself, so the resulting kernel will keep running on all other
+	  machines. When building a kernel that is supposed to run only
+	  on Cell, you should also select the POWER4_ONLY option.
+
 config 6xx
 	bool