powerpc/32: Don't always pass -mcpu=powerpc to the compiler

Since commit 4bf4f42a ("powerpc/kbuild: Set default generic
machine type for 32-bit compile"), when building a 32 bits kernel
with a bi-arch version of GCC, or when building a book3s/32 kernel,
the option -mcpu=powerpc is passed to GCC at all time, relying on it
being eventually overriden by a subsequent -mcpu=xxxx.

But when building the same kernel with a 32 bits only version of GCC,
that is not done, relying on gcc being built with the expected default
CPU.

This logic has two problems. First, it is a bit fragile to rely on
whether the GCC version is bi-arch or not, because today we can have
bi-arch versions of GCC configured with a 32 bits default. Second,
there are some versions of GCC which don't support -mcpu=powerpc,
for instance for e500 SPE-only versions.

So, stop relying on this approximative logic and allow the user to
decide whether he/she wants to use the toolchain's default CPU or if
he/she wants to set one, and allow only possible CPUs based on the
selected target.
Reported-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d4df724691351531bf46d685d654689e5dfa0d74.1657549153.git.christophe.leroy@csgroup.eu

arch/powerpc/Makefile

@@ -15,23 +15,6 @@ HAS_BIARCH	:= $(call cc-option-yn, -m32)
 # Set default 32 bits cross compilers for vdso and boot wrapper
 CROSS32_COMPILE ?=
 
-ifeq ($(HAS_BIARCH),y)
-ifeq ($(CROSS32_COMPILE),)
-ifdef CONFIG_PPC32
-# These options will be overridden by any -mcpu option that the CPU
-# or platform code sets later on the command line, but they are needed
-# to set a sane 32-bit cpu target for the 64-bit cross compiler which
-# may default to the wrong ISA.
-KBUILD_CFLAGS		+= -mcpu=powerpc
-KBUILD_AFLAGS		+= -mcpu=powerpc
-endif
-endif
-endif
-
-ifdef CONFIG_PPC_BOOK3S_32
-KBUILD_CFLAGS		+= -mcpu=powerpc
-endif
-
 # If we're on a ppc/ppc64/ppc64le machine use that defconfig, otherwise just use
 # ppc64_defconfig because we have nothing better to go on.
 uname := $(shell uname -m)
@@ -183,6 +166,7 @@ endif
 endif
 
 CFLAGS-$(CONFIG_TARGET_CPU_BOOL) += $(call cc-option,-mcpu=$(CONFIG_TARGET_CPU))
+AFLAGS-$(CONFIG_TARGET_CPU_BOOL) += $(call cc-option,-mcpu=$(CONFIG_TARGET_CPU))
 
 # Altivec option not allowed with e500mc64 in GCC.
 ifdef CONFIG_ALTIVEC
@@ -193,14 +177,6 @@ endif
 CFLAGS-$(CONFIG_E5500_CPU) += $(E5500_CPU)
 CFLAGS-$(CONFIG_E6500_CPU) += $(call cc-option,-mcpu=e6500,$(E5500_CPU))
 
-ifdef CONFIG_PPC32
-ifdef CONFIG_PPC_E500MC
-CFLAGS-y += $(call cc-option,-mcpu=e500mc,-mcpu=powerpc)
-else
-CFLAGS-$(CONFIG_E500) += $(call cc-option,-mcpu=8540 -msoft-float,-mcpu=powerpc)
-endif
-endif
-
 asinstr := $(call as-instr,lis 9$(comma)foo@high,-DHAVE_AS_ATHIGH=1)
 
 KBUILD_CPPFLAGS	+= -I $(srctree)/arch/$(ARCH) $(asinstr)

arch/powerpc/platforms/Kconfig.cputype

@@ -135,9 +135,9 @@ config GENERIC_CPU
 	select ARCH_HAS_FAST_MULTIPLIER
 	select PPC_64S_HASH_MMU
 
-config GENERIC_CPU
+config POWERPC_CPU
 	bool "Generic 32 bits powerpc"
-	depends on PPC32 && !PPC_8xx
+	depends on PPC32 && !PPC_8xx && !PPC_85xx
 
 config CELL_CPU
 	bool "Cell Broadband Engine"
@@ -196,11 +196,23 @@ config G4_CPU
 	depends on PPC_BOOK3S_32
 	select ALTIVEC
 
+config E500_CPU
+	bool "e500 (8540)"
+	depends on PPC_85xx && !PPC_E500MC
+
+config E500MC_CPU
+	bool "e500mc"
+	depends on PPC_85xx && PPC_E500MC
+
+config TOOLCHAIN_DEFAULT_CPU
+	bool "Rely on the toolchain's implicit default CPU"
+	depends on PPC32
+
 endchoice
 
 config TARGET_CPU_BOOL
 	bool
-	default !GENERIC_CPU
+	default !GENERIC_CPU && !TOOLCHAIN_DEFAULT_CPU
 
 config TARGET_CPU
 	string
@@ -215,6 +227,9 @@ config TARGET_CPU
 	default "e300c2" if E300C2_CPU
 	default "e300c3" if E300C3_CPU
 	default "G4" if G4_CPU
+	default "8540" if E500_CPU
+	default "e500mc" if E500MC_CPU
+	default "powerpc" if POWERPC_CPU
 
 config PPC_BOOK3S
 	def_bool y