linux-kernel - Re: [PATCH] powerpc/32: Don't always pass -mcpu=powerpc to the compiler

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Date:   Sun, 10 Jul 2022 20:11:31 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Arnd Bergmann <arnd@...db.de>,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH] powerpc/32: Don't always pass -mcpu=powerpc to the
 compiler

On Sunday 10 July 2022 19:36:58 Christophe Leroy wrote:
> Since commit 4bf4f42a2feb ("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@...nel.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Segher Boessenkool <segher@...nel.crashing.org>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

Tested-by: Pali Rohár <pali@...nel.org>

Works fine with SPE-based gcc for e500v2 platform Freescale P2020.

I have small suggestion for naming convention, see below.

> ---
>  arch/powerpc/Makefile                  | 26 +-------------------------
>  arch/powerpc/platforms/Kconfig.cputype | 24 ++++++++++++++++++++----
>  2 files changed, 21 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
> index a0cd70712061..d54e1fe03551 100644
> --- a/arch/powerpc/Makefile
> +++ b/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@...h,-DHAVE_AS_ATHIGH=1)
>  
>  KBUILD_CPPFLAGS	+= -I $(srctree)/arch/$(ARCH) $(asinstr)
> diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
> index 383ed4fe6013..197653442d9f 100644
> --- a/arch/powerpc/platforms/Kconfig.cputype
> +++ b/arch/powerpc/platforms/Kconfig.cputype
> @@ -117,7 +117,8 @@ endchoice
>  
>  choice
>  	prompt "CPU selection"
> -	default GENERIC_CPU
> +	default GENERIC_CPU if PPC64
> +	default POWERPC_CPU if PPC32
>  	help
>  	  This will create a kernel which is optimised for a particular CPU.
>  	  The resulting kernel may not run on other CPUs, so use this with care.
> @@ -135,9 +136,13 @@ 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 && !E500
> +
> +config TOOLCHAIN_DEFAULT_CPU
> +	bool "Rely on the toolchain's implicit default CPU"
> +	depends on PPC32
>  
>  config CELL_CPU
>  	bool "Cell Broadband Engine"
> @@ -196,11 +201,19 @@ config G4_CPU
>  	depends on PPC_BOOK3S_32
>  	select ALTIVEC
>  
> +config 8540_CPU
> +	bool "8540"
> +	depends on PPC_85xx && !PPC_E500MC

MPC8540 is name of the first (Motorola) SoC which had e500(v1) core.
e500 cores are used also in new Freescale P1/P2 series and also in
BSC913x and C29x products.

So I would suggest to call this 'config E500_CPU' with description
'bool "e500 (8540)"' because all other *_CPU options are named by the
CPU core, not by the SoC product which core contains. There is e.g.
E5500_CPU, E300C2_CPU or E500MC_CPU.

It is confusing that gcc named this option "-mcpu=8540" and not
"-mcpu=e500". For example GNU AS call this option "-me500" and I read
somewhere that clang/llvm wanted to add "-mcpu=e500" as an alias.

For example with this change:

diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index 6c2b2d2c8bfc..8c8dd3c066f9 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -202,8 +202,8 @@ config G4_CPU
 	depends on PPC_BOOK3S_32
 	select ALTIVEC
 
-config 8540_CPU
-	bool "8540"
+config E500_CPU
+	bool "e500 (8540)"
 	depends on PPC_85xx && !PPC_E500MC
 
 config E500MC_CPU
@@ -229,7 +229,7 @@ config TARGET_CPU
 	default "e300c2" if E300C2_CPU
 	default "e300c3" if E300C3_CPU
 	default "G4" if G4_CPU
-	default "8540" if 8540_CPU
+	default "8540" if E500_CPU
 	default "e500mc" if E500MC_CPU
 	default "powerpc" if POWERPC_CPU
 

> +
> +config E500MC_CPU
> +	bool "e500mc"
> +	depends on PPC_85xx && PPC_E500MC
> +
>  endchoice
>  
>  config TARGET_CPU_BOOL
>  	bool
> -	default !GENERIC_CPU
> +	default !GENERIC_CPU && !TOOLCHAIN_DEFAULT_CPU
>  
>  config TARGET_CPU
>  	string
> @@ -215,6 +228,9 @@ config TARGET_CPU
>  	default "e300c2" if E300C2_CPU
>  	default "e300c3" if E300C3_CPU
>  	default "G4" if G4_CPU
> +	default "8540" if 8540_CPU
> +	default "e500mc" if E500MC_CPU
> +	default "powerpc" if POWERPC_CPU
>  
>  config PPC_BOOK3S
>  	def_bool y
> -- 
> 2.36.1
>