linux-kernel - Re: [PATCH v1] powerpc/64: Set default CPU in Kconfig

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Date:   Thu, 15 Dec 2022 21:42:02 +0100
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
Subject: Re: [PATCH v1] powerpc/64: Set default CPU in Kconfig

Hello!

On Wednesday 07 December 2022 14:38:40 Christophe Leroy wrote:
> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
...
> @@ -166,11 +153,12 @@ CC_FLAGS_FTRACE += -mprofile-kernel
>  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))
> +CFLAGS-$(CONFIG_TARGET_CPU_BOOL) += -mcpu=$(CONFIG_TARGET_CPU)
> +AFLAGS-$(CONFIG_TARGET_CPU_BOOL) += -mcpu=$(CONFIG_TARGET_CPU)
>  
> -CFLAGS-$(CONFIG_E5500_CPU) += $(call cc-option,-mcpu=e500mc64,-mcpu=powerpc64)
> -CFLAGS-$(CONFIG_E6500_CPU) += $(call cc-option,-mcpu=e6500,$(E5500_CPU))
> +CFLAGS-$(CONFIG_POWERPC64_CPU) += $(call cc-option,-mtune=power10,	\
> +				  $(call cc-option,-mtune=power9,	\
> +				  $(call cc-option,-mtune=power8)))
>  
>  asinstr := $(call as-instr,lis 9$(comma)foo@...h,-DHAVE_AS_ATHIGH=1)
>  
> diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
...
> @@ -251,6 +249,10 @@ config TARGET_CPU
>  	default "power8" if POWER8_CPU
>  	default "power9" if POWER9_CPU
>  	default "power10" if POWER10_CPU
> +	default "e500mc64" if E5500_CPU

Now I'm looking at this change again... and should not E5500_CPU rather
enforce -mcpu=e5500 flag? I know that your patch moves e500mc64 flag
from the Makefile to Kconfig, but maybe it could be changed in some
other followup patch...

Anyway, do you know what is e500mc64 core? I was trying to find some
information about it, but it looks like some unreleased freescale core
which predates e5500 core. ISA (without extensions like altivec) seems
to be same for e500mc64, e5500 and e6500 cores and difference is only
pipeline definitions in gcc config files. So if my understanding is
correct then kernel binary compiled with any of these -mcpu= flag should
work on any of those cores. Just for mismatches core binary will not be
optimized for speed.

> +	default "e6500" if E6500_CPU
> +	default "power4" if POWERPC64_CPU && !CPU_LITTLE_ENDIAN
> +	default "power8" if POWERPC64_CPU && CPU_LITTLE_ENDIAN
>  	default "405" if 405_CPU
>  	default "440" if 440_CPU
>  	default "464" if 464_CPU
> -- 
> 2.38.1
>