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From: Segher Boessenkool <segher@kernel.crashing.org>
To: Pali =?iso-8859-1?Q?Roh=E1r?= <pali@kernel.org>
Subject: Re: [PATCH v1] powerpc/64: Set default CPU in Kconfig
Message-ID: <20221216191543.GE25951@gate.crashing.org>
References: <3fd60c2d8a28668a42b766b18362a526ef47e757.1670420281.git.christophe.leroy@csgroup.eu> <20221215204202.mbw2ij4ou7t2ttpv@pali>
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Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>
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Hi!

On Thu, Dec 15, 2022 at 09:42:02PM +0100, Pali Rohár wrote:
> On Wednesday 07 December 2022 14:38:40 Christophe Leroy wrote:
> >  	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.

It looks that way yes.  It was submitted at
<https://gcc.gnu.org/pipermail/gcc-patches/2009-November/273251.html>
and committed as <https://gcc.gnu.org/g:b17f98b1c541>.  It looks as if
it was based on the e500mc core, while e5500 is a new core (or
significantly different anyway).

> 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.

It appears the E500MC64 never made it outside of FSL, so it is best not
to use it at all, imo.


Segher

From mboxrd@z Thu Jan  1 00:00:00 1970
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Date:   Fri, 16 Dec 2022 13:15:43 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Pali =?iso-8859-1?Q?Roh=E1r?= <pali@kernel.org>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v1] powerpc/64: Set default CPU in Kconfig
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References: <3fd60c2d8a28668a42b766b18362a526ef47e757.1670420281.git.christophe.leroy@csgroup.eu> <20221215204202.mbw2ij4ou7t2ttpv@pali>
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Hi!

On Thu, Dec 15, 2022 at 09:42:02PM +0100, Pali Rohár wrote:
> On Wednesday 07 December 2022 14:38:40 Christophe Leroy wrote:
> >  	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.

It looks that way yes.  It was submitted at
<https://gcc.gnu.org/pipermail/gcc-patches/2009-November/273251.html>
and committed as <https://gcc.gnu.org/g:b17f98b1c541>.  It looks as if
it was based on the e500mc core, while e5500 is a new core (or
significantly different anyway).

> 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.

It appears the E500MC64 never made it outside of FSL, so it is best not
to use it at all, imo.


Segher