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

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Message-ID: <20221216222359.74i6otxszwanf76y@pali>
Date:   Fri, 16 Dec 2022 23:23:59 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v1] powerpc/64: Set default CPU in Kconfig

On Friday 16 December 2022 13:15:43 Segher Boessenkool wrote:
> > 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).

Just a two old Freescale PDF files which I found and mention e500mc-64:
https://web.archive.org/web/20121215191707/http://www.freescale.com/files/ftf_2010/Americas/FTF10_ENT_F0453.pdf
https://web.archive.org/web/20120905164305/http://www.freescale.com/files/ftf_2010/Americas/FTF10_ENT_F0273.pdf

On page 6 in both documents is described P5020 with e500mc-64 cores. But
production version of P5020 contains e5500 cores.

Some other documents are also on nxp.com website:
https://www.google.com/search?q=%22e500mc-64%22+site%3Anxp.com

https://www.nxp.com/files-static/training/doc/MULTICORE_MORE.pdf
https://www.nxp.com/docs/en/supporting-information/WBNR_FTF10_NET_F0707.pdf
https://www.nxp.com/docs/en/supporting-information/WBNR_FTF10_NET_F0704.pdf

> > 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.

Yes, it really makes sense to not use e500mc64 flag. Maybe gcc
documentation could be updated to mention this fact?