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Date:   Mon, 11 Jul 2022 12:12:26 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v1 4/5] powerpc/44x: Fix build failure with GCC 12 (unrecognized opcode: `wrteei')
Message-ID: <20220711171226.GG25951@gate.crashing.org>
References: <8abab4888da69ff78b73a56f64d9678a7bf684e9.1657549153.git.christophe.leroy@csgroup.eu> <b0d982e223314ed82ab959f5d4ad2c4c00bedb99.1657549153.git.christophe.leroy@csgroup.eu> <CAK8P3a33jf5CUpyzWmRHpoUDMWB7TqSw5afUW=rcmMVSK8BvRA@mail.gmail.com>
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On Mon, Jul 11, 2022 at 05:05:04PM +0200, Arnd Bergmann wrote:
> Is there any value in building for -mcpu=440 or -mcpu=464 when targeting a 476?

The original 440 had a very short pipeline.  Later IBM 4xx have a longer
pipeline.  Getting this right (with -mtune=, or just with -mcpu=) is
important for performance.  So, no?

> Maybe add another !PPC_47x dependency for the first two. Ideally we would also
> enforce that 440/464 based boards cannot be selected together with 476, though
> I guess that is a separate issue.
> 
> Is there a practical difference between 440 and 464 when building kernels?
> gcc seems to treat them the same way, so maybe one option for both is enough
> here.

-mcpu= is used as the default for -mtune=, so that is always a
consideration.  PPC464 is treated the same as PPC440 in binutils as
well, so I don't think there is any issue there.


Segher