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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Thu, 30 Mar 2023 10:12:00 +0200
Message-ID: <CACRpkdYuyzkvSQY0sQJei39d9ZoGM_DbJQ5LxUg3sXsBmzJyaQ@mail.gmail.com>
Subject: Re: [PATCH 18/21] ARM: drop SMP support for ARM11MPCore
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Vineet Gupta <vgupta@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Neil Armstrong <neil.armstrong@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Guo Ren <guoren@kernel.org>,
        Brian Cain <bcain@quicinc.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Michal Simek <monstr@monstr.eu>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Dinh Nguyen <dinguyen@kernel.org>,
        Stafford Horne <shorne@gmail.com>,
        Helge Deller <deller@gmx.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Rich Felker <dalias@libc.org>,
        John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
        "David S. Miller" <davem@davemloft.net>,
        Max Filippov <jcmvbkbc@gmail.com>,
        Christoph Hellwig <hch@lst.de>,
        Robin Murphy <robin.murphy@arm.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>,
        Conor Dooley <conor.dooley@microchip.com>,
        linux-snps-arc@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-oxnas@groups.io,
        linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org,
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        linux-xtensa@linux-xtensa.org, Daniel Golle <daniel@makrotopia.org>
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On Mon, Mar 27, 2023 at 2:16=E2=80=AFPM Arnd Bergmann <arnd@kernel.org> wro=
te:

> From: Arnd Bergmann <arnd@arndb.de>
>
> The cache management operations for noncoherent DMA on ARMv6 work
> in two different ways:
>
>  * When CONFIG_DMA_CACHE_RWFO is set, speculative prefetches on in-flight
>    DMA buffers lead to data corruption when the prefetched data is writte=
n
>    back on top of data from the device.
>
>  * When CONFIG_DMA_CACHE_RWFO is disabled, a cache flush on one CPU
>    is not seen by the other core(s), leading to inconsistent contents
>    accross the system.
>
> As a consequence, neither configuration is actually safe to use in a
> general-purpose kernel that is used on both MPCore systems and ARM1176
> with prefetching enabled.
>
> We could add further workarounds to make the behavior more dynamic based
> on the system, but realistically, there are close to zero remaining
> users on any ARM11MPCore anyway, and nobody seems too interested in it,
> compared to the more popular ARM1176 used in BMC2835 and AST2500.
>
> The Oxnas platform has some minimal support in OpenWRT, but most of the
> drivers and dts files never made it into the mainline kernel, while the
> Arm Versatile/Realview platform mainly serves as a reference system but
> is not necessary to be kept working once all other ARM11MPCore are gone.
>
> Take the easy way out here and drop support for multiprocessing on
> ARMv6, along with the CONFIG_DMA_CACHE_RWFO option and the cache
> management implementation for it. This also helps with other ARMv6
> issues, but for the moment leaves the ability to build a kernel that
> can run on both ARMv7 SMP and single-processor ARMv6, which we probably
> want to stop supporting as well, but not as part of this series.
>
> Cc: Neil Armstrong <neil.armstrong@linaro.org>
> Cc: Daniel Golle <daniel@makrotopia.org>
> Cc: Linus Walleij <linus.walleij@linaro.org>
> Cc: linux-oxnas@groups.io
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Yeah, we discussed this earlier, let's just drop it. Not worth the effort.
Acked-by: Linus Walleij <linus.walleij@linaro.org>

Yours,
Linus Walleij