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Date:   Thu, 30 Mar 2023 18:40:39 +0200
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Subject: Re: [PATCH 18/21] ARM: drop SMP support for ARM11MPCore
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To:     Arnd Bergmann <arnd@arndb.de>, Arnd Bergmann <arnd@kernel.org>,
        linux-kernel@vger.kernel.org
Cc:     Vineet Gupta <vgupta@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Linus Walleij <linus.walleij@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, guoren <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-oxnas@groups.io>,
        "linux-csky@vger.kernel.org" <linux-csky@vger.kernel.org>,
        linux-hexagon@vger.kernel.org, linux-m68k@lists.linux-m68k.org,
        linux-mips@vger.kernel.org,
        "linux-openrisc@vger.kernel.org" <linux-openrisc@vger.kernel.org>,
        linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-riscv@lists.infradead.org, linux-sh@vger.kernel.org,
        sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org,
        Daniel Golle <daniel@makrotopia.org>
References: <20230327121317.4081816-1-arnd@kernel.org>
 <20230327121317.4081816-19-arnd@kernel.org>
 <d7f36a28-0cae-a035-791b-363754aefeee@linaro.org>
 <6a5bd3a3-87e0-4a24-89ca-d265b0817a95@app.fastmail.com>
From:   Neil Armstrong <neil.armstrong@linaro.org>
In-Reply-To: <6a5bd3a3-87e0-4a24-89ca-d265b0817a95@app.fastmail.com>
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Le 30/03/2023 à 12:03, Arnd Bergmann a écrit :
> On Thu, Mar 30, 2023, at 09:48, Neil Armstrong wrote:
>> On 27/03/2023 14:13, Arnd Bergmann wrote:
>>> 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 written
>>>      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.
>>
>> Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
>>
>> It's sad but it's the reality, there's no chance full OXNAS support will
>> ever come upstream and no real work has been done for years.
>>
>> I think OXNAS support can be programmed for removal for next release,
>> it would need significant work to rework current support to make it acceptable
>> before trying to upstream missing bits anyway.
> 
> Ok, thanks for your reply!
> 
> To clarify, do you think we should plan for removal after the next
> stable release (6.3, removed in 6.4), or after the next LTS
> release (probably 6.6, removed in 6.7)? As far as I understand,
> the next OpenWRT release (23.x) will be based on linux-5.15,
> and the one after that (24.x) would likely still use 6.1, unless
> they skip an LTS kernel.

I think it's ok to remove it ASAP, or at least before the next LTS,
not having SMP makes the platform barely usable so the earliest is the best.

Neil

> 
>       Arnd