linux-kernel - Re: [RFC 1/1] ARM: Drop !ARCH_MULTIPLATFORM dependency from Cortex A8/9 erratas

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Message-Id: <b050a5b5-ca13-4ae2-baa3-41a3118e5e8a@app.fastmail.com>
Date:   Fri, 09 Dec 2022 22:36:17 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Russell King" <linux@...linux.org.uk>,
        "Sebastian Reichel" <sebastian.reichel@...labora.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, "Tony Lindgren" <tony@...mide.com>,
        "Rob Herring" <robh@...nel.org>
Subject: Re: [RFC 1/1] ARM: Drop !ARCH_MULTIPLATFORM dependency from Cortex A8/9
 erratas

On Fri, Dec 9, 2022, at 19:45, Russell King (Oracle) wrote:
> On Fri, Dec 09, 2022 at 07:20:00PM +0100, Sebastian Reichel wrote:
>> All the erratas for more recent CPUs (Cortex A15/A12/A17) do not
>> require !ARCH_MULTIPLATFORM, since there is runtime detection of
>> effected CPUs in arch/arm/mm/proc-v7.S.
>> 
>> Errata config options for the older Cortex A8/A9 CPUs have the
>> !ARCH_MULTIPLATFORM dependency, but the CPUs are also runtime
>> detected. Since there is runtime detection, it should be fine
>> to enable the erratas for a multi platform kernel.
>> 
>> Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
>> ---
>> For me the most likely explanation that this has not yet happened is
>> me missing an important detail, so I sent it as RFC. If you point it
>> out I will try to document the reason.
>
> IIRC, the reason these are marked with !ARCH_MULTIPLATFORM is for when
> the kernel is running in the non-secure world - the accessed registers
> may prevent platforms booting. However, it was a long time ago and I
> don't remember clearly now.

The git log confirms your memory, this is the description of
commit 62e4d357aae0 ("ARM: 7609/1: disable errata work-arounds
which access secure registers"):

    In order to support secure and non-secure platforms in multi-platform
    kernels, errata work-arounds that access secure only registers need to
    be disabled. Make all the errata options that fit in this category
    depend on !CONFIG_ARCH_MULTIPLATFORM.
    
    This will effectively remove the errata options as platforms are
    converted over to multi-platform.
    
    Signed-off-by: Rob Herring <rob.herring@...xeda.com>
    Acked-by: Tony Lindgren <tony@...mide.com>
    Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>

As far as I understand, these are all boot time chicken bits that
should already be set up by the boot loader before entering the
kernel. Sebastian, are you aware of any particular machines that
ship without the workaround code in the bootloader or in secure
world?

    Arnd