linux-kernel - Re: [PATCH v1 0/3] Meson A1 32-bit support

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Message-ID: <13cf3223-d3fb-1a3e-f13a-77db3b6d144c@linaro.org>
Date:   Mon, 27 Feb 2023 15:46:50 +0100
From:   neil.armstrong@...aro.org
To:     Dmitry Rokosov <ddrokosov@...rdevices.ru>, arm@...nel.org,
        soc@...nel.org
Cc:     Alexey Romanov <avromanov@...rdevices.ru>, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, khilman@...libre.com,
        jbrunet@...libre.com, martin.blumenstingl@...glemail.com,
        linus.walleij@...aro.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel@...rdevices.ru
Subject: Re: [PATCH v1 0/3] Meson A1 32-bit support

On 27/02/2023 15:28, Dmitry Rokosov wrote:
> Hello Neil!
> 
> On Mon, Feb 27, 2023 at 09:15:04AM +0100, neil.armstrong@...aro.org wrote:
> 
> [...]
> 
>> I'm aware Amlogic also runs their kernel as 32bit to gain a few kbytes
>> of memory, but those processors are ARMv8 and the arm64 arch code
>> has been designed for those CPUs.
>>
>> So far I didn't find a single good reason to add 32bit support for
>> ARMv8 Amlogic based SoCs, if you have a solid reason please share.
> 
> I totally agree with you, but I suppose it's fully related to 'big'
> Amlogic SoC like S905_ or A311_ series. A113L (aka 'a1') is
> a cost-efficient dual-core SoC which is used for small, cheap solutions
> with cheap components. Every cent is important during BoM development.
> That's why usually ODMs install small ROM and RAM capacity, and each
> megabyte is important for RAM/ROM kernel and rootfs footprints.

Do you have figures ? is 32bit ARM kernel really lighter when ARM64 one is correctly configured ?

> Why am I talking about rootfs? For such small projects a good
> choice is buildroot rootfs assembling framework. Unfortunatelly,
> buildroot doesn't support 'compat' mode when kernel and userspace have
> a different bitness. 

well this is a buildroot problem... the kernel itself is perfectly capable
of running an AArch32 userspace.

> In the internal project, we save several
> percents of ROM/RAM free space using 32-bit configuration (mostly rootfs
> ROM space, to be honest). Therefore, for such 'little' cost-efficient
> SoCs we can make an exception and support 32-bit configuration, from my
> point of view.

32bit ARM is now "legacy", I would need to have an advice from the ARM SoC
maintainers, but AFAIK new ARMv8 SoCs should stay in arm64 arch.

Arnd ? Olof ? do you have an opinion on this ?

> 
> What do you think about that?

>>
>> And as Krzysztof stated, the support is incomplete and cannot work
>> without a dts file.
> 
> Agreed, we shouldn't merge dead code. But there are several question to
> discuss there. Please check my reply to Krzysztof message.
> 
> [...]
>