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

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Message-ID: <20230227155100.hhl4yvkyfqfyoa6h@CAB-WSD-L081021>
Date:   Mon, 27 Feb 2023 18:51:00 +0300
From:   Dmitry Rokosov <ddrokosov@...rdevices.ru>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Neil Armstrong <neil.armstrong@...aro.org>,
        Alexey Romanov <avromanov@...rdevices.ru>,
        Rob Herring <robh+dt@...nel.org>,
        <krzysztof.kozlowski+dt@...aro.org>,
        Kevin Hilman <khilman@...libre.com>, <jbrunet@...libre.com>,
        <martin.blumenstingl@...glemail.com>,
        Linus Walleij <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

Hello Arnd,

On Mon, Feb 27, 2023 at 03:58:50PM +0100, Arnd Bergmann wrote:
> On Mon, Feb 27, 2023, at 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.
> > 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. 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.
> >
> > What do you think about that?
> 
> I would argue that is a problem with buildroot, and using a 32-bit
> kernel is not something we should encourage over fixing buildroot
> to do it right, or building the kernel separately from the rootfs.
> 
> We do allow building support for a couple of ARMv8 SoCs in 32-bit
> mode, but that is usually because they ship with a 32-bit bootrom
> and cannot actually run a 64-bit kernel.

To be honest, I didn't know about this principle. It looks like a very
rational approach "start from max supported bitness".
Based on overall maintainers opinion, we have to prepare a patch for
buildroot to support compat mode :)

[...]

-- 
Thank you,
Dmitry