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

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Message-ID: <7hedpxwq1i.fsf@baylibre.com>
Date:   Thu, 09 Mar 2023 13:52:41 -0800
From:   Kevin Hilman <khilman@...libre.com>
To:     Dmitry Rokosov <ddrokosov@...rdevices.ru>,
        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, 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

Dmitry Rokosov <ddrokosov@...rdevices.ru> writes:

> On Mon, Feb 27, 2023 at 07:19:38PM +0100, Arnd Bergmann wrote:
>> On Mon, Feb 27, 2023, at 17:50, Dmitry Rokosov wrote:
>> > On Mon, Feb 27, 2023 at 05:38:49PM +0100, Neil Armstrong wrote:
>> >> On 27/02/2023 17:15, Arnd Bergmann wrote:
>> >> > On Mon, Feb 27, 2023, at 16:51, Dmitry Rokosov wrote:
>> >> > 
>> >> > Most of these don't apply in userspace, so the incentive to
>> >> > run smaller 32-bit userland on systems with less than 1GB of
>> >> > RAM usually outweighs the benefits of 64-bit userspace.
>> >> 
>> >> Thanks for the details!
>> >
>> > Looks like Thomas has already prepared a basic patch series for buildroot,
>> > but maintainers declined it.
>> >
>> > https://lore.kernel.org/all/20220730194331.GA2515056@scaer/
>> 
>> I see. I know very little about buildroot, but it sounds like
>> there are other ways of doing the same thing here. In general,
>> this is pretty much an Arm specific problem. While you clearly
>> want compat mode for small userland on any architecture but don't
>> want 32-bit kernels, arm is the only one that has a different
>> kernel "ARCH=" value and needs a separate gcc toolchain.
>> 
>> If the problem is only the toolchain, an easy way out may
>> be to use clang instead of gcc as your compiler, as a single
>> clang binary can target both 32-bit userland and 64-bit kernel
>> on all supported architectures.
>
> Agreed with you. We will try different local approaches to support
> compat build configurations. For now, prebuilt toolchain (buildroot make
> sdk goal) is best way from my point of view. Anyway, we will try to
> solve this problem in the our sandbox and stay on the 64-bit kernel.
> Thank you for all the helpful details you shared, appreciate it!

Just to clarify one thing...

More specifically, this is a buildroot *build system* problem.  If you
build the kernel separately from the rootfs, it works fine. 

I use 32-bit buildroot (and debian) rootfs images all the time on
Amlogic SoCs with 64-bit kernels and it works fine.  

Kevin