linux-kernel - Re: [RFC} arm architecture board/feature deprecation timeline

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Message-ID: <CACRpkdYUHtfjL2jfGxjV1eMZGkk6NXaArbHpc5mhrY7C9rSi6g@mail.gmail.com>
Date: Thu, 1 Aug 2024 21:53:32 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Russell King <linux@...linux.org.uk>, Richard Earnshaw <richard.earnshaw@....com>, 
	Richard Sandiford <richard.sandiford@....com>, Ramana Radhakrishnan <ramanara@...dia.com>, 
	Nicolas Pitre <nico@...xnic.net>, Krzysztof Kozlowski <krzk@...nel.org>, Mark Brown <broonie@...nel.org>, 
	Kristoffer Ericson <kristoffer.ericson@...il.com>, Robert Jarzmik <robert.jarzmik@...e.fr>, 
	Aaro Koskinen <aaro.koskinen@....fi>, Janusz Krzysztofik <jmkrzyszt@...il.com>, 
	Tony Lindgren <tony@...mide.com>, linux-omap@...r.kernel.org, 
	Nikita Shubin <nikita.shubin@...uefel.me>, linux-samsung-soc@...r.kernel.org, 
	Andrew Lunn <andrew@...n.ch>, Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>, 
	Gregory Clement <gregory.clement@...tlin.com>, "Jeremy J. Peper" <jeremy@...emypeper.com>, 
	debian-arm@...ts.debian.org, Dmitry Torokhov <dmitry.torokhov@...il.com>, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>
Subject: Re: [RFC} arm architecture board/feature deprecation timeline

On Wed, Jul 31, 2024 at 7:29 PM Arnd Bergmann <arnd@...db.de> wrote:

> === ARMv4 ===
>
> This is used for both StrongARM and FA526 CPUs, which are still
> used on a small number of boards. Even the newest chips (moxa
> art, ) are close to 20 years olds but were still in use a few years
> ago. The last Debian release for these was Lenny (5.0).
>
> Dropping compiler support now would be appropriate IMHO, and
> we can drop kernel support in a few years.

I am actively using the Gemini as my NAS with OpenWrt and there
are several users of it in the OpenWrt community.

I don't know if there are enough of us to keep ARMv4 support in
GCC, but ARMv4 support has been added to CLANG (along
with ARMv4t), and I have tested to compile kernels for these
devices with CLANG (for testing CFI!) and they work fine, so if
GCC drops it, we can still build them with CLANG where it apparently
isn't a maintenance burden given that it was recently added.

Maybe CLANG has a more adaptive backend?

> === Highmem ===
>
> Most Arm machines are fine without highmem support and can
> use something like CONFIG_VMSPLIT_2GB to address up to 2GB
> of physical memory. Machines larger than only popped up
> around the time of the Cortex-A15 in 2012 and for the most
> part got replaced by 64-bit chips within a short time.
> In addition, there are also a handful of Cortex-A9 and
> Marvell CPU based machines that have either more than 2GB
> of RAM or a very sparse memory map that requires highmem
> support.
>
> Linus Walleij has done some work towards being able to use
> up to 4GB of RAM with LPAE (Cortex-A7/A15 and later)
> machines, which I think still needs to be finished before
> we can remove support for highmem.

This is either really hard, or I'm a bad developer. But I keep
churning it.

> === Gemini, Moxart ===
>
> These both use the Faraday FA526 CPU core that like
> StrongARM implements ARMv4 rather than ARMv4T with thumb.
>
> The chips are also over 20 years old, but the kernel
> code has been updated and is not a maintenance burden
> by itself, so there is no value in removing these
> machines until StrongARM is also gone.
>
> On the other hand, removing both FA526 and StrongARM
> platforms means we can probably remove ARMv4 (non-T),
> OABI and NWFPE support more quickly if we want, or
> we can wait until a few years after gcc drops ARMv4.
>
> OpenWRT lists the gemini platform as supported in
> https://openwrt.org/docs/techref/targets/gemini, but
> none of the individual machines have builds for the
> current release.
>
> Need input from Linus Walleij.

Yeah we use these devices. I don't know what counts as big
enough community to be considered, it's at least not just
me.

Gemini builds are in the official OpenWrt repos:
https://downloads.openwrt.org/releases/23.05.4/targets/gemini/generic/

Yours,
Linus Walleij