linux-kernel - Re: [TECH TOPIC] Reaching consensus on CONFIG

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Message-Id: <ea6562f4-ec8c-4e99-98ec-000c25c34a3e@app.fastmail.com>
Date: Thu, 18 Sep 2025 18:18:13 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Andrew Lunn" <andrew@...n.ch>
Cc: "Jason Gunthorpe" <jgg@...dia.com>, ksummit@...ts.linux.dev,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, linux-mips@...r.kernel.org,
 linux-mm@...ck.org, imx@...ts.linux.dev,
 "Christophe Leroy" <christophe.leroy@...roup.eu>,
 "Richard Weinberger" <richard@....at>,
 "Lucas Stach" <l.stach@...gutronix.de>,
 "Linus Walleij" <linus.walleij@...aro.org>,
 "Geert Uytterhoeven" <geert+renesas@...der.be>,
 "Ankur Arora" <ankur.a.arora@...cle.com>,
 "David Hildenbrand" <david@...hat.com>,
 "Mike Rapoport" <rppt@...nel.org>,
 "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>,
 "Matthew Wilcox" <willy@...radead.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 "Vlastimil Babka" <vbabka@...e.cz>,
 "Suren Baghdasaryan" <surenb@...gle.com>,
 "Ira Weiny" <ira.weiny@...el.com>, "Nishanth Menon" <nm@...com>,
 Heiko Stübner <heiko@...ech.de>,
 "Alexander Sverdlin" <alexander.sverdlin@...il.com>,
 "Chester A. Unal" <chester.a.unal@...nc9.com>,
 "Sergio Paracuellos" <sergio.paracuellos@...il.com>,
 "Andreas Larsson" <andreas@...sler.com>
Subject: Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

On Thu, Sep 18, 2025, at 15:34, Andrew Lunn wrote:
>> * Marvell mv78xx0 and kirkwood (armv5 pj1) were fairly
>>   powerful in 2008 and could support at least 1GB of RAM,
>>   but I only found one machine (OpenBlocks A7) that does
>>   this. It's unclear if anyone is still updating kernels
>>   on this machine, but they could /probably/ use
>>   VMSPLIT_3G_OPT if they do.
>
> If i remember correctly, there was a design issue with the OpenBlocks
> A7, and it would not run with its full amount of memory. To get a
> stable system you had to limit the RAM. I don't remember if that was
> just with the NULL series, and it was fixed for mass production
> devices, or they are all broken.

It's possible that the bug is in the exact bit of code that
Jason was suggesting to have removed, if this was the last
ARMv5 machine that actually tried to use highmem.

> I doubt there are any mv78xx0 machines left, why where never very
> popular, but there are still Kirkwood NAS boxes around.

https://github.com/1000001101000/Debian_on_Buffalo does support
some variations of the Terastation Duo, and they were planning
to do a DT conversion at some point, but I think there is not
much hope of that ever happening now that Debian armel has had
its last release, and I expect we can drop the entire platform
soon.

The reference system for mv78xx0 of course was the platform
that Nico used for implementing highmem on, with a full 2GB
of RAM. I think in the end this was around the same time
as dove and then armada xp.

> I keep mine up to date, put an LTS kernel on it once a year,
> update to the latest debian sid.

Do you know of any kirkwood machines beside the OpenBlocks A7
that have more than 512MB of RAM?

    Arnd