linux-kernel - Re: 32-bit HIGHMEM and game console downstreams

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Message-Id: <432e049f-886d-4734-ad59-52569a796046@app.fastmail.com>
Date: Sat, 13 Sep 2025 15:52:58 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Ash Logan" <ash@...quark.com>,
 "Christophe Leroy" <christophe.leroy@...roup.eu>
Cc: "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 officialTechflashYT@...il.com, "A. Wilcox" <AWilcox@...cox-tech.com>,
 "Michael Ellerman" <mpe@...erman.id.au>
Subject: Re: 32-bit HIGHMEM and game console downstreams

On Sat, Sep 13, 2025, at 12:53, Ash Logan wrote:

Hi Ash,

> All of these are flatmem devices, as that's all the 32-bit PowerPC arch 
> supports, with the Wii U additionally enabling highmem for its 2GB of 
> RAM. Both devices have a small memory area (MEM1) with the "bulk" of RAM 
> starting at 256MiB. The Wii U in particular sounds like a candidate 
> system for densemem - I would like to read up more about this if I can, 
> I was only able to find seemingly unrelated information about CXL when 
> searching online.

I would not expect densemem to make a difference, at least if we go
with 256MB chunks (the size has not been decided, and not much code
exists in the first place).

Like most other machines, this one is probably fine with a combination
of a custom LOWMEM_SIZE setting and using zram-highmem, even if we
end up removing support for highmem page cache.

The smaller devices are probable getting problematic sooner, 96MB
in the Wii is already really tight and this only gets worse over
time.

> I know I'm talking about hobbyist use - and mostly downstream use at 
> that - and I do suspect that in the event of a wider 32-bit deprecation 
> we'd be fine on the final LTS, whatever that ends up being. Still, I 
> think the Wii and Wii U represent a decent number of 32-bit users, so I 
> wanted to add to the conversation here.

Just to be clear: there is no general 32-bit deprecation going on. When
I talked about phasing out 32-bit platforms over time, that is purely
going to be those that have no users left, or the few ones that are
causing more work than they are worth. E.g. The ppc405 ones got
removed recently (after many years of discussion) because they were
making ppc440 maintenance harder and had no known users.

Highmem does get in the way, but unless more -mm folks make a strong
argument in favor of removing it all, it's more likely that we'll
go with Willy's suggestion of keeping highmem on page cache (anon
and file mappings) than just keeling zram, and even that would
still work.

     Arnd