linux-kernel - Highmem on AXM and K2

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Message-ID: <8f104eba-6805-46e0-90da-232ce18973c5@nokia.com>
Date: Fri, 19 Sep 2025 15:24:57 +0200
From: Stefan Wiehler <stefan.wiehler@...ia.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Highmem on AXM and K2

Hi Arnd,

You've been calling out for users still needing highmem, and here we are ;-) We
use both the Intel AXM 5516 and the TI KeyStone II with > 4 GB of RAM in our
networking products.

We use the AXM 5516 with 6 GB of RAM and highmem is therefore a must for us.
The latest estimate from product management for EOL is June 2029. We would
appreciate if this date (plus some buffer) could be kept in mind when choosing
the last LTS kernel with highmem.

With the TI K2, the situation is more complicated: The EOL for the latest
product hasn't even been defined yet, but most likely we need to support it
until 2037; it really depends on when the last 2G/3G networks will be shut
down. Obviously the community cannot wait that long with highmem removal. While
we have 5 GB of RAM there, a little bit less than half is used by Linux. I see
two options:

1. We'll need to evaluate if we could move away from current CONFIG_MEMSPLIT_3G
with our rather special memory configuration. My current understanding is that
there hasn't been a lot of interest in getting CONFIG_VMSPLIT_4G_4G into
mainline. As we cannot accept major performance degradation, I doubt that this
will be a viable path.
2. We'll need to switch over to the last highmem-enabled SLTS kernel, earliest
around 2028 (to keep some support buffer).

Kind regards,

Stefan