Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

Nicolas Ferre
2025-09-12T16:49:49Z

Arnd,

On 09/09/2025 at 23:23, Arnd Bergmann wrote:
> I'm still collecting information about which of the remaining highmem
> users plan to keep updating their kernels and for what reason.

We have 1GB of memory on our latest Cortex-A7 SAMA7D65 evaluation boards
[1] (full production announced beg. 2025). The wide range of DDR types
supported make some of these types interesting to use at such density.
Both our Cortex-A7 SoCs don't have IOMMU; core and DMAs can address the
full range of the 32 bit address space, so we're quite
standard/simplistic in this area. We use CMA with large chunks as our
camera or display interfaces address "modern-ish" resolutions (~1080p).

We use CONFIG_HIGHMEM and activated it for simplicity, conformance to
usual user-space workloads and planned to add it to our sama7_defconfig
[2]. I understand that we might reconsider this "by default" choice and
move to one of the solutions you highlighted in your message, lwn.net
article or recent talk at ELC-E.

Of course we plan to maintain these boards and keep updating our kernel
"offer" once a year for those associated SoCs (with maintaining
upstream, as usual). As you said, being ARMv7, we're quite confident for
now.

As you mentioned, we've recently released one ARMv5te arm926ejs-based
soc: the SAM9x75 family. But we don't have the intention to use too big
memory sizes on them, even if they do address large screens, with LVDS
and MIPI or modern camera interfaces...

I don't have too much info about our customer's use cases as they are
very, very diverse, but don't hesitate to reach out to me if you have
questions about a particular combination of use.
Thanks for your regular update on these topics.

Best regards,
Nicolas

[1]: for instance:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/microchip/at91-sama7d65_curiosity.dts#n29

[2]: