Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

Linus Walleij
2025-09-12T12:46:47Z

On Fri, Sep 12, 2025 at 12:31 PM Arnd Bergmann <arnd@arndb.de> wrote:
> On Wed, Sep 10, 2025, at 22:33, Richard Weinberger wrote:
> > ----- Ursprüngliche Mail -----
> >> Von: "Dave Hansen" <dave@sr71.net>
> >>> Even with a new memory split, which could utilize most of the
> >>> available memory, I expect there to be issues with various
> >>> applications and FPGA device drivers.
>
> I also remember driver problems on older Marvell NAS systems, which
> we never fully figured out, my best guess in retrospect is that these
> had devices with DMA address restrictions, and if lowmem is small
> enough it would always work, but any lowmem allocation above the
> hardware DMA address limit would cause data corruption.

This kind of mess is often also driver problems, in the Kirkwood MMC
driver there is some dated code traversing sglists iteratively
instead of using sg_miter() on !DMA which I think is not entirely
safe either.

I wanted to fix that driver but the hardware is in my NAS and
all the time someone is watching a movie from it :D

Yours,
Linus Walleij