Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

Richard Weinberger
2025-09-10T19:37:40Z

Christophe,

----- Ursprüngliche Mail -----
> Von: "Christophe Leroy" <christophe.leroy@csgroup.eu>
> Can you tell which PPC32 model/family you are using ? Is it mpc85xx or
> and/or other variants ? Maybe we can look at keeping CONFIG_HIGHMEM or
> find alternatives for that subset of PPC32 only.

Off the head, MPC8343 and QorIQ P1022.

>
> Could you also elaborate a bit on the kind of issues you forsee or fear
> with applications and FPGA device drivers.

Unfortunately, applications in this area often make certain assumptions
about the amount or layout of memory. This becomes a bigger problem when
using FPGA device drivers of questionable quality, which share memory
between the kernel, userspace, and the device itself.
These are mostly out-of-tree drivers, so that's my horse to ride,
I'm just mentally preparing myself for "interesting" bugs that will need
to be sorted out.

> FWIW I sent out today a patch that removes CONFIG_HIGHMEM complely on
> powerpc in order to get a better view of the impacted areas and allow
> people to test what it looks like on their system without
> CONFIG_HIGHMEM. See [1].

Oh, that was fast. :-)

Thanks,
//richard