linux-kernel - Re: [TECH TOPIC] Reaching consensus on CONFIG

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Message-ID: <1586788245.22320.1757533057730.JavaMail.zimbra@nod.at>
Date: Wed, 10 Sep 2025 21:37:37 +0200 (CEST)
From: Richard Weinberger <richard@....at>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Arnd Bergmann <arnd@...db.de>, ksummit <ksummit@...ts.linux.dev>, 
	linux-kernel <linux-kernel@...r.kernel.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, 
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>, 
	linux-mips <linux-mips@...r.kernel.org>, 
	linux-mm <linux-mm@...ck.org>, imx <imx@...ts.linux.dev>, 
	Lucas Stach <l.stach@...gutronix.de>, 
	Linus Walleij <linus.walleij@...aro.org>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, 
	Ankur Arora <ankur.a.arora@...cle.com>, 
	David Hildenbrand <david@...hat.com>, 
	Mike Rapoport <rppt@...nel.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Matthew Wilcox <willy@...radead.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, vbabka <vbabka@...e.cz>, 
	Suren Baghdasaryan <surenb@...gle.com>, 
	Ira Weiny <ira.weiny@...el.com>, Nishanth Menon <nm@...com>, 
	heiko <heiko@...ech.de>, 
	Alexander Sverdlin <alexander.sverdlin@...il.com>, 
	"Chester A. Unal" <chester.a.unal@...nc9.com>, 
	Sergio Paracuellos <sergio.paracuellos@...il.com>, 
	Andreas Larsson <andreas@...sler.com>
Subject: Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

Christophe,

----- Ursprüngliche Mail -----
> Von: "Christophe Leroy" <christophe.leroy@...roup.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