linux-kernel - Re: [PATCH 6/6] m68k/nommu: stop using GENERIC

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Message-Id: <a41f1b20-a76c-43d8-8c36-f12744327a54@app.fastmail.com>
Date: Mon, 24 Mar 2025 15:40:12 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Greg Ungerer" <gerg@...ux-m68k.org>, "Arnd Bergmann" <arnd@...nel.org>,
 Linux-Arch <linux-arch@...r.kernel.org>
Cc: "Richard Henderson" <richard.henderson@...aro.org>,
 "Matt Turner" <mattst88@...il.com>,
 "Geert Uytterhoeven" <geert@...ux-m68k.org>,
 "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
 "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
 "Helge Deller" <deller@....de>, "Madhavan Srinivasan" <maddy@...ux.ibm.com>,
 "Michael Ellerman" <mpe@...erman.id.au>,
 "Nicholas Piggin" <npiggin@...il.com>,
 "Christophe Leroy" <christophe.leroy@...roup.eu>,
 "Naveen N Rao" <naveen@...nel.org>,
 "Yoshinori Sato" <ysato@...rs.sourceforge.jp>,
 "Rich Felker" <dalias@...c.org>,
 "John Paul Adrian Glaubitz" <glaubitz@...sik.fu-berlin.de>,
 "Julian Vetter" <julian@...er-limits.org>,
 "Bjorn Helgaas" <bhelgaas@...gle.com>, linux-alpha@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
 linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org
Subject: Re: [PATCH 6/6] m68k/nommu: stop using GENERIC_IOMAP

On Mon, Mar 24, 2025, at 14:50, Greg Ungerer wrote:
> On 24/3/25 18:02, Arnd Bergmann wrote:
>> On Mon, Mar 24, 2025, at 02:33, Greg Ungerer wrote:
>>> On 15/3/25 20:59, Arnd Bergmann wrote:
>> 
>> Does this fixup work for you?
>
> Yes, this looks good, works for me.
> Feel free to add this if you like:
>
> Acked-by: Greg Ungerer <gerg@...ux-m68k.org>

Added now, thanks!

>> On a related note, I'm curious about how the MCF54xx chips are
>> used in practice, as I see that they are the only coldfire chips
>> with PCI and they all have an MMU. Are there actual users of these
>> chips that have PCI but choose not to use the MMU?
>
> No, I think everyone with these uses them with MMU enabled.
>
> It is probably more of an historical curiosity to use them with
> the MMU disabled. That supported pre-dated mainline kernels having
> full ColdFire MMU support by a good few years.

Ok, good to know. Given that there are no other chips that allow
PCI on !MMU kernels, I wonder if we should just make PCI itself
depend on MMU, and remove the "depends on MMU" for any PCI
drivers that currently have it.

There is no fundamental dependency here, but it is something that
breaks occasionally because of in-kernel interfaces that don't
work as expected on !MMU configurations, and with an extra
dependency we could stop fixing those.

      Arnd