linux-kernel - Re: [PATCH RESEND v11 10/13] um: nommu: a work around for MMU dependency to PCI driver

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Message-ID: <CAMuHMdW-417mPdhx8VzD5HK+OBzR6wOwgve7TQVaDB=B1ACQhQ@mail.gmail.com>
Date: Wed, 24 Sep 2025 10:02:55 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Hajime Tazaki <thehajime@...il.com>
Cc: johannes@...solutions.net, linux-um@...ts.infradead.org, 
	ricarkol@...gle.com, Liam.Howlett@...cle.com, linux-kernel@...r.kernel.org, 
	arnd@...db.de
Subject: Re: [PATCH RESEND v11 10/13] um: nommu: a work around for MMU
 dependency to PCI driver

Hi Hajime,

On Wed, 24 Sept 2025 at 01:51, Hajime Tazaki <thehajime@...il.com> wrote:
> On Wed, 24 Sep 2025 02:13:47 +0900,
> Johannes Berg wrote:
> > On Tue, 2025-09-23 at 17:42 +0200, Geert Uytterhoeven wrote:
> > > > currently, drivers/pci/Kconfig (CONFIG_PCI) marks as depends on MMU,
> > > > so we cannot select it when CONFIG_MMU=n.
> > >
> > > That is a fairly recent change, see commit 8fe743b5eba0abfb ("PCI:
> > > Add CONFIG_MMU dependency") in v6.16-rc1.  As this is not a "hard"
> > > dependency, perhaps it should be reverted, iff you are willing to take
> > > care of the casual breakage?
> >
> > Why though? UML with PCI can't really be a functional thing, only a
> > testing thing, and testing PCI on !MMU when that is actually impossible
> > in non-simulation is pointless?
>
> currently nommu UML doesn't come with using PCI except building under
> kunit (ARCH=um), but I have in my mind to use it under !MMU
> environment, so would be an option in the future.
>
> and this series doesn't include PCI w/ !MMU.
>
> so, I would propose the modification to revert the MMU dependency when
> time has come.
>
> btw, what do you mean by "hard" dependency in this context, Geert ?

I mean the PCI core does not really require an MMU, and PCI in-se
should work without it (e.g. on Coldfire MCF54xx).  Arnd added the
dependency to avoid having to deal with future build regressions.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds