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: <b55251bf-625e-48c3-8c60-eb4f8358f397@app.fastmail.com>
Date: Wed, 24 Sep 2025 13:03:11 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Hajime Tazaki" <thehajime@...il.com>,
 "Johannes Berg" <johannes@...solutions.net>
Cc: "Geert Uytterhoeven" <geert@...ux-m68k.org>, linux-um@...ts.infradead.org,
 "Ricardo Koller" <ricarkol@...gle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH RESEND v11 10/13] um: nommu: a work around for MMU dependency to
 PCI driver

On Wed, Sep 24, 2025, at 01:51, Hajime Tazaki 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.

The reason why it's currently disabled is that it was causing extra
work to fix build failures in random PCI drivers that individually
have a CONFIG_MMU dependency. Since we know that none of the NOMMU
boards we support uses PCI, this was an easy way to avoid work.

While there are still developers that care about NOMMU Linux
and test it on the platforms they use, the NOMMU build failures
usually end up in code that are irrelevant for their use cases,
so neither the platform owners nor the driver authors care deeply
about fixing that combination.

If you want to be able to use PCI drivers on UML-NOMMU,
you can probably use

config PCI
       depends on MMU || UML

so it will be ignored by the build bots on other architectures.
You'll still have to decide whether to fix driver code when
regressions in PCI drivers happen, add 'depends on !UML'
to individual drivers, or just live with randconfig failures.

      Arnd