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: <m2qzvv77p0.wl-thehajime@gmail.com>
Date: Thu, 25 Sep 2025 08:27:07 +0900
From: Hajime Tazaki <thehajime@...il.com>
To: arnd@...db.de
Cc: johannes@...solutions.net,
	geert@...ux-m68k.org,
	linux-um@...ts.infradead.org,
	ricarkol@...gle.com,
	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


Hello Arnd,

On Wed, 24 Sep 2025 20:03:11 +0900,
Arnd Bergmann wrote:

> > 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.

thanks for the detail information.
yes, when nommu UML wish to use PCI drivers in the future, I'll take
this approach to be able to use it.

-- Hajime