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: <a58620ecefa207e141a435c36492647c3d5bd3df.camel@sipsolutions.net>
Date: Thu, 18 Sep 2025 10:30:22 +0200
From: Johannes Berg <johannes@...solutions.net>
To: Hajime Tazaki <thehajime@...il.com>, linux-um@...ts.infradead.org
Cc: ricarkol@...gle.com, Liam.Howlett@...cle.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v11 10/13] um: nommu: a work around for MMU
 dependency to PCI driver

On Thu, 2025-09-18 at 16:39 +0900, Hajime Tazaki wrote:
> The commit 8fe743b5eba0 ("PCI: Add CONFIG_MMU dependency") restricts the
> PCI base driver to depend on MMU.  While nommu UML _can_ implement PCI
> drivers over PCI devices (e.g., virtio-pci), the current nommu UML
> doesn't implement it.
> 
> But without PCI drivers kunit complains as config for kunit
> (arch_uml.config) defines the dependency to PCI drivers.
> 
> This commit fixes the issue of this compile failures when building PCI
> drivers with nommu UML.  In particular, the fix is to undefine the
> constant PCI_IOBASE to be able to bypass pci_unmap_iospace() call.

This doesn't make a lot of sense to me. Why would we even want to build
PCI on NOMMU-UML if PCI in general is dependent on MMU now?

It's not like ARCH=um with PCI and NOMMU has any value even for testing
if such a configuration cannot exist in reality?

johannes