linux-kernel - Re: [PATCH] PCI: apple: Add depends on PAGE_SIZE

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Message-ID: <20250211195601.GB810942@robin.jannau.net>
Date: Tue, 11 Feb 2025 20:56:01 +0100
From: Janne Grunau <j@...nau.net>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Alyssa Rosenzweig <alyssa@...enzweig.io>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: apple: Add depends on PAGE_SIZE_16KB

On Tue, Feb 11, 2025 at 12:38:59PM -0600, Bjorn Helgaas wrote:
> On Tue, Feb 11, 2025 at 01:03:52PM -0500, Alyssa Rosenzweig wrote:
> > From: Janne Grunau <j@...nau.net>
> > 
> > The iommu on Apple's M1 and M2 supports only a page size of 16kB and is
> > mandatory for PCIe devices. Mismatched page sizes will render devices
> > useless due to non-working DMA. While the iommu prints a warning in this
> > scenario, it seems a common and hard to debug problem, so prevent it at
> > build-time.
> 
> Can we include a sample iommu warning here to help people debug this
> problem?

I don't remember and it might have changed in the meantime due to iommu
subsystem changes. What currently happens is that
apple_dart_attach_dev_identity() fails with -EINVAL. I can't say whether
that results in a failure to probe now. I'll test and report back.

Janne