Re: [PATCH] PCI: apple: Add depends on PAGE_SIZE_16KB

[Manivannan Sadhasivam <mani@kernel.org>]

On Tue, Feb 11, 2025 at 11:00:19PM +0100, Janne Grunau wrote:
> On Tue, Feb 11, 2025 at 08:56:04PM +0100, Janne Grunau wrote:
> > 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@jannau.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.
> 
> Using a kernel with 4K page size It results now in following warning per
> PCI device:
> 
> | ------------[ cut here ]------------
> | WARNING: CPU: 7 PID: 260 at drivers/iommu/iommu.c:2979 iommu_setup_default_domain+0x348/0x590

This should be added to the patch description.

- Mani

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