linux-kernel - Re: [RFC] iommu: Fix virtio-iommu probing

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Message-ID: <e3935099-6e29-493d-8587-64ceca8a20e9@arm.com>
Date: Thu, 14 Aug 2025 15:53:43 +0100
From: Robin Murphy <robin.murphy@....com>
To: Eric Auger <eric.auger@...hat.com>, eric.auger.pro@...il.com,
 rafael@...nel.org, bhelgaas@...gle.com, jgg@...pe.ca, lpieralisi@...nel.org,
 linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org, lenb@...nel.org,
 jean-philippe@...aro.org, jsnitsel@...hat.com
Subject: Re: [RFC] iommu: Fix virtio-iommu probing

On 14/08/2025 3:17 pm, Eric Auger wrote:
> Commit bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper
> probe path") broke virtio-iommu probing and no iommu group are
> produced anymore.
> 
> When probe_iommu_group() gets called viommu_probe_device() fails
> because viommu_get_by_fwnode(fwspec->iommu_fwnode) returns NULL.

...which it's not supposed to. And *now* I remember, we never got this 
finished, did we?

https://lore.kernel.org/linux-iommu/9beaed48da83a0882dba153e65e6cfd0a8e21482.1742484773.git.robin.murphy@arm.com/

Thanks,
Robin.

> So it seems we need to restore the original iommu_probe_device
> call site in acpi_iommu_configure_id() to get a chance to probe
> the device again.
> 
> Maybe this defeats the whole purpose of the original commit but
> at least it fixes the virtio-iommu probing.
> 
> Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
> Cc: stable@...r.kernel.org # v6.15+
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> 
> ---
> 
> I also tested smmu probing and this seems to work fine.
> ---
>   drivers/acpi/scan.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
> index fb1fe9f3b1a3..9f4efa8f75a6 100644
> --- a/drivers/acpi/scan.c
> +++ b/drivers/acpi/scan.c
> @@ -1632,6 +1632,13 @@ static int acpi_iommu_configure_id(struct device *dev, const u32 *id_in)
>   		err = viot_iommu_configure(dev);
>   	mutex_unlock(&iommu_probe_device_lock);
>   
> +	/*
> +	 * If we have reason to believe the IOMMU driver missed the initial
> +	 * iommu_probe_device() call for dev, replay it to get things in order.
> +	 */
> +	if (!err && dev->bus)
> +		err = iommu_probe_device(dev);
> +
>   	return err;
>   }
>