linux-kernel - [RFC 2/8] iommu: Add a helper to check if any iommu device is registered

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Message-Id: <20251201173012.18371-3-jacob.pan@linux.microsoft.com>
Date: Mon,  1 Dec 2025 09:30:06 -0800
From: Jacob Pan <jacob.pan@...ux.microsoft.com>
To: linux-kernel@...r.kernel.org,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Jason Gunthorpe <jgg@...dia.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Nicolin Chen <nicolinc@...dia.com>,
	"Tian, Kevin" <kevin.tian@...el.com>,
	"Liu, Yi L" <yi.l.liu@...el.com>
Cc: skhawaja@...gle.com,
	pasha.tatashin@...een.com,
	Jacob Pan <jacob.pan@...ux.microsoft.com>,
	Zhang Yu <zhangyu1@...ux.microsoft.com>,
	Jean Philippe-Brucker <jean-philippe@...aro.org>,
	David Matlack <dmatlack@...gle.com>
Subject: [RFC 2/8] iommu: Add a helper to check if any iommu device is registered

The dummy IOMMU driver for No-IOMMU mode should only be active when
no real IOMMU devices are present in the system. Introduce a helper
to check this condition, ensuring that the dummy driver does not
interfere when hardware-backed IOMMU support is available.

Signed-off-by: Jacob Pan <jacob.pan@...ux.microsoft.com>
---
 drivers/iommu/iommu.c | 10 ++++++++++
 include/linux/iommu.h |  1 +
 2 files changed, 11 insertions(+)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 0df914a04064..958f612bf176 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2895,6 +2895,16 @@ static const struct iommu_device *iommu_from_fwnode(const struct fwnode_handle *
 	return ret;
 }
 
+bool iommu_is_registered(void)
+{
+	bool registered;
+
+	spin_lock(&iommu_device_lock);
+	registered = !list_empty(&iommu_device_list);
+	spin_unlock(&iommu_device_lock);
+	return registered;
+}
+
 const struct iommu_ops *iommu_ops_from_fwnode(const struct fwnode_handle *fwnode)
 {
 	const struct iommu_device *iommu = iommu_from_fwnode(fwnode);
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index c30d12e16473..4191ae7312dd 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -933,6 +933,7 @@ extern void iommu_put_resv_regions(struct device *dev, struct list_head *list);
 extern void iommu_set_default_passthrough(bool cmd_line);
 extern void iommu_set_default_translated(bool cmd_line);
 extern bool iommu_default_passthrough(void);
+extern bool iommu_is_registered(void);
 extern struct iommu_resv_region *
 iommu_alloc_resv_region(phys_addr_t start, size_t length, int prot,
 			enum iommu_resv_type type, gfp_t gfp);
-- 
2.34.1