iommu: Introduce device_iommu_capable()

iommu_capable() only really works for systems where all IOMMU instances
are completely homogeneous, and all devices are IOMMU-mapped. Implement
the new variant which will be able to give a more accurate answer for
whichever device the caller is actually interested in, and even more so
once all the external users have been converted and we can reliably pass
the device pointer through the internal driver interface too.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/8407eb9586677995b7a9fd70d0fd82d85929a9bb.1650878781.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu.c

@@ -1906,6 +1906,29 @@ bool iommu_present(struct bus_type *bus)
 }
 EXPORT_SYMBOL_GPL(iommu_present);
 
+/**
+ * device_iommu_capable() - check for a general IOMMU capability
+ * @dev: device to which the capability would be relevant, if available
+ * @cap: IOMMU capability
+ *
+ * Return: true if an IOMMU is present and supports the given capability
+ * for the given device, otherwise false.
+ */
+bool device_iommu_capable(struct device *dev, enum iommu_cap cap)
+{
+	const struct iommu_ops *ops;
+
+	if (!dev->iommu || !dev->iommu->iommu_dev)
+		return false;
+
+	ops = dev_iommu_ops(dev);
+	if (!ops->capable)
+		return false;
+
+	return ops->capable(cap);
+}
+EXPORT_SYMBOL_GPL(device_iommu_capable);
+
 bool iommu_capable(struct bus_type *bus, enum iommu_cap cap)
 {
 	if (!bus->iommu_ops || !bus->iommu_ops->capable)

include/linux/iommu.h

@@ -417,6 +417,7 @@ static inline const struct iommu_ops *dev_iommu_ops(struct device *dev)
 extern int bus_set_iommu(struct bus_type *bus, const struct iommu_ops *ops);
 extern int bus_iommu_probe(struct bus_type *bus);
 extern bool iommu_present(struct bus_type *bus);
+extern bool device_iommu_capable(struct device *dev, enum iommu_cap cap);
 extern bool iommu_capable(struct bus_type *bus, enum iommu_cap cap);
 extern struct iommu_domain *iommu_domain_alloc(struct bus_type *bus);
 extern struct iommu_group *iommu_group_get_by_id(int id);
@@ -689,6 +690,11 @@ static inline bool iommu_present(struct bus_type *bus)
 	return false;
 }
 
+static inline bool device_iommu_capable(struct device *dev, enum iommu_cap cap)
+{
+	return false;
+}
+
 static inline bool iommu_capable(struct bus_type *bus, enum iommu_cap cap)
 {
 	return false;