iommu/arm-smmu: Report IOMMU_CAP_CACHE_COHERENCY better

Assuming that any SMMU can enforce coherency for any device is clearly
nonsense. Although technically even a single SMMU instance can be wired
up to only be capable of emitting coherent traffic for some of the
devices it translates, it's a fairly realistic approximation that if the
SMMU's pagetable walker is wired up to a coherent interconnect then all
its translation units probably are too, and conversely that lack of
coherent table walks implies a non-coherent system in general. Either
way it's still less inaccurate than what we've been claiming so far.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/106c9741415f0b6358c72d53ae9c78c553a2b45c.1660574547.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Author: rmurphy-arm

drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c

@@ -1994,9 +1994,12 @@ static const struct iommu_flush_ops arm_smmu_flush_ops = {
 /* IOMMU API */
 static bool arm_smmu_capable(struct device *dev, enum iommu_cap cap)
 {
+	struct arm_smmu_master *master = dev_iommu_priv_get(dev);
+
 	switch (cap) {
 	case IOMMU_CAP_CACHE_COHERENCY:
-		return true;
+		/* Assume that a coherent TCU implies coherent TBUs */
+		return master->smmu->features & ARM_SMMU_FEAT_COHERENCY;
 	case IOMMU_CAP_NOEXEC:
 		return true;
 	default:

drivers/iommu/arm/arm-smmu/arm-smmu.c

@@ -1332,13 +1332,12 @@ static phys_addr_t arm_smmu_iova_to_phys(struct iommu_domain *domain,
 
 static bool arm_smmu_capable(struct device *dev, enum iommu_cap cap)
 {
+	struct arm_smmu_master_cfg *cfg = dev_iommu_priv_get(dev);
+
 	switch (cap) {
 	case IOMMU_CAP_CACHE_COHERENCY:
-		/*
-		 * Return true here as the SMMU can always send out coherent
-		 * requests.
-		 */
-		return true;
+		/* Assume that a coherent TCU implies coherent TBUs */
+		return cfg->smmu->features & ARM_SMMU_FEAT_COHERENT_WALK;
 	case IOMMU_CAP_NOEXEC:
 		return true;
 	default: