iommu: Add a capability for flush queue support

Passing a special type to domain_alloc to indirectly query whether flush
queues are a worthwhile optimisation with the given driver is a bit
clunky, and looking increasingly anachronistic. Let's put that into an
explicit capability instead.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Tested-by: Jerry Snitselaar <jsnitsel@redhat.com> # amd, intel, smmu-v3
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/f0086a93dbccb92622e1ace775846d81c1c4b174.1683233867.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd/iommu.c

@@ -2293,6 +2293,8 @@ static bool amd_iommu_capable(struct device *dev, enum iommu_cap cap)
 		return amdr_ivrs_remap_support;
 	case IOMMU_CAP_ENFORCE_CACHE_COHERENCY:
 		return true;
+	case IOMMU_CAP_DEFERRED_FLUSH:
+		return true;
 	default:
 		break;
 	}

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

@@ -2008,6 +2008,7 @@ static bool arm_smmu_capable(struct device *dev, enum iommu_cap cap)
 		/* Assume that a coherent TCU implies coherent TBUs */
 		return master->smmu->features & ARM_SMMU_FEAT_COHERENCY;
 	case IOMMU_CAP_NOEXEC:
+	case IOMMU_CAP_DEFERRED_FLUSH:
 		return true;
 	default:
 		return false;

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

@@ -1325,6 +1325,7 @@ static bool arm_smmu_capable(struct device *dev, enum iommu_cap cap)
 		return cfg->smmu->features & ARM_SMMU_FEAT_COHERENT_WALK ||
 			device_get_dma_attr(dev) == DEV_DMA_COHERENT;
 	case IOMMU_CAP_NOEXEC:
+	case IOMMU_CAP_DEFERRED_FLUSH:
 		return true;
 	default:
 		return false;

drivers/iommu/intel/iommu.c

@@ -4369,6 +4369,7 @@ static bool intel_iommu_capable(struct device *dev, enum iommu_cap cap)
 
 	switch (cap) {
 	case IOMMU_CAP_CACHE_COHERENCY:
+	case IOMMU_CAP_DEFERRED_FLUSH:
 		return true;
 	case IOMMU_CAP_PRE_BOOT_PROTECTION:
 		return dmar_platform_optin();

include/linux/iommu.h

@@ -127,6 +127,11 @@ enum iommu_cap {
 	 * this device.
 	 */
 	IOMMU_CAP_ENFORCE_CACHE_COHERENCY,
+	/*
+	 * IOMMU driver does not issue TLB maintenance during .unmap, so can
+	 * usefully support the non-strict DMA flush queue.
+	 */
+	IOMMU_CAP_DEFERRED_FLUSH,
 };
 
 /* These are the possible reserved region types */