xen/grant-dma-iommu: Introduce stub IOMMU driver

In order to reuse generic IOMMU device tree bindings by Xen grant
DMA-mapping layer we need to add this stub driver from a fw_devlink
perspective (grant-dma-ops cannot be converted into the proper
IOMMU driver).

Otherwise, just reusing IOMMU bindings (without having a corresponding
driver) leads to the deferred probe timeout afterwards, because
the IOMMU device never becomes available.

This stub driver does nothing except registering empty iommu_ops,
the upper layer "of_iommu" will treat this as NO_IOMMU condition
and won't return -EPROBE_DEFER.

As this driver is quite different from the most hardware IOMMU
implementations and only needed in Xen guests, place it in drivers/xen
directory. The subsequent commit will make use of it.

Signed-off-by: Oleksandr Tyshchenko <[e-mail]>
Reviewed-by: Stefano Stabellini <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/Kconfig

···
 	  having to balloon out RAM regions in order to obtain physical memory
 	  space to create such mappings.
 
+config XEN_GRANT_DMA_IOMMU
+	bool
+	select IOMMU_API
+
 config XEN_GRANT_DMA_OPS
 	bool
 	select DMA_OPS

drivers/xen/Makefile

···
 obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF)	+= xen-front-pgdir-shbuf.o
 obj-$(CONFIG_XEN_UNPOPULATED_ALLOC)	+= unpopulated-alloc.o
 obj-$(CONFIG_XEN_GRANT_DMA_OPS)		+= grant-dma-ops.o
+obj-$(CONFIG_XEN_GRANT_DMA_IOMMU)	+= grant-dma-iommu.o

drivers/xen/grant-dma-iommu.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Stub IOMMU driver which does nothing.
+ * The main purpose of it being present is to reuse generic IOMMU device tree
+ * bindings by Xen grant DMA-mapping layer.
+ *
+ * Copyright (C) 2022 EPAM Systems Inc.
+ */
+
+#include <linux/iommu.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+
+struct grant_dma_iommu_device {
+	struct device *dev;
+	struct iommu_device iommu;
+};
+
+/* Nothing is really needed here */
+static const struct iommu_ops grant_dma_iommu_ops;
+
+static const struct of_device_id grant_dma_iommu_of_match[] = {
+	{ .compatible = "xen,grant-dma" },
+	{ },
+};
+
+static int grant_dma_iommu_probe(struct platform_device *pdev)
+{
+	struct grant_dma_iommu_device *mmu;
+	int ret;
+
+	mmu = devm_kzalloc(&pdev->dev, sizeof(*mmu), GFP_KERNEL);
+	if (!mmu)
+		return -ENOMEM;
+
+	mmu->dev = &pdev->dev;
+
+	ret = iommu_device_register(&mmu->iommu, &grant_dma_iommu_ops, &pdev->dev);
+	if (ret)
+		return ret;
+
+	platform_set_drvdata(pdev, mmu);
+
+	return 0;
+}
+
+static int grant_dma_iommu_remove(struct platform_device *pdev)
+{
+	struct grant_dma_iommu_device *mmu = platform_get_drvdata(pdev);
+
+	platform_set_drvdata(pdev, NULL);
+	iommu_device_unregister(&mmu->iommu);
+
+	return 0;
+}
+
+static struct platform_driver grant_dma_iommu_driver = {
+	.driver = {
+		.name = "grant-dma-iommu",
+		.of_match_table = grant_dma_iommu_of_match,
+	},
+	.probe = grant_dma_iommu_probe,
+	.remove = grant_dma_iommu_remove,
+};
+
+static int __init grant_dma_iommu_init(void)
+{
+	struct device_node *iommu_np;
+
+	iommu_np = of_find_matching_node(NULL, grant_dma_iommu_of_match);
+	if (!iommu_np)
+		return 0;
+
+	of_node_put(iommu_np);
+
+	return platform_driver_register(&grant_dma_iommu_driver);
+}
+subsys_initcall(grant_dma_iommu_init);