[PATCH V3 6/8] xen/grant-dma-iommu: Introduce stub IOMMU driver

[Oleksandr Tyshchenko <olekstysh@gmail.com>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

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 <oleksandr_tyshchenko@epam.com>
---
According to the discussion at:
https://lore.kernel.org/xen-devel/c0f78aab-e723-fe00-a310-9fe52ec75e48@gmail.com/

Change V2 -> V3:
   - new patch
---
 drivers/xen/Kconfig           |  4 +++
 drivers/xen/Makefile          |  1 +
 drivers/xen/grant-dma-iommu.c | 78 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 83 insertions(+)
 create mode 100644 drivers/xen/grant-dma-iommu.c

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index a7bd8ce..35d20d9 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -335,6 +335,10 @@ config XEN_UNPOPULATED_ALLOC
 	  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
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 1a23cb0..c0503f1 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -40,3 +40,4 @@ xen-privcmd-y				:= privcmd.o privcmd-buf.o
 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
diff --git a/drivers/xen/grant-dma-iommu.c b/drivers/xen/grant-dma-iommu.c
new file mode 100644
index 00000000..16b8bc0
--- /dev/null
+++ b/drivers/xen/grant-dma-iommu.c
@@ -0,0 +1,78 @@
+// 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);
-- 
2.7.4